Kinetics of thyroxine and triiodothyronine in a case of thyroxine-binding globulin deficiency with hypothyroidism.

Abstract

Kinetics of T3 and T4 were studied in a case of TBG deficiency with hypothyroidism. Serum total and free T4 of 35-year-old man were 0.6 μg/100ml and 0.54 ng/100ml, respectively. Serum total and free T3 were 18 ng/100ml and 74 pg/100ml, respectively. BMR was -27%. Thyroidal 24-hr uptake of 131I was 7%. Resin-sponge 131I-T3 uptake was 52%. Binding capacity of TBG was O. Binding capacities of TBPA measured by two different methods were 240 and 223 μg/100ml. The half-life of T4 was shortened, the T4 distribution space, turnover rate and clearance rate increased. Extrathyroidal organic iodine pool and degradation rate decreased. The half-life of T3 was slightly shortened, turnover rate, distribution space and clearance rate increased. Extrathyroidal T3 pool and degradation rate decreased. During the treatment with 75 μg/day of l-T3, the patient's complaints disappeared. When administration of l-T3 was switched to 400 μg/day of l-T4, elevated serum T3 remained higher than the preadministration level. His mother and a younger sister had decreased T4-binding capacity of TBG.