Familial hyperthyroxinemia due to abnormal thyroid hormone binding.

A patient had an elevated serum total thyroxine (T4) free thyroxine (free T4), free T4 index, and T4/thyroxine-binding globulin ratio. The serum triiodothyronine (T3), as well as the thyrotrophin response to thyrotrophin-releasing hormone was normal and consistent with the clinical euthyroid status. The basis for the elevated serum total T4, free T4 index, and T4/thyroxine binding globulin is the abnormally enhanced binding of thyroid hormone by albumin, or by an anomalous protein migrating with a mobility similar to albumin as determined in paper electrophoretic systems. The abnormal T4 protein binding was also seen in several members of the patient's family indicating that the condition was familial. A new type of serum T4 protein binding abnormality that results in hyperthyroxinemia may falsely indicate thyrotoxicosis in clinically euthyroid subjects.