TeacherOfMedicine: Thyroid hormones

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.

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.

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.

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.

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