Relationship between serum thyroxine concentration and dialyzable triiodothyronine fraction in patients with hyper- and hypothyroidism.

The interrelationship between the dialyzable triiodothyronine (T3) fraction (DFT3), T3, and thyroxine (T4) was examined in various groups of patients with hyper- and hypothyroidism. Partial correlational analysis showed that DFT3 correlated significantly with T4 (r = 0.383, N = 114, P less than 0.001), but not with T3 (r = 0.132). DFT3 exceeded the normal range when T4 was supranormal. The relation between T3 and free T3 (FT3) was quadratic (R2 = 0.944, N = 112, P less than 0.001). When the patients were divided into two groups according to T4 levels, the regression line between T3 and FT3 in patients with supranormal T4 was more than twice as steep as that obtained in patients with normal or subnormal T4 levels. In addition, the DFT3 and FT3 values were higher in patients with hyperthyroidism and both elevated T4 and T3 than in patients with hyperthyroidism and only elevated T3. The results indicate that T4 is a major factor influencing the DFT3 value, except when the T4-binding globulin concentrations vary markedly, and that the T4 level should be taken into account for a clinical evaluation of the FT3 concentration in hyper- and hypothyroidism.