Anthony P. Weetman
Chronic autoimmune thyroiditis is the original paradigm for autoimmune diseases in general. Only foreign proteins were considered to be antigens until Witebsky and Rose demonstrated that rabbits immunized with a saline extract of homologous thyroid tissue and Freund's adjuvant produced thyroid antibodies and had lymphocytic infiltration in the thyroid gland (1). In the same year, 1956, Doniach and Roitt reported that many patients with chronic goitrous (Hashimoto's) thyroiditis had high serum concentrations of antithyroglobulin antibodies (2). A year later, a second autoantigen was identified in the microsomal fraction of thyroid homogenates; it proved to be thyroid peroxidase (TPO) (3). We know now that other components of thyroid tissue, including the thyrotropin (TSH) receptor and, rarely, the sodium/iodide cotransporter, can serve as autoantigens.
The term autoimmune thyroiditis encompasses several different entities whose interrelationships remain unclear (Table 47.1). The most important are chronic goitrous thyroiditis and chronic atrophic thyroiditis. The pathology of these disorders, considered in detail in Chapter 21, consists of varying degrees of lymphocytic infiltration, fibrosis, and loss of follicular epithelium; occasionally some follicular hyperplasia is seen if the thyroid gland is enlarged. Repeat biopsies, performed at intervals of up to 20 years, reveal little alteration in thyroid histology, even in patients treated with thyroxine (T4) (4). There is no good evidence that goitrous thyroiditis precedes atrophic thyroiditis. Among patients with hypothyroidism, the severity of fibrosis correlates with age (5). The atrophic and goitrous variants share many clinical and biochemical, as well as pathologic, features, suggesting that their pathogenesis overlaps.
TABLE 47.1. TYPES OF AUTOIMMUNE THYROIDITIS
Goitrous (Hashimoto's) thyroiditis
Goiter, lymphocytic infiltration, fibrosis, thyroid cell hyperplasia
Atrophic thyroiditis (primary myxedema)
Usually lymphocytic infiltration
Transient; may progress to chronic thyroiditis
Small goiter, some lymphocytic infiltration
Silent (painless) thyroiditis
Small goiter, some lymphocytic infiltration
Progressive in some patients
Present in the thyroid gland of 20% of people at autopsy
EXPERIMENTAL AUTOIMMUNE THYROIDITIS
Studies of various types of experimental autoimmune thyroiditis have provided major insights into the etiology and pathogenesis of autoimmune thyroiditis, although it is not completely satisfactory as a model of chronic autoimmune thyroiditis in humans. Immunization of mice with thyroglobulin (Tg) and an adjuvant causes transient thyroiditis, but its severity correlates only moderately well with Tg antibody production (6), and there is little change in thyroid function. Genetic susceptibility is determined predominantly but not exclusively by class I and II major histocompatibility complex (MHC) genes (6). A critical T-cell epitope in Tg includes conserved hormonogenic sites, and iodination of these may increase the antigenicity of Tg (7).
Experimental autoimmune thyroiditis can be transferred by T cells but not by serum, and both CD4 and CD8 T cells are required for optimal effects. However, a subpopulation of CD4 T cells has an important regulatory role in maintaining tolerance to Tg under normal circumstances (8). Cytotoxic CD8 T cells are important mediators of thyroid-cell damage in this type of experimental thyroiditis (9). A variation of experimental autoimmune thyroiditis is that which occurs in mice carrying human leukocyte antigen (HLA)-DR3 transgenes. These mice are susceptible to the induction of thyroiditis when immunized with Tg, unlike HLA-DR2 transgenic mice, thus confirming a role for the HLA haplotype in determining susceptibility to autoimmune thyroiditis (10).
Changes in the T-cell repertoire of mice or rats, as, for example, induced by thymectomy with or without sublethal irradiation, causes many organ-specific, genetically determined autoimmune diseases, including autoimmune thyroiditis (11). It can be transferred by CD4 cells, like when it is immunization induced, but a subfraction of normal CD4 cells prevents it, either directly or by inducing regulatory (suppressor) T cells. At least two explanations are offered for the etiology of this type of thyroiditis, related to different mechanisms for maintaining self-tolerance (Table 47.2). One is that autoreactive T cells escape tolerance because they do not encounter thyroid autoantigen in the appropriate setting in the athymic animals. However, intrathymic tolerance is never complete, and other mechanisms are required to prevent autoimmunity, such as T cell–mediated immune regulation. Thymectomy can therefore induce experimental autoimmune thyroiditis if performed at a stage of T-cell development when autoreactive, effector T cells have left the thymus for the periphery but the regulatory T cells have not. There has been intense recent interest in these regulatory T cells, one distinct population of which expresses CD4 and high levels of CD25 and may sample self antigens presented by dendritic cells in the thymus (12).
