Total and Free Triiodothyronine and Thyroid-Binding Globulin Concentration in Elderly Human Persons

Abstract

Total thyroxine (TT⁴) and triiodothyronine (TT³) were found to be low in healthy elderly subjects with a preferential decrease of triiodothyronine. In order to determine the importance of these findings 22 healthy elderly subjects were examined. Free triiodothyronine (FT³), thyroid binding globulin (TBG) concentration and basal thyroid stimulating hormone (TSH) were measured by radioimmunoassay. Liver enzymes, cholesterol and total protein concentration were also assayed. TBG was significantly increased compared to a middle-aged group and did not correlate with TT⁴, TT³ and TSH. Basal TSH values were in the normal range and could be detected in all the elderly subjects in contrast to undetectable values in 40 % of the younger subjects. PT³ determined directly did not correlate with the values calculated according to the law of mass action. According to the FT³ values the elderly subjects could be subdivided into three groups independent of their TT⁴, TT³, TBG and TSH values. FT³ was undetectable in one group, in the low normal to normal range in another and elevated in the third group. Our results suggest that 1) there is no correlation between TT⁴, TT³, elevated TBG and FT³ determined directly or by calculation, 2) basal TSH values seem to indicate possible hypothyroidism in elderly persons which is correlated with elevated cholesterol levels and 3) FT³ measured directly subdivides this metabolic state into three groups possibly depending on the intracellular concentration of T⁴.