Hypothyroidism
A state of thyroid hormone deficiency that slows metabolic rate in virtually every tissue, so the whole clinical picture (from bradycardia to cold intolerance to slowed cognition) can be predicted simply by asking what happens when metabolism runs too slowly.
In a nutshell
Thyroid hormone sets metabolic rate everywhere, so deficiency slows every system at once: cold intolerance, weight gain, constipation, bradycardia and cognitive slowing. Primary (gland) failure shows high TSH with low T4; central failure shows low or normal TSH with low T4.
Classic presentation
A middle-aged woman with fatigue, weight gain, cold intolerance, constipation and dry skin, found to have a raised TSH and low free T4.
Key points
- TSH rises before T4 clearly falls in early primary hypothyroidism, making TSH the most sensitive screening test.
- Hashimoto's thyroiditis (anti-TPO positive) is the leading cause of primary hypothyroidism in iodine-replete countries like the UK.
- A low TSH with a low T4 points to central (pituitary/hypothalamic) hypothyroidism, not primary disease, and warrants pituitary work-up.
- Coexisting adrenal insufficiency must be treated before or alongside thyroxine, since thyroxine can precipitate an adrenal crisis if started alone.
- Levothyroxine is titrated against TSH, rechecked roughly 6-8 weeks after each dose change.
- Levothyroxine requirements typically increase in pregnancy.
First-line investigation
TSH and free T4 together, to confirm hypothyroidism and localise it as primary (gland) or central (pituitary) disease.
First-line management
Oral levothyroxine replacement, started cautiously in older or cardiac patients and titrated against TSH.
Exam traps
- Do not start levothyroxine before excluding or treating coexisting adrenal insufficiency: it can precipitate an adrenal crisis.
- A low TSH does not always mean hyperthyroidism; with a low T4 it signals central hypothyroidism.
- In older patients or those with ischaemic heart disease, start levothyroxine at a low dose and titrate slowly to avoid provoking angina.
Educational content pending clinical review. Not medical advice.