TABLE 47.2. T-CELL MECHANISMS INVOLVED IN IMMUNOLOGIC SELF-TOLERANCE
Intrathymic (central) deletion of autoreactive T cells
Intrathymic (central) anergy due to autoantigen presentation to CD4 cells in the absence of a costimulatory signal
Peripheral tolerance, usually by anergy, but deletion may occur
Active suppression of T-helper cells by several possible pathways, including cytokines and regulatory T cells
Sequestration of autoantigen, particularly by means of tissue expression of Fas ligand (immunologic privilege): autoreactive T cells expressing Fas undergo apoptosis
Absence of the activated CD4 cells required for expansion of CD8 cells or antigen sequestered by an anatomic barrier (immunologic ignorance)
Chronic thyroiditis occurs spontaneously in several animal species. A low proportion of female Buffalo strain rats develops chronic thyroiditis and high serum anti-Tg antibody concentrations, and the proportion can be increased by thymectomy or administration of methylcholanthrene (13). About 50% of diabetes-prone Biobreeding (BB) rats develop Tg antibodies and have lymphocytic infiltration of thyroid tissue, particularly when fed diets containing large amounts of iodide, but thyroid follicular destruction does not occur. The incidence of chronic thyroiditis and high serum antithyroid antibody concentrations is even higher in nonobese diabetic (NOD) mice (14).
Chronic thyroiditis in Obese strain (OS) chickens is perhaps the most like chronic goitrous thyroiditis, because these animals develop overt hypothyroidism as well as appropriate immunologic abnormalities (15). The MHC contributes slightly in determining susceptibility, but the major contribution is from uncharacterized immunoregulatory genes outside the MHC that affect T-cell responsiveness and control Tg antibody production. Additional genes increase the susceptibility of the thyroid to autoimmune damage, although no analogous genetic influence is apparent in other forms of experimental autoimmune thyroiditis.
Environmental factors are also important in the pathogenesis of autoimmune thyroiditis (16). The best characterized of these factors is iodide, which can cause thyroid injury through the generation of reactive oxygen intermediates and which may also increase the immunogenicity of Tg and directly stimulate the immune system, particularly dendritic cell function.
PREDISPOSITION TO CHRONIC AUTOIMMUNE THYROIDITIS
Autoimmune diseases usually occur when there is a failure of T-cell tolerance as a result of a combination of genetic and nongenetic factors (Table 47.3). The detailed genetics of chronic goitrous and chronic atrophic thyroiditis are considered in Chapter 20. Nongenetic factors may be endogenous or exogenous. The high prevalence of chronic autoimmune thyroiditis in women is partly related to the effects of sex hormones, and is best demonstrated by manipulation of estrogen and androgen concentrations in experimental autoimmune thyroiditis and other animal models of autoimmune disease (6), but a role for a gene on the long arm of the X chromosome is suggested by the high frequency of autoimmune thyroid disease in women with Turner's syndrome (17). Postpartum thyroiditis (see Chapter 27) also may result from hormonal effects on the immune system and is an important predictor of subsequent hypothyroidism. An alternative explanation for the effect of pregnancy in thyroid autoimmunity is the occurrence of intrathyroidal fetal microchimerism, demonstrated in both animals with experimental autoimmune thyroiditis during pregnancy and in women with Hashimoto's thyroiditis (18,19). Fetal cells may provoke an intrathyroidal alloimmune reaction that would be especially pronounced in those with preexisting subclinical autoimmune thyroiditis.
TABLE 47.3. ROLES OF GENETIC AND NONGENETIC FACTORS IN AUTOIMMUNITY
Major histocompatibility complex
Encode products that delete autoreactive T cells, select for presentation of autoantigenic peptides, or activate suppressor T cells
Other MHC genes
Potential effects on antigen presentation; the cytokine tumor necrosis factor and some complement components also encoded in the MHC
Cytotoxic T-lymphocyte antigen-4 (CTLA-4) gene
CTLA-4 is important in terminating T-cell activation; polymorphic variants may fail to control autoreactive T cells fully
Control cytokine production
T-cell antigen receptor and immunoglobulin genes
Uncertain—some reports of association with autoimmunity
May release autoantigens, alter expression of surface molecules, directly affect the immune system, or contain immunogenic sequences (epitopes) that mimic autoantigens
Diverse potential effects; e.g., iodide may enhance the immunogenicity of thyroglobulin, alter thyroid cell function, or form toxic metabolites with oxygen
Diverse potential effects; e.g., methylcholanthrene enhances thyroiditis in Buffalo strain rats and toxins in cigarette smoke affect cytokine production
Diverse potential effects; estrogens enhance most immune responses, whereas glucocorticoids and androgens suppress the responses
May alter neuroendocrine interactions with the immune system
Diverse potential effects on the immune system; e.g., lithium, interferon-α, and some other cytokines exacerbate autoimmune thyroiditis
A role for stress is not apparent in chronic autoimmune thyroiditis, possibly because the disease evolves over such a long period, but stress-related variations in cortisol secretion could affect susceptibility, as suggested by observations in OS chickens (15). Low birth weight increases the prevalence of high serum antithyroid antibody concentrations in monozygotic twins discordant for birth size (20), demonstrating the potential effect of early nutritional and hormonal factors.
