Levothyroxine

Levothyroxine is the standard replacement for thyroxine when the thyroid gland no longer makes enough of it. Low thyroid hormone slows the body down, often causing tiredness, feeling cold, weight gain, constipation, dry skin and low mood. The UK SmPC lists the control of hypothyroidism in adults, congenital hypothyroidism in infants, acquired hypothyroidism in children and juvenile myxoedema. Treatment is usually lifelong, and the aim is a TSH level within the normal range together with relief of symptoms.

The page offers 25, 50, 100 and 200 microgram tablets in packs of 100, 200, 300 or 400, and 75 and 125 microgram tablets in packs of 100 or 200. Many people combine strengths to reach their exact daily dose, for example 100 plus 25 micrograms. In the UK, licensed tablets mostly come as 12.5, 25, 50, 75 and 100 micrograms; 125 and 200 microgram strengths are licensed here as soft capsules rather than tablets, so the tablets above are not identical to UK-licensed packs.

The tablets are taken once a day, ideally before breakfast or the first meal. For adults without heart disease the usual starting dose is 50–100 micrograms daily, adjusted in steps of 25–50 micrograms every three to four weeks, with a usual maintenance dose of 100–200 micrograms. People over 50 generally start at no more than 50 micrograms, and those with heart disease at 25 micrograms daily, with small steps every four weeks. Children’s doses depend on age and weight. Blood tests (TSH and sometimes free T4) are repeated after each change, because too much or too little hormone both cause problems.

Levothyroxine sodium is licensed in the UK and classed as a prescription-only medicine (POM). It is widely supplied under its generic name, and branded versions include Eltroxin and Vencamil. Oral solutions and soft capsules are also licensed for people who need very fine dose steps or cannot take tablets. Switching between makes can occasionally alter TSH results, so a repeat test may be advised after a change.

The SmPC lists untreated thyrotoxicosis, adrenal insufficiency or an adrenal gland disorder that has not been corrected, and hypersensitivity to any ingredient. Treatment must not be started during an acute heart attack, acute myocarditis or acute pancarditis. Care is needed in angina, coronary disease, high blood pressure, diabetes and older age, where an ECG and a slow start are recommended. Thyroid hormone must never be used to lose weight.

Calcium and iron supplements, antacids, colestyramine and some other binding agents reduce absorption, so they should be taken several hours apart. Enzyme-inducing drugs such as rifampicin, carbamazepine and phenytoin can increase the dose needed, and so can oestrogens. Levothyroxine may strengthen the effect of warfarin and change insulin or tablet requirements in diabetes. Amiodarone and lithium can affect thyroid function in their own right.

Unwanted effects usually point to a dose that is too high: a fast or irregular heartbeat, chest pain, tremor, restlessness, sleeplessness, sweating, heat intolerance, diarrhoea and weight loss. Contact your GP if these appear, or call 999 or go to A&E with chest pain. Any change of dose or brand of Levothyroxine is for your doctor or prescriber to decide.