Correlation between antithyroid effect and serum concentrations of propylthiouracil in patients with hyperthyroidism.

1 Correlation between the kinetics of propylthiouracil and its antithyroid effect was studied in 17 hyperthyroid patients. The serum concentration of propylthiouracil 1 h after an oral dose of 400 mg of hyperthyroid patients. The serum concentration of propylthiouracil 1 h after an oral dose of 400 mg of the drug was used as kinetic parameter as this concentration from the previous study was found to correlate significantly (r = 0.84, P less than 0.01) with the area under the serum concentration-time curve. 2 After 3 weeks of treatment with 200 mg propylthiouracil three times daily the serum concentration of propylthiouracil was correlated to the decrease in various thyroid parameters such as total and free indexes of serum thyroxine, triiodothyronine and reverse triiodothyronine. 3 Significant correlations were found between the serum concentration of propylthiouracil and all the measured thyroid variables except reverse triiodothyronine. The highest degree of correlation was obtained between serum propylthiouracil and the percentage decrease in total and free indexes of triiodothyronine (r = 0.63 and 0.70, respectively, P less than 0.01). 4 It is suggested that a serum concentration of propylthiouracil above 4 to 5 micrograms/ml 1 h after an oral dose of 400 mg of the drug will secure a sufficient and rapid antithyroid effect during continuous therapy.