Many drugs can interfere with biochemical tests of thyroid functions by interfering with the synthesis, transport and metabolism of thyroid hormones, or by altering the synthesis and secretion of thyrotrophin (thyroid-stimulating hormone; TSH). Only rarely, however, do these effects cause overt, clinically apparent thyroid disease. Lithium therapy causes overt hypothyroidism in 5 to 15% of patients, and goitre in up to 37%. Thyroid function tests should be performed prior to initiating lithium therapy, and at 6-monthly intervals thereafter. Iodine and iodine-containing drugs (e.g. radiographic contrast media, iodinated glycerol and amiodarone) can have profound and variable effects on thyroid function.