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Hypothyroidism
A high TSH with a low FT4 is primary hypothyroidism (usually Hashimoto), treated with levothyroxine adjusted by TSH after 6–8 weeks; but a "normal" TSH with a low FT4 is central hypothyroidism, where TSH is not a monitoring tool.
Why it matters in the exam
Questions present fatigue, weight gain, constipation, cold intolerance and dry skin, sometimes with indirect laboratory findings — hyponatraemia, high cholesterol, high CK and a high MCV. The candidate must read the TSH and FT4 pair: primary — high TSH, low FT4; subclinical — high TSH and normal FT4; central — low FT4 with an inappropriately low or "normal" TSH, prompting a pituitary work-up. Anti-TPO antibodies confirm Hashimoto, which also raises the risk of thyroid lymphoma.
Treatment questions: full-dose levothyroxine of about 1.6 µg/kg a day in a healthy young adult, but in an older patient or one with heart disease start at 25–50 µg and increase slowly. It is taken on an empty stomach, apart from calcium, iron and PPIs. Subclinical hypothyroidism is treated when TSH is 10 or more, in pregnancy or when pregnancy is planned. In pregnancy the dose is raised by about 25%–30% as soon as it is confirmed. In myxoedema coma (hypothermia, hyponatraemia, reduced consciousness) intravenous levothyroxine is given together with hydrocortisone until adrenal insufficiency is excluded. Drugs that cause hypothyroidism: amiodarone, lithium and checkpoint inhibitors.
Key points
- Primary: high TSH, low FT4; central: low FT4 with a TSH that is not high.
- Levothyroxine about 1.6 µg/kg; older or cardiac patients 25–50 µg.
- TSH 6–8 weeks after every dose change.
- Subclinical: treat if TSH is 10 or more or in pregnancy.
- Pregnancy: raise the dose by about 25%–30%; myxoedema coma: hydrocortisone and intravenous levothyroxine.
Summary table
| Situation | TSH / FT4 | Treatment | Exam note |
|---|---|---|---|
| Primary | High TSH, low FT4 | Levothyroxine about 1.6 µg/kg a day | TSH check after 6–8 weeks |
| Subclinical | High TSH, normal FT4 | Treat if TSH 10 or more, pregnancy or planned pregnancy | Repeat the test before deciding |
| Central | Low FT4, low or "normal" TSH | Levothyroxine; follow by FT4 | Investigate the pituitary and exclude adrenal insufficiency first |
| Older or cardiac patient | Primary | Start at 25–50 µg and increase slowly | Risk of ischaemia |
| Pregnancy | All hypothyroidism is treated | Raise the dose by about 25%–30% at diagnosis | Trimester-specific TSH target |
| Myxoedema coma | Hypothermia, hyponatraemia, reduced consciousness | Intravenous levothyroxine + hydrocortisone | Intensive care |
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Sources
- American Thyroid Association Guidelines for the Treatment of Hypothyroidism, 2014 — Levothyroxine dosing, monitoring and special populations
- ATA Guidelines for the Diagnosis and Management of Thyroid Disease During Pregnancy and the Postpartum, 2017 — Levothyroxine in pregnancy
- Harrison's Principles of Internal Medicine, 21st edition (2022) — Hypothyroidism
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-02. The text may contain errors; when in doubt, check the source.