A New Inherited Abnormality of Thyroxine-Binding Globulin (TBG-San Diego) With Decreased Affinity for Thyroxine and Triiodothyronine*

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Abstract
Evaluation of a family in which males were clinically euthyroid despite having low serum total T4 (TT4) and free T4 index (FT4I) values revealed the presence of a new inherited T4-binding globulin (TBG) variant (TBG-San Diego). Two brothers had low TT4 (39 and 49 nmol/L; normal range, 64–154 nmol/L) and FT4I (4.0 and 4.4; normal range, 6.0–10.5) values, while their grandfather, despite treatment with T4, had a low TT4 (53 nmol/L) and normal FT4I (7.2) in the presence of suppressed TSH (4 binding assay at saturation (TBG-CAP) was similar, giving a mean TBGRIA/ TBG-CAP ratio not significantlydifferent from 1.0. In these males, the TBG affinity for T4 (Ka = 0.48 ± 0.04 × 1010 mol−1) was less than that in subjects with the common type TBG (TBG-C; Ka = 1.10 ± 0.14 × 1010 mol−1; P< 0.0001) and similar to that in Aboriginal males from Australia with the variant TBG-A (0.52 ± 0.10 × 1010 mo−1). TBG affinity for T3 in the affected males (Ka = 0.68 ± 0.05 × 109 mol−1) was less than that in subjects with TBG-C (1.39 ± 0.12 × 109 mol−1; P < 0.00001), but greater than that in males with TBG-A (0.44 ± 0.03 × 109 mol−1; P < 0.0005). The rate of TBG denaturation at 56 C was increased in the affected males (t½ = 28.4 and 26.9 min) compared to that in subjects with TBG-C (t½ = 54.1 ± 7.1 min), but was lower than that in males with TBG-A (t½ = 20.8 ± 1.9). A value intermediate between those in affected males and normal subjects was found in serum from an obligatory heterozygote. Microheterogeneity on isoelectric focusing was normal. The exact nature of this structural TBG variant is not yet known. The presence of a TBG mutant should be considered in healthy, clinically euthyroid patients with low serum TT4 and free T4 index values, especially when such low values are found in several members of the same family.

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