BMA Concise Guide to Medicine & Drugs


Brand names Eltroxin, Evotrox

Used in the following combined preparations None


Drug group Thyroid hormone

Overdose danger rating Medium

Dependence rating Low

Prescription needed Yes

Available as generic Yes


Levothyroxine is the main hormone produced by the thyroid gland. A deficiency of the natural hormone causes hypothyroidism, which is associated with symptoms such as weight gain and slowing of body functions. A synthetic preparation is given to replace the natural hormone when it is deficient. It is also sometimes given in combination with carbimazole or propylthiouracil in the treatment of an overactive thyroid gland. Levothyroxine is also given (in higher doses) to people who have had thyroid cancer. Doses of levothyroxine are usually increased gradually to help prevent adverse effects, and particular care is required in patients with heart problems such as angina.


Your drug prescription is tailored for you. Do not alter dosage without checking with your doctor.

How taken/used Tablets, liquid.

Frequency and timing of doses Once daily.

Dosage range Adults Doses of 25–100mcg daily, increased at 3–4-week intervals as required. The usual maximum dose is 200mcg daily.

Onset of effect Within 48 hours. Full beneficial effects may not be felt for several weeks.

Duration of action 1–3 weeks.

Diet advice None.

Storage Keep in original container at room temperature out of the reach of children. Protect from light.

Missed dose Take as soon as you remember. If your next dose is due within 8 hours, take a single dose now and skip the next.

Stopping the drug Do not stop the drug without consulting your doctor; symptoms may recur.

Exceeding the dose An occasional unintentional extra dose is unlikely to cause problems. Large overdoses may cause palpitations in next few days. Notify your doctor.


Adverse effects are rare and are usually due to overdosage, causing thyoid overactivity. They include anxiety, agitation, diarrhoea, weight loss, sweating, flushing, muscle cramps, insomnia, and tremors. These effects diminish as the dose is lowered, but if they occur, discuss with your doctor. If you experience palpitations or chest pain, seek immediate medical advice. Too low a dose of levothyroxine may cause signs of thyroid underactivity.


Oral anticoagulants Levothyroxine may increase the effect of these drugs.

Colestyramine This drug may reduce the absorption of levothyroxine.

Amiodarone may affect thyroid activity; levothyroxine dosage may need adjustment.

Antiepileptic drugs These drugs may reduce the effect of levothyroxine.

Antidiabetic agents The doses of these drugs may need increasing once levothyroxine treatment is started.

Sucralfate Absorption of levothyroxine may be reduced by sucralfate.

Antidepressants Levothyroxine may enhance the effects of tricyclic antidepressants.

Oral contraceptives may increase levothyroxine requirements.


Be sure to tell your doctor if:

· You have high blood pressure.

· You have heart problems.

· You have diabetes.

· You are taking other medicines.

Pregnancy No evidence of risk, but dosage adjustment may be necessary.

Breast-feeding The drug passes into the breast milk, but at normal doses adverse effects on the baby are unlikely. Discuss with your doctor.

Infants and children Dosage depends on age and weight.

Over 60 Reduced dose usually necessary.

Driving and hazardous work No known problems.

Alcohol No known problems.


No special problems.

Monitoring Periodic tests of thyroid function are required.