Thyroid Function Tests in Subjects with a Genetic Isoelectric Focusing Variant TBG

Abstract

A variant thyroxine-binding globulin (TBG) with slower migration on isoelectric focusing (TBG-S) has been found in some American Blacks. It is inherited as an X-chromosome-linked trait. Hemizygous males with TBGS exhibit TBG bands shifted cathodally. Heterozygous females with TBG-CS, show the summation pattern of TBG bands corresponding to sera from subjects with only TBG-S and those with only the common type of TBG (TBG-C). Because the effect of TBG-S on thyroid function has not been studied in detail, we compared data of thyroid function tests in sex and age matched subjects belonging to four TBG genotypes (10 males each with TBG-S vs. TBG-C, and 10 females each with TBG-CS vs. TBG-CC). The mean serum T₄ level of the group of males with TBG-S was 6.79 + 1.52 (SD) μg/dl, significantly lower than that of the group of males with TBG-C (8.49 + 1.29 lig/μl, p<0.02). A significant concomitant reduction in the mean serum TBG concentration was found in the group of males with TBG-S as compared to that with TBG-C (1.41 + 0.30 mg/dl vs. 1.72 + 0.29 mg/dl, p<0.05). Mean values of serum free T4 concentrations and other tests of thyroid function, including TSH levels, were not significantly different among the four groups matched by age and paired by sex. Although the reason for the reduced serum TBG concentration in subjects with TBG-S remains unclear the associated reduction of serum T₄ levels is not accompanied by hypothyroidism.

Supported in part by United States Public Health Service Grant AM 15070