Thyroid hormone preparations, especially thyroxine, are widely used either at replacement doses to correct hypothyroidism or at suppressive doses to abolish thyrotropin (thyroid-stimulating hormone) secretion in patients with differentiated thyroid carcinoma after total thyroidectomy or with diffuse/ nodular nontoxic goitre. In order to suppress thyrotropin secretion, it is necessary to administer slightly supraphysiological doses of thyroxine. Possible adverse effects of this therapy include cardiovascular changes (shortening of systolic time intervals, increased frequency of atrial premature beats and, possibly, left ventricular hypertrophy) and bone changes (reduced bone density and bone mass), but the risk of these adverse effects can be minimised by carefully monitoring serum free thyroxine and free liothyronine (triiodothyronine) measurements and adjusting the dosage accordingly. Thionamides [thiamazole (methimazole), carbimazole, propylthiouracil] are the most widely used antithyroid drugs. They are given for long periods of time and cause adverse effects in 3 to 5% of patients. In most cases, adverse effects are minor and transient (e.g. skin rash, itching, mild leucopenia). The most dangerous effect is agranulocytosis, which occurs in 0.1 to 0.5% of patients. This life-threatening condition can now be effectively treated by granulocyte colony-stimulating factor administration. Other major adverse effects (aplastic anaemia, thrombocytopenia, lupus erythematosus-like syndrome, vasculitis) are exceedingly rare.