Euthyroid familial hyperthyroxinemia due to abnormal thyroid hormone-binding protein.

A family is described in which three members had an elevated total serum thyroxine level and free thyroxine index. Each affected subject was clinically euthyroid and had a normal pulse wave arrival time (QKd), serum triiodothyronine and free thyroxine levels, and a normal serum thyroxine-binding globulin (TBG) concentration. Electrophoresis of their serum with 125I-labeled thyroxine revealed increased thyroxine binding in the albumin region. In addition, this abnormal protein, like thyroxine-binding globulin, bound 125I-labeled triiodothyronine and 125I-labeled reverse triiodothyronine. However, electrophoresis of serum treated by sialidase (neuraminidase) digestion suggested that this abnormal protein is not an anomalous form of thyroxine-binding globulin "buried" in the albumin area. These cases of euthyroid familial hyperthyroxinemia due to an abnormal thyroid hormone-binding protein show that an elevated serum thyroxine level or free thyroxine index is not always sufficient to confirm the presence of thyrotoxicosis.