Pharmacokinetics of methimazole in children and adolescents with Graves' disease

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Abstract
Methimazole concentrations in plasma and in the thyroid glands were measured by means of high-performance liquid chromatography. Pharmacokinetics of methimazole were studied after a single oral dose (175 .mu.mol/m2) in nine children and adolescent who were in the thyrotoxic state. Plasma levels of methimazole showed peak concentrations of 4.4 to 12.6 (median 9.2) .mu.mol/l at 0.5 to 4 h after drug administration. Plasma half-life, area under the curve, and distribution volume ranged from 2.73 to 6.04 h, 32.8 to 77.9 .mu.mol .cntdot. l-1 .cntdot. h-1, and 0.516 to 0.913 l/kg, respectively. These pharmacokinetic parameters showed a wide variation among the patients, but were quite reproducible in the same subject. Intrathyroid concentrations of methimazole were measured in another nine subjects including four adolescents and five adults who underwent thyroidectomy. The drug concentrations in the thyroid glands ranged between 3.5 and 23.8 .mu.mol/kg tissue and were far higher than those in the plasma obtained at the time of surgery. In this series of experiments, the dose of the drug varied from 76 to 319 .mu.mol/m2, time after the last dose to surgery from 5 to 24 h, and the mode of drug administration from a single to three divided doses. Among these variable factors, only the daily dose of methimazole corrected by body surface area showed significant correlation with the intrathyroidal concentration, whereas the time after the last dose of the drug and the mode of drug administration did not. Our results revealed that methimazole was concentrated in the thyroid gland and that the intrathyroidal concentrations were maintained for 16 to 24 h in spite of a short plasma half-life. It is suggested that a single daily dose of methimazole is adequate for the treatment of Graves'' disease in children and adolescents.

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