Synopsis
Distinguish thyroid-gland failure from pituitary or hypothalamic disease, replace hormone safely, and protect patients with possible adrenal insufficiency or special physiological needs.
- Primary hypothyroidism produces raised TSH with low free T4; central hypothyroidism produces low free T4 with TSH that is low, normal or biologically inadequate.
- Autoimmune thyroiditis, thyroidectomy and radioiodine are frequent primary causes; pituitary tumours, surgery, radiotherapy, apoplexy and infiltrative disease underlie many central cases.
- Symptoms such as tiredness, weight gain, constipation, dry skin, cold intolerance and cognitive slowing are common but nonspecific, so diagnosis requires credible biochemistry.
- Levothyroxine is NICE first-line treatment for primary hypothyroidism; routine liothyronine and natural thyroid extract are not recommended.
- Adults under 65 without cardiovascular disease can usually start near full replacement, while older people and those with cardiac disease need lower initiation and careful titration.
- Never use TSH to exclude or titrate central hypothyroidism; specialist follow-up uses free T4, clinical response and the status of other pituitary axes.
- Before treating central hypothyroidism, assess adrenal function and give glucocorticoid cover first if adrenal insufficiency is possible, because levothyroxine can precipitate crisis.
- Iron, calcium, food, antacids and several gastrointestinal conditions can reduce levothyroxine absorption; structured administration review is more useful than repeated unexplained dose increases.
- Pregnancy, planned conception, childhood and severe decompensation need rapid specialist pathways rather than ordinary adult titration.
Key red flags
Headache, visual-field symptoms, amenorrhoea, infertility, reduced libido, loss of body hair, post-partum haemorrhage history, polyuria or previous pituitary treatment should shift attention from the thyroid gland to the sellar axis.
Postural hypotension, weight loss, nausea, abdominal pain, hyponatraemia, hypoglycaemia or unexplained eosinophilia in pituitary disease makes cortisol deficiency a priority; pigmentation usually indicates primary rather than central adrenal failure.
Altered consciousness with hypothermia, hypoventilation, bradycardia, hypotension and metabolic disturbance suggests myxoedema coma, which may occur without literal coma and needs immediate critical care.
Investigation priorities
Confirm hypothyroidism and locate dysfunction within the axis.
Management branches
Raised TSH with free T4 below the local reference interval.
- Assess severity, pregnancy possibility, cardiovascular disease, previous thyroid intervention, medicines and features of adrenal or pituitary disease before prescribing.
- Start levothyroxine using NICE age and cardiac-risk principles, explain consistent administration and separate it from relevant food, iron and calcium exposure.
- Review TSH about every three months until stable, adjusting gradually and avoiding suppression; check free T4 as well if symptoms remain unexplained.
Symptoms continue after TSH normalises on a stable levothyroxine regimen.
- Verify the original overt diagnosis, adherence, free T4, and absence of over-replacement, then seek non-thyroid explanations through focused history and examination.
- Treat relevant anaemia, sleep disorder, depression, menopause, cardiopulmonary disease or nutritional deficiency rather than using thyroid hormone as nonspecific therapy.
- If substantial symptoms persist after optimisation, refer to an endocrinologist; any liothyronine trial should follow current BTA/Society guidance, shared decision-making and specialist monitoring.