Thyroid Overactivity and TBG Deficiency Simulating “T3Hyperthyroidism”

A case of toxic nodular goiter with a deficiency of thyroxine-binding globulin (TBG) is described. Laboratory test results fulfilled the usually suggested criteria for the diagnosis of “T3 hyperthyroidism” except for the low TBG binding capacity. In the presence of reduced TBG binding capacity, serum T4 is low, since thyroxine is predominantly bound to this carrier. The diagnosis of T3 hyperthyroidism should not be made without proof of normal levels of TBG.