Pharmacokinetics of Methimazole in Normal Subjects and Hyperthyroid Patients

Serum and urinary concentrations of methimazole (MMI) were measured by high-performance liquid chromatography (HPLC) with an electrochemical detector (ECD) in 10 normal subjects and 43 hyperthyroid patients after intravenous and oral administration of the drug. The pharmacokinetic parameters of MMI were estimated in 5 normal subjects and 15 hyperthyroid patients according to a two-compartment model after intravenous injection of a 10 mg dose. The mean half-life of the distribution phase (T1/2α) was 2.7 ±1.0 h (mean±SD) and 3.1 ± 1.4 h and that of the slower-phase (T1/2β) was 20.7±9.6 h and 18.5±12.9 h in normal subjects and hyperthyroid patients, respectively.There were no significant differences between pharmacokinetic parameters of normal subjects and those of hyperthyroid patients.No correlations between free T4 index (FT4I) and pharmacokinetic parameters were observed. Maximum serum MMI concentrations (Cmax)(213±84 and 299±92 ng/ml) were attained 1.8±1.4 h and 2.3±0.8 h after a single dose of 10 mg in 5 normal subjects and in 15 hyperthyroid patients, respectively.In hyperthyroid patients the time taken to reach the peak concentration (Tmax) after a single dose of 10 mg was similar to that after a single 15 mg and 30 mg dose.The pharmacokinetic parameters, except Cmax and the area under the curve (AUC), were not affected by the administered dose and those, except Cmax, were not affected by the thyroid function. All urine was collected at intervals of 3 h for the first 12 h and then at 24 h and 48 h after intravenous and oral administration of MMI.In all subjects, MMI rapidly appeared in the urine and the rate of excretion was highest in the first 3 h.The cumulative urinary excretion of MMI was 5.5-8.5% of administered doses in normal subjects and hyperthyroid patients. These findings in the present study are compatible with the assumption that the extent of absorption of MMI is high, if not complete, and hyperthyroidism does not affect the kinetics of MMI, and that interindividual variation is observed in the time taken to reach the peak concentration after oral administration.