Despite the plausibility of the hypothesis that infectious agents cause autoimmunity via tissue damage or molecular mimicry, there is no evidence that infection plays a role in chronic autoimmune thyroiditis; this may be due to the difficulty in proving the influence of any environmental agent that may operate only at the beginning of what is a long preclinical course of disease (21). Normal gut microflora are required for the appearance of experimental autoimmune thyroiditis after thymectomy and irradiation in rats, although the mechanism is unclear (22). Polyclonal activation and molecular mimicry are both possible. There is an increase in autoimmune thyroid disease in patients with the congenital rubella syndrome (23), and hepatitis C virus infection has been associated with chronic goitrous thyroiditis in some but not other studies (24); on the other hand, chronic autoimmune thyroiditis is not a sequel of subacute granulomatous thyroiditis, which is presumed to be caused by a viral infection.
Observations in animals provide the most compelling evidence that a high dietary iodine intake can exacerbate thyroid autoimmunity. A high iodine intake increases the frequency of spontaneous thyroiditis in rats and OS chickens, as does a normal amount given after a period of iodine restriction (16). The effects of iodine supplementation on thyroid autoimmunity in humans are contradictory, probably due to the existence of immunoregulatory mechanisms that render any effects transient, and also genetic and environmental differences. Nonetheless, iodine administration can increase the frequency of high serum antithyroid antibody concentrations and lymphocytic thyroiditis in patients with goiter (25).
In patients with cancer or chronic hepatitis who have subclinical chronic autoimmune thyroiditis, treatment with interferon-α (IFN-α), interleukin-2 (IL-2), or granulocyte-macrophage colony-stimulating factor worsens the autoimmune process and may cause transient thyrotoxicosis, hypothyroidism, or both, and sometimes permanent hypothyroidism (26,27,28). The frequency of chronic autoimmune thyroiditis was increased in atomic bomb survivors in Japan and in children exposed to fallout after the Chernobyl nuclear reactor accident in the Ukraine (29,30).
Lithium also exacerbates autoimmune thyroiditis, probably by a direct effect on regulatory T cells (31).
PATHOGENESIS OF CHRONIC AUTOIMMUNE THYROIDITIS
The initial event in the autoimmune process is presentation of antigen, in which an antigen is taken up by cells and processed into peptide epitopes, and the peptides then combine with MHC class II molecules on those cells to form bimolecular complexes (32) (Fig. 47.1). The binding of these complexes to T-cell antigen receptors leads to T-cell activation and the release of cytokines, which, for instance, can provide help for B cells to make antibodies. To stimulate T cells, particularly na▪ve T cells not previously exposed to antigen, costimulatory signals are required, the best characterized of which is a family of proteins called B7 found on antigen-presenting cells (33). These proteins bind to CD28 receptors on T cells to augment or induce T-cell activation by the complexes of MHC class II and peptide (antigen) molecules. By binding, instead, to cytotoxic lymphocyte antigen-4 (CTLA-4) expressed on activated T cells, B7 can terminate T-cell activation. Other costimulatory signal receptors exist, some with structural similarity to CD28, such as inducible costimulator, and others that are members of the tumor necrosis factor (TNF) receptor family, such as CD40 (34). Cytokines can increase the production of inducible costimulator, but not that of B7, in nonlymphoid tissues; thus, inflamed tissues may be able to stimulate memory but not na▪ve T cells and enhance an ongoing immune response.
FIGURE 47.1. Major molecular interactions in antigen presentation. A: In a typical antigen-presenting cell (APC), such as a dendritic cell, protein antigen is processed into a peptide epitope of about 12 to 20 amino acids that binds to the polymorphic region of a major histocompatibility complex (MHC) class II molecule. This is recognized by the heterodimeric T-cell antigen receptor, composed of an α and a β chain, and the interaction is stabilized by a CD4 molecule that binds to a nonpolymorphic region of the MHC class II molecule. Full activation of the T cell also requires a costimulatory signal, such as that supplied by B7 through CD28. B: If B7 engages with cytotoxic lymphocyte antigen-4 (CTLA-4) on the T cell, the result is anergy, this pathway being responsible for termination of an immune response. C: Antigen presentation by a nonprofessional APC, such as a thyroid cell, can have alternate outcomes depending on prior exposure of the T cell to costimulation. (Reprinted from Weetman AP. Autoimmune thyroid disease: propagation and progression. Eur J Endocrinol 2003;148:1, with permission.)
