Drug class · Endocrine
Thyroid hormones
Also known as: thyroid replacement
Synthetic thyroxine (levothyroxine) replaces what an underactive thyroid cannot make, restoring metabolic rate; the dose is tuned by the TSH blood test.
- Biological target
- Thyroid hormone receptor
- Also called
- thyroid replacement
Mechanism of action
Levothyroxine is identical to the thyroid's own T4. Tissues convert it to active T3, which enters cell nuclei and switches on genes controlling metabolic rate, heat production, heart rate, gut motility, growth and brain development. As levels normalise, the pituitary's TSH falls back into range, which is how the dose is checked. Liothyronine (T3) is used rarely, on specialist advice.
Members of the class
levothyroxineliothyronine
Members with a page on this site are linked.
Conditions treated
Underactive thyroid (hypothyroidism)Overactive thyroid (hyperthyroidism)Atrial fibrillationOsteoporosis
Class effects
- None at the right dose
- Palpitations, sweating, weight loss and insomnia if the dose is too high
Cautions
- Take on an empty stomach, 4 hours apart from calcium, iron, antacids and proton pump inhibitors
- Start low in the elderly and in heart disease
- Dose often rises in pregnancy: check TSH early
- Not for weight loss in people with a normal thyroid
Class warnings
- Over-replacement causes palpitations, atrial fibrillation, weight loss, anxiety and bone loss; under-replacement leaves hypothyroid symptoms. The dose is adjusted by TSH, rechecked 6-8 weeks after any change. UK
- Started at a low dose in cardiac disease and older people, because a sudden rise in metabolic rate can precipitate angina or arrhythmia. UK
Class interactions
| With | Effect | Source |
|---|---|---|
| Iron, calcium, antacids, proton pump inhibitors, sucralfate, colestyramine | Reduce levothyroxine absorption; doses are separated by several hours and TSH is monitored. | UK |
| Enzyme inducers (carbamazepine, phenytoin, rifampicin) | Increase levothyroxine metabolism; dose may need to rise. | UK |
| Warfarin | Thyroid status changes warfarin sensitivity; INR monitoring when the dose changes. | UK |
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