Thyroxine

Question 1

Interactions

Answer 1

1. Primary hypothyroidism.

2. Hypothyroidism secondary to hypopituitarism.

Question 2

Mechanisms of action

Answer 2

The thyroid gland produces thyroxine (T4), which is converted to the more active triiodothyronine (T3) in target tissues. Thyroid hormones regulate metabolism and growth. Deficiency of these hormones causes hypothyroidism, with clinical features including lethargy, weight gain, constipation and slowing of mental processes. Hypothyroidism is treated by long-term replacement of thyroid hormones, most usually as levothyroxine (synthetic T4). Liothyronine (synthetic T3) has a shorter half-life and quicker onset (a few hours) and offset (24–48 hours) of action than levothyroxine. It is therefore reserved for emergency treatment of severe or acute hypothyroidism.

Question 3

Adverse effects

Answer 3

The adverse effects of levothyroxine are usually due to excessive doses, so are predictably similar to symptoms of hyperthyroidism. These include gastrointestinal (e.g. diarrhoea, vomiting, weight loss), cardiac (e.g. palpitations, arrhythmias, angina) and neurological (e.g. tremor, restlessness, insomnia) manifestations.

Question 4

Warnings

Answer 4

Thyroid hormones increase heart rate and metabolism. They can therefore precipitate cardiac ischaemia in people with coronary artery disease, in whom replacement should be started cautiously at a low dose and with careful monitoring. In hypopituitarism, corticosteroid therapy must be initiated before thyroid hormone replacement to avoid precipitating an Addisonian crisis.

Question 5

Interactions

Answer 5

As gastrointestinal absorption of levothyroxine is reduced by antacids, calcium or iron salts, administration of these drugs needs to be separated by about 4 hours. An increase in levothyroxine dose may be required in patients taking cytochrome P450 inducers, e.g. phenytoin, carbamazepine. Levothyroxine-induced changes in metabolism can increase insulin or oral hypoglycaemic requirements in diabetes mellitus and enhance the effects of warfarin.

Question 6

Prescription

Answer 6

Levothyroxine is only available for oral administration. A starting dose of 50–100 micrograms daily is recommended, except in the elderly or people with cardiac disease, who should start on 25 micrograms daily. The dose is adjusted monthly in 25–50-microgram increments according to monitoring (see below) to a usual maintenance dose of 50–200 micrograms once daily. Remember to write micrograms in full to reduce the risk of dosing errors. Liothyronine is available for IV administration in emergency care. It should be prescribed only after consultation with senior and specialist colleagues.

Question 7

Administration

Answer 7

Levothyroxine is available in 25, 50 and 100 microgram tablets, so a combination is often required for adequate dosing (e.g. patients requiring 175 micrograms will need to take one of each strength daily).

Question 8

Communication

Answer 8

Explain that treatment will replace a natural hormone that their body has stopped making and this will give them more energy.

Question 9

Monitoring

Answer 9

Review monthly then three-monthly.