The pharmacokinetics of methimazole following therapeutic doses were studied in healthy subjects, in thyrotoxic and hypothyroid patients before and after treatment to euthyroidism, and in patients with renal or hepatic insufficiency, using a highly sensitive gas chromatographic-mass spectrometric assay. Following intravenous administration of 10mg to healthy subjects, methimazole had an initial distribution half-life (t1/2&agr;) of 0.10 to 0.23 hours and an elimination half-life (t1/2&bgr;) of 4.9 to 5.7 hours. The absolute bioavailability after oral administration of 10mg methimazole in the fasting state was high, with a mean of 93%.The pharmacokinetic profiles showed small interindividual variations, although one of the hypothyroid patients had a rapid elimination half-life, in both the hypothyroid and euthyroid state (2.6 and 2.4 hours, respectively). The elimination rate was not enhanced in the thyrotoxic patients but was slightly prolonged in the hypothyroid patients. There was no influence of renal insufficiency, but a prolonged elimination half-life was observed in patients with hepatic failure, the prolongation being proportional to the degree of impairment.Thus, the pharmacokinetics of methimazole are relatively simple with small interindividual variations. In general, there are no pharmacokinetic reasons to adjust dosage in the treatment of thyrotoxicosis, except in the rare case of concomitant advanced hepatic insufficiency.