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EVALUATION OF T4 AND T3 BINDING KINETICS IN THE THYROXINE BINDING GLOBULIN ABNORMALITY OF AUSTRALIAN ABORIGINES
Author(s) -
MOHR V. S.,
BARLOW J. W.,
TOPLISS D. J.,
O'DEA K.,
STOCKIGT J. R.
Publication year - 1987
Publication title -
clinical endocrinology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.055
H-Index - 147
eISSN - 1365-2265
pISSN - 0300-0664
DOI - 10.1111/j.1365-2265.1987.tb00808.x
Subject(s) - euthyroid , medicine , endocrinology , thyroxine binding proteins , thyroxine binding globulin , globulin , chemistry , triiodothyronine , hormone
SUMMARY Low serum total T4 associated with subnormal concentrations of thyroxine binding globulin (TBG) has been reported in up to 40% of euthyroid Australian aborigines. It has been suggested that these subjects show both diminished concentration of TBG and reduced TBG affinity for T4 (Same et al. , 1985). We have compared 12 euthyroid aborigines with low T4 (total T4 44±5 nmol/1) and aborigines with normal T4 (T4 99±9 nmol/1, n = 12) using measurements of free T4 and T3 by equilibrium dialysis. TBG was measured both by RIA (Henning, Berlin, FRG) and a method dependent on T4 binding (Corning Immophase). Aborigines with low T4 showed lower levels of free T4 (12.6±0.6 cf. 18.7±1.0 pM), free T4 index (66±8 cf. 98±13), total T3 (1.1±0.2 cf. 1.6±0.3 nmol/1), TBG RIA (140±0.6 cf. 25.0±1.2 ng/1), and TBG Immophase (9.0±0.5 cf. 22.0±1.2 mg/1) (P±0.01), but free T3 (5.3±0.4 cf. 4.7±0.4 pM) and TSH (1.9±0.2 cf. 1.8±0.2 mU/1) were not significantly different from the values found in aborigines with normal T4. Scatchard analysis of T4 and T3 binding was performed using serum diluted 1:20 000 for T4 and 1:500 for T3 (barbitone buffer pH 8.6, 4°C, dextran‐coated charcoal separation). In euthyroid low T4 aborigines compared to those with normal T4, both T4 capacity (106±14 cf. 238±13 nM, P ±0.01) and affinity (5–05±10 10 cf. 8.47±10 10 M −1 , P ±0.05) were significantly reduced. Similarly, both T3 capacity (62±10 cf. 154±16 nM, P±0.01) and affinity (1.67×10 9 cf. 2.28×10 9 M −1 , p <0.02) were reduced. A substantial minority of euthyroid Australian aborigines have a TBG variant characterized by both reduced capacity and affinity of T4 and T3. These findings suggest that TBG may be both qualitatively and quantitatively abnormal in these subjects.