Sex-Related Differences in the Binding in Serum of Thyroid Hormones

Studies were conducted to determine whether there occur any sex-related differences inthe binding of thyroid hormones in the serums of normal young adult males and females. Paper electrophoretic studies conducted in both Tris-maleate and glycine-acetate buffers revealed a higher thyroxine (T4)-binding capacity of thyroxine-binding globulin(TBG) and a lower binding capacity of T4-binding prealbumin (TBPA) in specimens from females than in those from males. Studies of the distribution of small “tracer” concentrations of 131I-labeled T4 revealed similar sex-related differences, a greater proportion being associated with TBG and a lower proportion with TBPA in serums from females. Studies were also conducted in a barbital buffer in which binding of T4 by TBPA was completely inhibited. Here, too, at standard concentrations of added T4, a greater proportion was associated with TBG in specimens from females. The resin-sponge uptake of 131I-labeled 3,5,3′- triiodothyronine was slightly, but significantly, lower in serums from females than from males. No significant sex-related differences were observed in the serum PBI, the per cent or the absolute concentration of free T4, perhaps because of the small magnitude and the reciprocal nature of the differences in the 2 major binding proteins. It is suggested that thesesex-related differences in T4-binding may reflect a tonic physiologic influence of gonadal steroids.