Introduction to hypothyroidism

Hypothyroidism refers to the clinical condition that results from insufficient production of thyroid hormones. Primary hypothyroidism—hypothyroidism resulting from a disorder within the thyroid gland—is the most common form of hypothyroidism. Other types of hypothyroidism include central hypothyroidism (caused by a pituitary or hypothalamic disorder), drug-induced hypothyroidism and congenital hypothyroidism.

Typical symptoms of hypothyroidism include fatigue, weight gain, cold intolerance, constipation and dry skin. Thyroid hormones affect almost all tissue and organ systems in the body, so atypical presentations are common and diagnosis requires confirmation with laboratory testing. For information about testing thyroid function and interpreting test results, see Initial testing for thyroid disorders. In the absence of known or palpable thyroid nodules, thyroid ultrasound has no role in the diagnosis of hypothyroidism.

Hypothyroidism is associated with an increased incidence of cardiovascular risk factors, such as hypercholesterolaemia, high blood pressure, and left ventricular systolic dysfunction. These abnormalities may improve with treatment of hypothyroidism.

Severe hypothyroidism can alter the pharmacokinetics of some drugs, which can cause increased drug activity or toxicity (eg sedatives, analgesics, anaesthetics, digoxin), or decreased drug activity (eg anticoagulants). If the patient is taking other drugs, review the doses when treatment for hypothyroidism is started or adjusted.