Most antigen-presenting cells such as dendritic cells constitutively express MHC class II molecules and costimulatory molecules. Normal thyroid cells do not express class II molecules, but those from patients with chronic autoimmune thyroiditis do (35). A close spatial correlation exists between class II molecules on thyroid cells and IFN-γ-containing T cells in thyroid tissue (36); this T cell–derived cytokine is the only known initiator of class II xpression by normal thyroid cells in vitro. In animals with experimental autoimmune thyroiditis, MHC class II–positive thyroid cells only appear late in the disease, after T-cell infiltration (13). Therefore, MHC class II expression on thyroid cells is probably the consequence rather than the cause of thyroid autoimmunity, and as a result, antigen presentation by thyroid cells is likely to be important only in the perpetuation of the autoimmune response.
Class II–positive thyroid cells can stimulate T cells, but it is unclear whether the thyroid cells also express costimulators: B7–1, but not B7–2, was identified on the remaining thyroid follicles in goitrous thyroiditis (37). In vitro, however, class II–positive thyroid cells do not stimulate T cells that require B7, although T cells not requiring costimulation via B7 can be activated (38). Thus, in the absence of B7, which seems likely to be the case early in the pathogenesis of autoimmune thyroiditis, thyroid cells will not stimulate naïve, autoreactive T cells (Fig. 47.1). Instead, these T cells might actually acquire tolerance through the operation of anergy (39), which can be viewed as a protective mechanism (Table 47.2). Once T cells have encountered an appropriately presented antigen, they are less dependent on costimulation for subsequent activation and therefore could respond to class II–positive thyroid cells.
Thus, the extent of antigen presentation by thyroid cells in autoimmune thyroiditis should depend at least in part on the timing of the appearance of class II molecules on these cells. Prompt appearance of class II molecules on thyroid cells, for example, in response to a viral infection, could be an important protective mechanism because this might render potentially autoreactive T cells anergic (Fig. 47.2). However, in chronic autoimmune thyroiditis, these T cells are presumably first activated by dendritic cells and may then be further activated by class II–expressing thyroid cells, depending on their requirement for costimulation.
FIGURE 47.2. Alternative outcomes after expression of major histocompatibility complex class II molecules by thyroid cells. Naïve T cells undergo peripheral tolerance through anergy because the thyroid follicular cells do not supply costimulatory signals. However, T cells that have already encountered autoantigen presented by a macrophage or dendritic cell may be stimulated in the absence of further costimulation, exacerbating the autoimmune response. IFN, interferon.
Phenotypic changes in peripheral blood and thyroid-infiltrating T cells provide only indirect information on T-cell function in chronic autoimmune thyroiditis. In the circulation, the number of activated T cells expressing HLA-DR is increased (40) and the number of CD8 cells is decreased, but only during active thyroid damage (41). Of more direct relevance is the phenotype of T cells infiltrating the thyroid, which are mainly CD4 T cells that express activation markers such as MHC class II molecules (42).
The clonality of these T cells is of major interest, because in some animal models of autoimmune disease the T-cell response is clonally restricted in the early phase of the disease, as marked by the utilization of particular variable (Vα) region genes encoding the T-cell antigen receptor of pathogenic T cells (43). With time, however, the immune response spreads, so that the autoantigens are recognized by multiple T-cell antigen receptors (44). Restriction of Vα gene usage in T-cell antigen receptors was found among T cells infiltrating the thyroid gland in patients with Graves' disease and chronic autoimmune thyroiditis in one study, with use of an average of only 4 to 5 of the 18 Vα gene families (45). However, subsequent studies found no evidence for restriction of Vα gene usage by recently activated, IL-2 receptor–positive intrathyroidal T cells from patients with Graves' disease or chronic autoimmune thyroiditis, although a limited degree of restriction in the CD8 T-cell population was found in patients with goitrous thyroiditis (46,47). The role of these cells is unclear, but such restriction is compatible with expansion of T cells that may be involved in thyroid cell destruction. Moreover, in experimental autoimmune thyroiditis in NOD mice, marked restriction in V gene usage is not apparent even in early thyroiditis (48), and considerable heterogeneity of V gene usage also was found in the thyroiditis induced by T-cell depletion (49).
Circulating and intrathyroidal T cells from patients with chronic autoimmune thyroiditis are weakly stimulated in vitro by Tg and TPO (50). Attempts have been made using synthetic peptides or recombinant fusion proteins to identify the T-cell epitopes within TPO; based on these studies, the epitopes of TPO seem to cluster between residues 415 and 589 of the protein. However, the antibody responses are quite heterogeneous, with patients having antibodies that react with varying numbers of epitopes of TPO and with marked variation of the epitope specificity of the antibodies among different patients (51,52,53). These results are also compatible with a polyclonal T-cell response that has spread to involve multiple epitopes, some of which are “cryptic” in the sense that they are recognized by the immune system only during the course of an autoimmune response. Furthermore, TPO seems to be processed differently by thyroid follicular cells and professional antigen-presenting cells, and different T-cell clones respond differently to peptide fragments of TPO presented by the two types of cells (54); these differences could play an important role in the activity of the disease.
