Thioamides Drugs propylthiouracil (PTU) methimazole Mechanism inhibit thyroid hormone synthesis by blocking peroxidase-catalyzed reactions iodination of tyrosine residues of thyroglobulin coupling of DIT and MIT propylthiouracil ↓ peripheral conversion of T4 to T3 Clinical use HYPERthyroidism Toxicity skin rash agranulocytosis (rare) aplastic anemia hepatotoxicity (PTU) PTU not used first line in non-pregnant adults and children for this reason Pregnancy use PTU in the first trimester methimazole crosses the placenta and enters breast milk (may be teratogenic) Thyroid hormones Drugs levothyroxine synthetic T4 (thyroxine) triiodothyronine T3 Mechanism replaces endogenous thyroid hormones Clinical use HYPOthyroidism myxedema Toxicity thyrotoxicosis heat intolerance tachycardia dyspnea ↑ appetite tremors arrhythmias