Peripheral Metabolism of Thyroid Hormones in Congenital Thyroid Hormone Transport Abnormalities: Familial Dysalbuminemic Hyperthyroxinemia and TBG Increase

Abstract

Familial dysalbuminemic hyperthyroxinemia (FDH) and inherited increase of thyroxine-binding globulin (TBG) in plasma are two very uncommon thyronine serum-binding protein abnormalities that cause hyperthyroxinemia without hyperthyroidism. In FDH, subjects have an abnormal albumin molecule which shows increased affinity only for thyroxine (T₄). As a result, serum total T₄ (TT₄) value is increased, while the serum total triiodothyronine (TT₃) concentration is normal or slightly elevated. Nevertheless, the increase in the free T₄ index frequently leads to an erroneous diagnosis of thyrotoxicosis. On the contrary, in the inherited high TBG syndrome, proportionate increments in serum TT₄ and TT₃ concentrations are seen because TBG has a high affinity for T₄, as well as T₃, whereas the free T₄ index remains normal.