Triiodothyronine toxicosis developing during antithyroid drug therapy for hyperthyroidism

Twelve patients with toxic diffuse goiter on thioamides became euthyroid 42 to 56 days after commencement of the drug and their serum triiodothyronine (T3) and thyroxine (T4) levels fell to normal. Twelve additional patients with hyperthyroidism on long term antithyroid drugs were euthyroid and had normal serum T3 and T4 levels. A patient who demonstrated thyrotoxicity on a suboptimal dose of methylthiouracil had low serum T4 and a high T3 concentration. It is postulated that localized relative intrathyroidal iodide deficiency in the small, markedly hyperfunctioning thyroid follicles accounts for the observation. Whatever its explanation, the finding underscores the importance of T3 measurements in some patients with hyperthyroidism.