PassFinals
Endocrinology & Metabolic

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 deficiency slows metabolism across the body: cold intolerance, weight gain, constipation, bradycardia and cognitive slowing. Confirm the biochemical pattern, replace primary deficiency with levothyroxine, and recognise myxoedema coma, central disease, adrenal insufficiency, pregnancy and over-treatment as safety-critical branches.

Classic presentation

A person with fatigue, cold intolerance, constipation, dry skin, weight gain and slow-relaxing reflexes, with a raised TSH and low FT4 indicating primary hypothyroidism.

Key points

  • When pituitary disease is not suspected, TSH is the first test; add FT4 in the same sample when TSH is outside the reference range.
  • Primary hypothyroidism produces a raised TSH with low FT4; low FT4 with a low or inappropriately normal TSH suggests central disease.
  • TPO antibodies support autoimmune thyroid disease but are measured once, not serially, to monitor response.
  • Levothyroxine is first-line treatment; do not routinely use liothyronine or natural thyroid extract for primary hypothyroidism.
  • Start more cautiously in older adults and people with cardiovascular disease, then titrate to biochemical and clinical response.
  • Pregnancy, suspected adrenal insufficiency, myxoedema coma and pituitary disease need early specialist involvement.
  • Persistent symptoms despite a normal TSH should prompt adherence, interaction and alternative-diagnosis review rather than automatic dose escalation.

First-line investigation

TSH with FT4 according to the NICE cascade, or TSH and FT4 together when pituitary disease is suspected; ask about biotin and review medicines.

Management

Recognise the emergency and protect the adrenal axis

  • Reduced consciousness, hypothermia, bradycardia, hypotension, hypoglycaemia, hyponatraemia or respiratory failure in severe hypothyroidism suggests myxoedema coma: admit urgently, provide ABCDE and cardiorespiratory support, involve endocrinology and critical care, treat the precipitant, and use hydrocortisone and specialist-directed IV levothyroxine according to BNF and local protocol.4,5,6
  • If adrenal insufficiency is possible, assess and treat it before or alongside levothyroxine: thyroid hormone can increase cortisol clearance and precipitate adrenal crisis.2,3,4

Confirm the pattern and start appropriate replacement

  • Raised TSH with low FT4 indicates overt primary hypothyroidism. Low FT4 with a low or inappropriately normal TSH indicates possible central disease: refer for pituitary assessment and do not use TSH alone to guide treatment.1,2,3
  • Offer levothyroxine first-line. NICE recommends considering 1.6 micrograms/kg/day, rounded to the nearest 25 micrograms, for adults under 65 without cardiovascular disease; consider 25 to 50 micrograms/day with titration for adults aged 65 or over or with cardiovascular disease.1,6

Handle subclinical disease without overtreatment

  • Consider levothyroxine for adults with TSH 10 mIU/L or higher on 2 occasions 3 months apart. For adults under 65 with symptoms and persistent TSH above range but below 10 mIU/L, consider a 6-month trial; stop and reassess other causes if symptoms remain after TSH normalises.1
  • Do not routinely offer liothyronine, alone or with levothyroxine, or natural thyroid extract for primary hypothyroidism. Persistent symptoms with a normal TSH need review of adherence, interactions and alternative diagnoses.1,8

Protect pregnancy and medicine absorption

  • Levothyroxine is safe in pregnancy, but requirements commonly rise. Arrange prompt thyroid-function testing and endocrine-obstetric follow-up when pregnancy is planned or confirmed, with regular monitoring during pregnancy and after birth.9,10
  • Take levothyroxine consistently on an empty stomach, usually 30 to 60 minutes before breakfast, caffeine or other medicines. Check adherence and formulation changes before escalating; iron, calcium, antacids and some foods or supplements can reduce absorption.7,8,6

Titrate, monitor and safety-net

  • Aim for TSH within the reference range and avoid suppression or thyrotoxicosis. Check TSH from about 6 weeks after starting or changing treatment, then every 3 months until 2 similar measurements are within range 3 months apart; once stable, monitor annually. Add FT4 if symptoms continue.1,3
  • Refer suspected central disease, adrenal insufficiency, pregnancy or post-partum disease, ischaemic heart disease, severe hypothyroidism, and difficult biochemical control. Escalate urgently for myxoedema features, chest pain, severe bradycardia, hypotension, hypothermia or reduced consciousness.2,3,4

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.
  • Do not treat a single mild TSH elevation as permanent subclinical hypothyroidism: confirm persistence before applying the treatment pathway.
  • TSH is not the treatment target in central hypothyroidism; FT4 and clinical response guide replacement with specialist input.
  • Persistent symptoms with a normal TSH are not automatically an indication for liothyronine or a higher levothyroxine dose.
  • Hypothermia, reduced consciousness and cardiovascular or respiratory compromise in severe hypothyroidism are an emergency, not a routine outpatient dose-adjustment problem.

Illustrations

Metabolic slowdown mechanismDiagram mapping reduced thyroid hormone to reduced metabolic rate and onward to each organ-system feature of hypothyroidism.PassFinals · original
Hashimoto's thyroiditis histologyPhotomicrograph showing lymphocytic infiltration and germinal centre formation typical of autoimmune thyroiditis.Librepath, Wikimedia Commons · CC-BY-SA-3.0
Myxoedema faciesClinical photograph illustrating periorbital puffiness, dry skin and hair thinning characteristic of significant hypothyroidism.Herbert L. Fred, MD and Hendrik A. van Dijk, Wikimedia Commons · CC-BY-2.5

Key sources

  1. NICE, Thyroid disease: assessment and management (NG145)Published 20 Nov 2019 | Updated 12 Oct 2023
  2. NICE CKS, Hypothyroidism
  3. NHS Specialist Pharmacy Service, Levothyroxine monitoringPublished 15 Jul 2021 | Updated 3 Jun 2024
  4. Norfolk and Norwich University Hospitals NHS Foundation Trust, Suspected Thyroid Emergencies Management guideline v3.2 (CA5060 v3.2)
  5. Society for Endocrinology, Medicine Supply Notification: liothyronine injection and levothyroxine injection
  6. BNF, Levothyroxine sodium
  7. NHS, Levothyroxine: how and when to take itUpdated 6 Jul 2026
  8. NHS England, Liothyronine: advice for prescribers
  9. RCOG, Management of Thyroid Disorders in Pregnancy (Green-top Guideline No. 76)
  10. NHS England North West, Thyroid Disease in Pregnancy guideline

This page is exam revision material, not medical advice, and must not be used for patient care. Always check drug doses against the BNF and current guidance. Full disclaimer.