Dichotomy between Serum Free Triiodothyronine and Free Thyroxine Concentrations in Familial Thyroxine- Binding Globulin Deficiency

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Abstract
The mean serum total T4 (3.2 ± 1.0 jug/100 ml) and T3 levels (83 ± 29 ng/100 ml) in eight euthyroidpatients with T4-binding globulin (TBG) deficiency from two families were significantly lower than those in either the unaffected relatives (P< 0.05) or the control subjects (P 4 and T3 values were found in the four hemizygotes (2.4 ± 0.2 jxg/100 ml and 69 ±25 ng/100 ml, respectively).In contrast to the absolute T4 and T3 levels, the mean percentages of free T4 and free T3 in the affected members were about twice as high a those in either the unaffected relatives or the controls. Despite an elevated percentage of free T4 in each of the TBG-deficient members, the mean absolute free T4 (FT4) level (0.89 ± 0.20 ng/100 ml) was significantly lower than those in both control groups [1.31± 0.2 ng/100 ml (P < 0.02) and 1.24 ± 0.30 ng/100 ml {P < 0.01) respectively]. Serum FT4 levels were subnormal in three of four hemizygotes.In contrast to FT4, the mean FT3 level in the affecte members (0.33 ± 0.04 ng/100 ml) was significantly higher than that in either the nondeficient relatives (0.25 ± 0.02 ng/100 ml; P < 0.02) or the control subjects (0.26 ± 0.04 ng/100 ml; P < 0.02), whereas the ratio of FT3 to FT4 was about twice a high The data suggest that in TBG deficiency, euthyroidism is maintained by a subtle equilibrium between low FT4 and high FT3 levels.