Low serum thyroxine in phenothiazine-treated psychiatric patients

Five patients treated with phenothiazines for serious psychiatric disease had markedly depressed serum thyroxine concentrations. There were no clinical or other laboratory findings to suggest hypothyroidism. Additional studies revealed normal levels of thyroxine-binding globulin as assessed by triiodothyronine resin uptake, thyroid-binding globulin capacity, and free thyroxine concentration. In contrast to the low serum thyroxine, serum triiodothyronine concentration was nornal.