T cells regulate the immune response. A defect in immune suppression (usually now called immune regulation) mediated by T cells has been proposed to play a major role in the pathogenesis of autoimmune thyroiditis (55). Several in vitro techniques were used to identify this defect, including the migration inhibition assay and mitogen-dependent autoantibody production (56,57). The results suggested that a subset of circulating T cells from normal subjects inhibits the function of T cells from patients with chronic autoimmune thyroiditis, and that cells of this subset are lacking in the patients. However, the physiologic relevance of these assay systems has been questioned, because the systems involve coculture of allogeneic cells and because such effects are not always antigen specific (58). The effects also do not correspond with those expected of CD4, CD25 regulatory cells, but it remains possible that as yet unexplored abnormalities occur in these cells that would induce thyroid autoimmunity. It is also possible that immunoregulatory function could be impaired by changes in dendritic cell–mediated induction of regulatory T cells or in the expression of the chemokines that guide these cells to their targets (59).
Nonetheless, there is little doubt from studies of experimental autoimmune thyroiditis that some form of antigen-specific T cell–mediated regulation can be critical in determining the outcome of an immune response; how these T cells mediate this effect is unclear (6). The particular cytokines released by subsets of antigen-stimulated T-helper (Th) cells may confer suppressor function, with the products of the Th1 subset (cells involved primarily in cell-mediated immunity) suppressing Th2 cells (cells involved primarily in antibody-mediated immunity) and vice versa (60). Stimulation of Th2 responses in animal models of autoimmunity results in deviation of the immune response away from the more harmful Th1 responses (61,62). On this basis, genetically determined differences in cytokine production could result in variations in immunoregulation and contribute to autoimmune disease. There are several other explanations for suppressor-like effects, including idiotypic interaction between T-cell receptors, direct cytotoxicity, and secretion of antigen-specific suppressor factors (63,64). With such complexity, it is not surprising that there remain questions regarding the specificity and time of appearance of any immunoregulatory defect in patients with chronic autoimmune thyroiditis.
The production of thyroid antibodies in patients with chronic autoimmune thyroiditis is considered in detail in Chapter 15. The variable heavy- and light-chain composition of Tg and TPO autoantibodies indicates that production of these antibodies is the result of a polyclonal B-cell response (65). However, the range of heavy- and light-chain combinations of TPO and, to a lesser extent, Tg antibodies (66,67,68), is limited, which could be a result of the genetic control of thyroid antibody production noted previously. Far less is known about the molecular structure of TSH receptor antibodies; at present all one can say is that TSH receptor–blocking antibodies appear to be more heterogeneous than TSH receptor–stimulating antibodies (69). The recent cloning of human TSH receptor antibodies should permit further analysis of the functional effects of stimulating and blocking antibodies (70).
The thyroid gland is an important source of thyroid antibodies in autoimmune thyroid disease, although the antibodies also are produced in the lymph nodes adjacent to the thyroid and in the bone marrow (71). Secondary lymphoid follicles, similar to those found in lymph nodes, are often present in thyroid tissue of patients with autoimmune thyroid disease. These follicles are capable of generating high-affinity antibodies, and their number correlates with the serum antibody concentration (72). Intrathyroidal B cells are polyclonal, but B-cell lymphomas that arise as a complication of chronic autoimmune thyroiditis are monoclonal (73).
Both humoral and cell-mediated effector mechanisms cause tissue injury in chronic autoimmune thyroiditis (Fig. 47.3). Intrathyroidal complement fixation by thyroid antibodies is revealed by the presence of terminal complement complexes around the thyroid follicles in chronic autoimmune thyroiditis; these complexes are also found in the circulation (74). Although TPO antibodies can activate complement, the correlation between the extent of complement fixation in thyroid tissue and serum TPO antibody concentrations is poor in patients with chronic autoimmune thyroiditis, indicating the presence of additional complement-fixing antibodies (75). Thyroid cells are not necessarily lysed by complement attack, because they are protected by several inhibitors of complement activation (76). Nonlethal complement attack may lead to impaired responsiveness to TSH (77), and to the release of proinflammatory molecules including IL-1, IL-6, prostaglandin E2, and reactive oxygen metabolites, all of which could exacerbate thyroid injury (78). Transplacental passage of complement-fixing antibodies does not affect the fetal thyroid, presumably because the antibodies do not gain sufficient access to their target antigen. TPO antibodies can be detected within thyroid follicular cells in patients with thyroid autoimmune disease (79). It is possible that tight junctions between thyroid cells limit antibody binding to TPO. IL-1 causes disruption of these junctions, suggesting that intrathyroidal cytokine production by lymphocytes and macrophages may be needed to expose hidden thyroid autoantigens and render them accessible to antibodies (80).
