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.
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.
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.
Symptoms continue after TSH normalises on a stable levothyroxine regimen.
Key medicines
Levothyroxine for primary hypothyroidismAdults under 65 without cardiovascular disease: consider 1.6 micrograms/kg once daily, rounded to the nearest 25 micrograms; age 65 or over, or cardiovascular disease: consider 25 to 50 micrograms once daily with titration.
Levothyroxine for central hypothyroidismIndividualised once-daily replacement prescribed and titrated by endocrinology using free T4, symptoms, age and cardiac risk rather than a TSH target.