Serum thyroid-stimulating hormone (TSH) testing is the best screening tool for thyroid dysfunction. When TSH levels are in the reference range, additional tests such as free thyroxine, free triiodothyronine or thyroid antibodies rarely add value, except in patients with pituitary disease, when TSH is unreliable. Overt hypothyroidism and subclinical hypothyroidism with TSH levels > 10 mU/L can be treated without further investigation. The health impact of subclinical hypothyroidism with mildly elevated levels of TSH (4-10 mU/L) remains uncertain, particularly in older people; treatment or observation are reasonable options. Thyroxine remains standard treatment for hypothyroidism, with optimal dosage determined by clinical response and serum TSH. Hyperthyroidism is commonly caused by Graves' disease, thyroiditis or toxic nodular goitre. The cause should be established before offering treatment. Radionuclide scanning is the imaging modality of choice. Positive TSH-receptor antibodies indicate Graves' disease. Thyroid ultrasound is indicated for assessment of palpable goitre and thyroid nodules. It is not part of routine assessment of hyperthyroidism or hypothyroidism. Overzealous use of ultrasound identifies clinically unimportant thyroid nodules and can lead to overdiagnosis of thyroid cancer. For thyroid nodules, the key investigation is ultrasound-guided fine needle aspiration biopsy, depending on size and sonographic appearance. Biopsy should not be performed routinely on small nodules < 1 cm. It remains controversial whether pregnant women should be screened for thyroid disease. Reference intervals for thyroid function tests during pregnancy are not well established, and it is uncertain whether thyroxine treatment for pregnant women with serum TSH levels between 2.5 and 4.0 mU/L is beneficial. Iodine supplementation is recommended during pregnancy.