FIGURE 47.3. Effector mechanisms leading to thyroid follicular cell injury. Expression of major histocompatibility complex class I molecules and adhesion molecules by thyroid follicular cells in patients with chronic autoimmune thyroiditis may increase the potential for cytotoxicity by allowing binding of cytotoxic T cells to thyroid follicular cells. Cytokines produced by the infiltrating macrophages and T cells can alter thyroid cell function and also increase expression of complement regulatory proteins, such as CD59, which prevent lethal complement-mediated injury, but sublethal complement attack impairs thyroid cell metabolism. Cytokines such as interleukin-1 also upregulate Fas expression by thyroid cells. ADCC, antibody-dependent cell-mediated cytotoxicity. (Reprinted from Weetman AP. Autoimmune thyroid disease. In: Wass JAH, Shalet SM, eds. Oxford textbook of endocrinology and diabetes. Oxford, UK: Oxford University Press 2002:404, with permission.)
Autoantibodies play a key role in the process of antibody-dependent cell-mediated cytotoxicity, in which natural killer (NK) cells kill thyroid cells because the latter have autoantibodies bound to their surface that engage the immunoglobulin Fc receptors of the NK cells. Both TPO and Tg antibodies can mediate antibody-dependent cell-mediated cytotoxicity in vitro, and other uncharacterized thyroid autoantibodies also may be involved (81). The importance of this mechanism in vivo is unclear: although NK cells can be detected in thyroid tissue of patients with chronic autoimmune thyroiditis (43), NK cell activity in peripheral blood is reduced in comparison with normal subjects (82).
In addition, autoantibodies may have direct functional effects, considered in detail in Chapter 15. TSH receptor–blocking antibodies, directed against unique determinants on the TSH receptor (83), can cause hypothyroidism, although the relative contribution of this mechanism, as compared with the others described in this section, is not clear. In Japan and Korea, these antibodies are present in up to 75% of patients with hypothyroidism caused by atrophic thyroiditis; they are detected less often in whites, and the frequency is similar in white patients with goitrous and atrophic thyroiditis (84,85). In occasional patients, disappearance of TSH receptor-blocking antibodies is associated with remission of hypothyroidism, indicating that chronic thyroiditis does not invariably result in thyroid destruction. Transplacental passage of TSH receptor–blocking antibodies may cause transient neonatal hypothyroidism (see Chapter 75).
The existence of growth-promoting and growth-blocking antibodies, acting independently of the TSH receptor, has been postulated to account for goitrous and atrophic thyroiditis, respectively (86). These antibodies have not been well characterized, and most data indicate that thyroid growth is mediated via the TSH receptor.
T cell–mediated cytotoxicity is demonstrable in experimental autoimmune thyroiditis (9), and it is likely to be important in chronic autoimmune thyroiditis in humans. One CD8 cell clone established from a patient with chronic autoimmune thyroiditis killed thyroid cells in vitro, but the nature of the thyroid antigen recognized was not established (87). Cytotoxic T cells kill their targets by the action of soluble cytolytic mediators (perforin and granzymes) and by engaging their Fas ligand molecules with Fas molecules on the surface of the target cells. Many of the CD8 T cells and some CD4 T cells in the thyroid in goitrous thyroiditis express perforin, indicating that it is an important mediator of thyroid cell destruction (88).
Fas is expressed by thyroid cells in goitrous thyroiditis, apparently induced by IL-1, but not by normal thyroid cells (89). Expression of Fas renders thyroid cells liable to apoptosis by T cells expressing Fas ligand, although such T cells are not common in thyroid infiltrates. IL-1 also induces expression of Fas ligand on thyroid cells, leading to the possibility of cell death by interaction of Fas and its ligand on adjacent thyroid cells without any requirement for T cells (89). In addition, a substantial number of Fas-expressing T cells appear to undergo apoptosis in autoimmune thyroiditis, possibly through interaction with Fas ligand on thyroid cells (90). High levels of Bcl-2, an apoptosis regulatory protein, in the remaining lymphocytes protect them from apoptosis (91), but the content of Bcl-2 in thyroid cells is low in patients with autoimmune thyroiditis, rendering the thyroid cells prone to apoptosis. In contrast, the effect of Fas ligand expression in experimental autoimmune thyroiditis is opposite, so that infiltrating T cells are more susceptible to apoptosis, thereby minimizing tissue damage (92). An alternative or possibly complementary mode of thyroid cell destruction through apoptosis may involve a death ligand known as tumor necrosis factor–related apoptosis-inducing ligand (TRAIL), which can be induced on thyroid cells by cytokines (93), although others have not found this molecule to be important in thyroid cell killing (94). At present, the extent to which these different pathways account for tissue destruction in autoimmune thyroiditis is not known, and the importance of these different pathways probably varies among different patients.
