FAMILIAL INCREASE IN THE THYROXINE-BINDING SITES IN SERUM ALPHA GLOBULIN*

Abstract

Because the serum protein-bound iodine (PBI) has been found elevated in euthyroid states associated with an increase of thyroxine-binding capacity of plasma thyroxine-binding a globulin (TBP) due to known causes, a search was made for individuals with unexplained elevation of the serum PBI. A euthyroid man was found with serum PBI and butanol extractable iodine (BEI) levels ranging between 2 and 3 times normal and maintained over an observation period of 9 months. A subnormal I131 triiodothyronine erythrocyte uptake (I131 T3 rbc uptake) suggested that the serum PBI elevation was related to increased affinity of the plasma proteins for thyroid hormone. This suspicion was confirmed by the finding of an increase in thyroxine-binding capacity of TBP by reverse-flow serum electrophoresis. No abnormality was detected in thyroxine binding to albumin or prealbumin. Although the extrathyroidal organic I pool was expanded to roughly the same size as that found in patients with untreated thyrotoxicosis, the degradation rate in micrograms of thyroxine per day was within the normal range. The finding that one of this man''s 3 children also had elevated serum PBI and BEI levels, a subnormal I131 t3 RBC uptake and an elevated TBP capacity constitutes evidence that the abnormality is familial and could conceivably be hereditary. No other cause for this abnormality was detected. The finding of such markedly increased levels of thyroid hormone in the serum of euthyroid individuals supports the contention that the total serum thyroxine concentration does not alone govern the metabolic status of the individual.