Hyperthyroidism with normal serum total T4, T3, and thyroid binding protein concentration

A 60-year-old woman, free of other disease, with severe clinical hyperthyroidism had normal total serum T4 with elevated free T4 concentration and T3 resin uptake characteristic of hyperthyroidism, with thyroid binding globulin (TBG) deficiency. However, serum TBG concentration as determined by immunoassay was normal, suggesting that the biochemical defect affecting the T4 binding sites did not affect is antigenicity. Her total and free serum T3 levels were low-normal with a markedly elevated reverse T3 concentration. These findings, in the presence of clinical hyperthyroidism, are consistent with an impairment in peripheral conversion of T4 to T3, apparently, in this case, due only to long-standing severe hyperthyroidism.