Familial Dysalbuminemic Hyperthyroxinemia: A Persistent Diagnostic Challenge

Familial dysalbuminemic hyperthyroxinemia (FDH)1 is a well-characterized condition associated with increased circulating total thyroxine (T4) concentrations and normal physiological thyroid function. It is caused by mutations in the ALB (albumin) gene that increase the affinity of albumin for T4 by approximately 60-fold. When measured by a technique that minimally disturbs the equilibria between T4 and its serum binding proteins, such as equilibrium or symmetric dialysis (SyD) performed in a near-physiological medium, the free T4 (FT4) value is characteristically within the reference interval. Assays that rely on the competition of a T4 analog with unbound T4 in the sample can give spuriously high results in FDH patients, because albumin binding of the T4 analog is enhanced by the FDH mutation. “Two step” methods, in which the T4 analog never comes into contact with serum albumin owing to a wash step immediately after capture, avoid this problem. Such assay methods are expected to give FT4 results within the reference interval in FDH patients, but this expectation has been questioned (1)(2).