ENDO + REPRO: THYROID HORMONES

Question 1

THs: INDICATIONS

Answer 1

• Primary Hypothyroidism

- Hypothyroidism secondary to hypopituitarism

Question 2

THs: MOA

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

THs: 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

THs: 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

THs: 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

THs: EXAMPLES

Answer 6

• levothyroxine

- liothyronine