HYPERTHYROIDISM AND DECREASED THYROXINE BINDING BY SERUM PROTEINS*

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Abstract
In 2 sisters, hyperthyroidism was associated with normal levels of serum protein-bound iodine (PBI). A decrease in thyroxine (T4) binding by serum alpha-globulin (TBG) and pre-albumin (TBPA) was demonstrated in both cases by paper electrophoresis in ammonium carbonate buffer. TBG capacities were 0.08 and 0.11 μg. of T4 per ml., and the serum PBI levels were 5.1 and 6.6 μg. per 100 ml., respectively. TBG capacity among normal subjects ranged from 0.13 to 0.25 μg. of T4 per ml. The reduction in the TBG level is offered as the most likely explanation for the normal concentration of serum PBI in these patients. In one of the cases, repeated studies following radioiodine therapy revealed an increase in TBG and TBPA levels to low-normal and a fall in PBI concentration to an average value of 4.0 μg. per 100 ml. Chromatographic anatysis of the serum I131 compounds showed a normal ratio of T4 to T3. An incidental finding in both patients was the presence in the serum of an iodinated protein comprising nearly one-sixth of the proteinbound I131. A genetically determined defect is postulated to account for the occurrence in these sisters of hyperthyroidism and decreased serum thyroxine binding.