The infiltrating T cells also release a wide array of cytokines, including IFN-γ, IL-2, IL-4, IL-6, and TNF, indicating a mixed Th1 and Th2 response (95). Although direct injury of thyroid cells has been difficult to detect in vitro, these cytokines (plus those derived from the thyroid cells themselves) may impair the functional activity of thyroid cells, and therefore contribute to the development of hypothyroidism (96) (Table 47.4). Another consequence of intrathyroidal cytokine production is the expression of intercellular adhesion molecule-1 (ICAM-1) on thyroid cells (97). This increases the ability of thyroid cells to bind lymphocyte function-associated antigen-1 (LFA-1), a receptor for ICAM-1 present on T cells, and therefore increases cytotoxicity mediated by T cells. In addition to increasing the expression of these and other surface molecules, such as HLA class II molecules, on thyroid cells, cytokines induce the production of other mediators by thyroid cells, including IL-1, IL-6, nitric oxide, and an array of chemokines (96,98).
TABLE 47.4. EFFECT OF CYTOKINES ON THYROID CELL FUNCTION
Increased expression of MHC class I and II molecules, complement regulatory proteins, and ICAM-1; growth inhibition; inhibition of Na+/I- cotransporter expression and function; decreased Tg and thyroid hormone synthesis; increased synthesis of IL-1 and IL-6
Increased expression of complement regulatory proteins and ICAM-1; inhibition of Na+/I- cotransporter expression and function; inhibition of cAMP, Tg, and TPO production; enhanced growth; increased IL-6 and IL-8 synthesis; increased production of nitric oxide and Fas
Decreased cAMP and TPO production; variable effects on growth
Increased expression of MHC class I molecules; enhanced effect of IFN-γ on MHC class II expression; inhibition of Na+/I- cotransporter expression and function; increased production of IL-6 and complement regulatory proteins; growth inhibition; decreased thyroid hormone release
cAMP, cyclic adenosine monophosphate; ICAM-1; intercellular adhesion molecule-1; IFN-γ, interferon-γ IL-1, interleukin-1; MHC, major histocompatibility complex; Tg, thyroglobulin; TPO, thyroid peroxidase.
The mechanisms involved in thyroid injury are therefore diverse. It is likely that quantitative differences in their contributions, rather than simply the presence or absence of a particular antibody or cytokine, account for the different clinical types of autoimmune thyroiditis (Table 47.1). The participation of the thyroid cells in the autoimmune response is of particular interest because these cells express cytokines and cell surface molecules with the potential to promote or protect against autoimmune damage (99). Genetic or environmentally induced alterations in these responses could be important in the initiation and progression of chronic autoimmune thyroiditis.
Patients with chronic autoimmune thyroiditis may present with hypothyroidism, painless thyroid enlargement (goiter), or both. However, the widespread use of thyroid tests identifies many patients with subclinical chronic autoimmune thyroiditis (e.g., high serum concentrations of anti-TPO or anti-Tg antibodies), some of whom also have high serum TSH and normal serum T4 concentrations (subclinical hypothyroidism) (See Chapters 19 and 78).
When present, the goiter is usually firm, with size varying from small to very large, and is often lobulated; the latter characteristic sometimes makes it difficult to distinguish from a multinodular goiter. The goiter can cause some discomfort, but is rarely overtly painful and tender (100); the presence of pain should suggest the presence of lymphoma, although it can be caused by thyroiditis alone (see Chapter 72). Fine-needle-aspiration biopsy or even surgical biopsy may be needed to distinguish between these conditions. Biopsy is also warranted if there is a dominant nodule or rapid change in goiter size. In most patients, however, biopsy is not indicated, the diagnosis being made on clinical and biochemical grounds and supported by the presence of high serum anti-TPO (or anti-Tg) antibody concentrations. Ophthalmopathy occurs in a few patients, and there is evidence of chronic autoimmune thyroiditis in a large percentage of patients with euthyroid (ophthalmic) Graves' disease, who represent 5% to 10% of cases of Graves' ophthalmopathy (101) (see Chapter 23). Localized myxedema is rare in patients with chronic autoimmune thyroiditis (102).
The rate of decline in thyroid secretion in patients with chronic autoimmune thyroiditis is slow. For example, in a study of white American women over 20 years of age (103), the majority of whom did not have a goiter, about 40% had focal thyroiditis, a far higher frequency than that of overt hypothyroidism. Similarly, among women in Whickham (UK) with subclinical hypothyroidism and high serum anti-TPO antibody concentrations, the rate of progression to overt hypothyroidism was 5% per year (104) (see Chapter 19). In about 25% of adolescent subjects with goitrous thyroiditis, there was spontaneous resolution over a 20-year period, although another 33% had become hypothyroid during this time (105). Among women with postpartum thyroiditis, about 25% developed autoimmune hypothyroidism during a 4-year follow-up period (106).
