Thyroxine

Question 1

How does Thyroxine work?

Answer 1

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 2

Indications for Thyroxine

Answer 2

Primary hypothyroidism.

Hypothyroidism secondary to hypopituitarism.

Question 3

Contraindications for Thyroxine

Answer 3

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 4

Side effects of Thyroxine

Answer 4

usually due to excessive doses, so are predictably similar to symptoms of hyperthyroidism - GI upset; diarrhoea, vomiting, weight loss), cardiac (e.g. palpitations, arrhythmias, angina) and neurological (e.g. tremor, restlessness, insomnia) manifestations.

Question 5

Interactions of thyroxine

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

Elimination of thyroxine

Answer 6

Renal

Question 7

Patient information on thyroxine

Answer 7

Explain that treatment will replace a natural hormone that their body has stopped making and that this will give them more energy and make them feel better. Advise them that it may take some time (months in some cases) for them to feel ‘back to normal’. It is important to emphasise (for most people) that treatment is for life and that they should not stop taking it. Warn them of the signs of too much treatment (e.g. shakiness, anxiety, sleeplessness, diarrhoea) and advise them to see a doctor if these occur, as their treatment may need to be reduced. If they take calcium or iron replacement, advise them to leave a gap of about 4 hours between these treatments and levothyroxine.