Familial dysalbuminaemic hyperthyroxinaemia and inherited partial TBG deficiency: first report

Inherited defects of thyroxine-binding proteins

Thyroid hormones (TH) are bound to three major serum transport proteins, thyroxine-binding globulin (TBG), transthyretin (TTR) and human serum albumin (HSA). TBG has the strongest affinity for TH, whereas HSA is the most abundant protein in plasma. Individuals harboring genetic variations in TH transport proteins present with altered thyroid function tests, but are clinically euthyroid and do not require treatment. Clinical awareness and early recognition of these conditions are important to prevent unnecessary therapy with possible untoward effects. This review summarizes the gene, molecular structure and properties of these TH transport proteins and provides an overview of their inherited abnormalities, clinical presentation, genetic background and pathophysiologic mechanisms.

A possible new syndrome of familial euthyroid dysproteinemia associated with elevated thyroxine and triiodothyronine

Increased thyroxine (T4) binding to thyroid binding prealbumin (TBPA) or albumin causes a biochemical picture suggestive of thyrotoxicosis with increased total T4 (TT4). Previously described familial syndromes are characterized by increased binding of T4 to thyroid binding globulin (TBG), TBPA, or albumin. Increased T3 binding to TBG has been noted, but we are not aware of any kindreds reported with increased T3 binding to TBPA or albumin. We wish to report a family in which there is increased binding of T3 to TBG, TBPA and albumin. All family members were clinically euthyroid and tested negative for T3 antibodies. This family appears to demonstrate two abnormalities: first, elevated TBG, and second, normal levels of TBPA and albumin with increased binding of T3 and T4 to these proteins. We believe this may represent a new syndrome of familial dysproteinemia in which multiple proteins show enhanced binding of T3 and T4. This could reflect increased binding sites, increased binding affinity, or a combination of both.