Among a group of hypothyroid patients with goitrous thyroiditis, therapy with T4 for 2 years led to a 32% reduction in thyroid volume (as determined by ultrasonography), but the extent of the reduction varied considerably in different patients (107). Serum concentrations of TPO or other antithyroid antibodies may decrease (108,109) or not change (107) during T4 therapy. In a group of French patients with goitrous thyroiditis treated with T4 for 1 year, the serum concentration of TSH receptor–blocking antibodies decreased, but no patient remained euthyroid after T4 was discontinued (109). In a study of 21 Japanese patients with with hypothyroidism and TSH receptor–blocking antibodies, T4 treatment for 4 to 8 years was associated with disappearance of these antibodies in 71% of the patients, and over 50% remained euthyroid after T4 therapy was discontinued (110); the possibility of remission in patients without TSH receptor antibodies before treatment was not assessed. In a unselected group of 79 Canadian patients with goitrous thyroiditis, T4 was successfully withdrawn in 11% for at least 1 year (111). In a group of Japanese children with atrophic thyroiditis, hypothyroidism was irreversible, but thyroid function remained normal after cessation of T4 therapy in some with goitrous thyroiditis (112). The permanence of such remissions is unclear. Withdrawal of T4 for a month is the only practical method of assessing whether hypothyroidism persists (113), but at present there is little justification for this because of the low frequency of remission among unselected patients.
Graves' disease may precede or follow chronic autoimmune thyroiditis in the same patient, presumably related to the similar autoimmune processes in the two disorders. This is highlighted by observations in five patients with Graves' ophthalmopathy and biopsy evidence of chronic goitrous thyroiditis in whom TSH receptor–stimulating antibodies characteristic of Graves' disease were detected (114). These patients had a fluctuating clinical course thought to be due to changes in the balance between antibody-mediated TSH receptor stimulation and the effector mechanisms discussed above. Hypothyroidism, due to chronic autoimmune thyroiditis, ultimately supervenes in about 20% of patients with Graves' thyrotoxicosis treated with an antithyroid drug who remain euthyroid after cessation of therapy. TSH receptor–blocking antibodies contribute to the hypothyroidism in about one third of cases (115), and their presence in patients with Graves' disease is also associated with a greater prevalence of ophthalmopathy (116).
There is a clear association between chronic autoimmune thyroiditis and primary B-cell lymphoma of the thyroid. In one series of 119 patients with lymphoma, all had chronic autoimmune thyroiditis (117) (see Chapter 72). Presumably, prolonged stimulation of intrathyroidal B cells ultimately results in the emergence of a malignant clone. The frequency of other thyroid tumors is not increased in patients with chronic autoimmune thyroiditis, although cancer may be overlooked in those patients with an irregular goiter. Chronic fibrous (Riedel's) thyroiditis has been suggested to have an autoimmune etiology because some patients have high serum antithyroid antibody concentrations and a lymphocytic infiltrate of the thyroid (118), but the relationship of this condition to chronic autoimmune thyroiditis is unclear. Chronic autoimmune thyroiditis is a well-recognized component of both the type 1 and type 2 autoimmune polyglandular syndromes (119,120) and occurs with increased prevalence in a variety of other disorders (121) (Table 47.5).
TABLE 47.5. AUTOIMMUNE DISORDERS OCCURRING WITH INCREASED FREQUENCY IN PATIENTS WITH CHRONIC AUTOIMMUNE THYROIDITIS
Type 1 autoimmune polyglandular syndrome
The major componentsa are hypoparathyroidism, Addison's disease, and chronic mucocutaneous candidiasis; chronic autoimmune thyroiditis occurs in 10%–15% of cases.
Type 2 autoimmune polyglandular syndrome
The major componentsa are autoimmune thyroid disease, type 1 diabetes mellitus, and Addison's disease. Other components are premature ovarian failure, lymphocytic hypophysitis, vitiligo, alopecia areata, celiac disease, pernicious anemia, serositis, and myasthenia gravis; chronic autoimmune thyroiditis occurs with increased frequency in any of these disorders in their isolated form.
Rheumatoid arthritis, systemic lupus erythematosus, Sjögren's syndrome, polymyalgia rheumatica, temporal arteritis, relapsing polychondritis, and systemic sclerosis.
Chronic active hepatitis, primary biliary cirrhosis, dermatitis herpetiformis, autoimmune thrombocytopenia
aTwo major components are required to qualify as the syndrome.
Depression in menopausal women is associated with a threefold higher frequency of high serum antithyroid antibody concentrations than in normal subjects, which is independent of thyroid status (122). Infertility in women is associated with a twofold increase in the frequency of high serum anti-TPO antibody concentrations, with a particularly high frequency occurring in those women with endometriosis (123). There is also an association between goitrous thyroiditis and breast cancer (124). The reasons for these associations are unclear, but may be related to some interaction between the neuroendocrine and immune systems. The remarkably low prevalence of high serum antithyroid antibody concentrations in healthy centenarians suggests that the absence of autoimmune thyroid disease is associated with protection against senescence (125).
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