Thyroid Function Tests in Thyroxine-Binding Globulin Deficiency

Abstract

SUMMARY Results are presented on two patients with complete and two with partial thyroxinebinding globulin (TBG) deficiency. All four subjects had lowered serum thyroxine but were clinically euthyroid. While thyroid hormone uptake tests or TBG assay were effective in the recognition of such individuals, indices based on these tests were misleading in assessing their thyroid status. Results within the reference range were obtained with the Immophase Free Thyroxine assay.More than 99 % of circulating thyroxine is protein bound, but it is the small free thyroxine concentration (free-T4) that determines thyroid status (Ingbar and Freinkel, 1960).The serum total thyroxine (T4) concentration has been used to measure thyroid function, although it changes independently of thyroid activity when the concentration of binding protein is altered.Attempts to compensate for binding abnormalities include the free-thyroxine index (FTI) (Howorth and MacLagen, 1969) and T4: TBG ratio (Burr et al., (1977), in both of which the T4 concentration is divided by a measure of binding protein levels. The Immophase Free-T4 kit (Corning Medical, Halstead) gives absolute free-T4 results by a rate method.In the present paper, these three approaches are compared in four patients with very low or undetectable TBG concentrations. MethodsT4 and serum triiodothyronine (T3) (Challand et al., 1975) and thyrotropin (TSH) (Burrin, 1976) were measured by radioimmunoassay, while TBG was determined by rocket immunoelectrophoresis (Bradwell et al., 1976).Commercial kits were used for the serum thyroid hormone uptake (Thyopac-3) (Radiochemical Centre Amersham) and Immophase Free T4. FTI = (T4 x 1(0) --:-Thyopac-3, and T4 : TBG ratio were both calculated.