A case of total thyroxine-binding globulin deficiency with Graves' disease: fluctuations of plasma triiodothyronine/thyroxine ratio.

A 37-year-old male with total thyroxine-binding globulin (TBG) deficiency associated with Graves' disease is described. Both TBG immunoreactivity and TBG capacity were not detectable in his serum. Serum concentrations of thyroxine-binding prealbumin and albumin were normal. He was initially hyperthyroid. During methimazole-treatment he was maintained in an euthyroid state except for two short hypothyroid periods. His plasma triiodothyronine/thyroxine (T3/T4) ratios during both the untreated hyperthyroid and the methimazole-induced hypothyroid states were higher than those during his methimazole-induced euthyroid state. These findings on changes in his T3/T4 ratio accompanying thyroidal dysfunction were qualitatively comparable with those in patients with Graves' disease with normal TBG levels: that both untreated hyperthyroid and methimazole-induced hypothyroid patients showed higher T3/T4 ratios than methimazole-induced euthyroid patients. These results may provide indirect evidence that changes in hormonal secretion and conversion that raise T3/T4 ratio can occur in thyroidal dysfunctions even in the complete absence of TBG.