Euthyroid hyperthyroxinemia due to familial excess of thyroxine-binding globulin.

The correct diagnosis of benign hyperthyroxinemia in this patient and his family members will spare them the unnecessary testing and treatment for thyrotoxicosis that has befallen some such patients. Results of the usual blood tests for assessment of thyroid function, such as T4, T3, and thyrotropin determinations, were not uniformly diagnostic, and were potentially misleading. An increased T4 level, a nonsuppressed TSH level, normal levels of FT4 and FT4D, and a low level of T3RU were clues that led to a request for specific measurement of serum TBG levels in multiple family members; family testing was essential for the diagnosis of euthyroid hyperthyroxinemia due to familial hepatic overproduction of TBG.