Hyperthyroxinemia associated with high thyroxine binding to albumin in euthyroid subjects.

Hyperthyroxinemia, and a high free-T4 index (calculated from T4 levels and T3 Resin uptake), was observed in an euthyroid boy. Thyroxine-binding globulin and prealbumin levels were normal. Direct assay of free-T4 yielded a normal value. Upon electrophoretic separation of a serum sample enriched with labeled T4 abnormal binding of T4 was detected in the albumin fraction. This binding was sensitive to barbital, and affected T3 to a lesser degree than T4. The excessive binding of T4 to albumin accounts for the normal free-T4 levels in the presence of increased total T4 concentration. Two additional cases of abnormal binding of T4 to albumin were detected in the family of the propositus. The correct evaluation of the thyroid status of these subjects should be important to prevent a misdiagnosis of hyperthyroidism.