Altered plasma half-lives of antipyrine, propylthiouracil, and methimazole in thyroid dysfunction

In normal, nonmedicated volunteers and in patients with thyroid disorders the plasma half-lives of antipyrine, propylthiouracil, and methimazole were determined after single oral doses. The plasma half-liver plus or minus S.D. of antipyrine, propylthiouracil, and methimazole were 11.9 plus or minus 1.4 hr, 6.7 plus or minus 1.0 hr, and 9.3 plus or minus 1.4 hr, respectively, in normal volunteers, but were shortened to 7.7 plus or minus 1.2 hr, 4.3 plus or minus 0.7 hr, and 6.9 plus or minus 0.6 hr, respectively, in hyperthyroid patients. In hypothyroid patients the plasma half-lives of these drugs were prolonged to 26.4 plus or minus 4.0 hr, 24.7 plus or minus 34.5 hr, and 13.6 plus or minus 4.8 hr, respectively. Return to the euthyroid state restored plasma half-lives to or toward normal. Alterations in plasma drug half-lives during thyroid dysfunction appear to result mainly from accelerated hepatic microsomal drug metabolism in hyperthyroidism and retarded drug biotransformation during hypothyroidism.