Low Thyroidal Iodine Uptake with Euthyroidism Associated with Deficient Thyroid-Binding Globulin But Normal Cortisol Binding1

A deficiency of thyroxine-binding globulin (TBG) has been recognized in euthyroid subjects but has not been reported in conjunction with a depressed thyroidal I131 uptake. We have studied a 35-yr-old Indonesian male physician who lacked TBG but showed the normal localization of thyroxine (T4) binding in the albumin and prealbumin serum protein fractions. No physical abnormalities were present and mild serum hepatitis was the only prior illness of note. The subject had a consistently low protein-bound iodine (PBI), an increased red blood cell (RBC) uptake of triiodothyronine, and a normal basal metabolic rate (BMR) and serum cholesterol. The thyroidal I131 uptake was markedly low but showed a 7-fold increase after thyroidstimulating hormone (TSH); an abnormal release of trapped I131 was not induced by KSCN. The half-life of radioiodinated thyroxine (T4-I131) was shortened and the turnover rate was increased with a proportionally reduced extrathyroidal organic iodine pool. These findings imply a normal production of T4. No concomitant defect in cortisol binding was found.