Hypothyroidism
(Common Endocrine Disorders)
Introduction
Hypothyroidism is the clinical syndrome caused by inadequate thyroid hormone (T4/T3). Presentation ranges from asymptomatic biochemical abnormalities (subclinical hypothyroidism) to life‑threatening decompensation (myxoedema coma).
Emergency department care focuses on recognising disease and emergencies, arranging appropriate investigations, initiating or resuming levothyroxine when indicated, and early escalation to endocrine or critical‑care teams for severe presentations (NICE NG145; RCEM).
Classification and pathophysiology
Thyroid hormone production is regulated by the hypothalamic-pituitary-thyroid axis. Important clinical categories include:
- Primary hypothyroidism: This accounts for approximately 95% of cases and is due to a defect in the thyroid gland (for example autoimmune thyroiditis, surgery, radioiodine therapy, iodine deficiency or drug‑induced). Biochemically it produces a raised TSH and a low free T4 in overt disease; a raised TSH with normal free T4 denotes subclinical disease.
- Secondary (central) hypothyroidism: This accounts for about 5% of cases and is caused by pituitary or hypothalamic failure (for example pituitary adenoma or hypophysitis). Biochemically it produces a low or inappropriately normal TSH with a low free T4.
- Subclinical hypothyroidism: This is defined by an elevated TSH with free T4 within the reference range.
Common consequences of chronic hypothyroidism include dyslipidaemia, accelerated atherosclerosis, infertility, sensory impairment, reduced exercise tolerance and heart failure. Untreated severe hypothyroidism may progress to myxoedema coma.