Diagnosis of familial dysalbuminemic hyperthyroxinemia and investigation of the nature of the variant albumin.

A variant albumin with abnormally increased thyroxine-binding affinities has been identified, and termed as familial dysalbuminemic hyperthyroxinemia (FDH). The aims of this study are to seek for a method of diagnosis to distinguish FDH from other causes of hyperthyroxinemia and to investigate the nature of this abnormal albumin. Percent T4 bound to albumin was high in FDH, ranging 29-48% (normal range 3-8%), as measured in [125I] T4 loaded serum using a single immunoprecipitation by anti-human albumin antibody. A subject with T4 autoantibody in serum had a lower value. Only a single subject with TBG deficiency had an overlapped value (33%) with that of FDH, but the level of serum T4 could differentiate TBG deficiency from FDH. Isoelectric focusing (IEF) showed a distint albumin band with an isoelectric point of pH 5.05 in all FDH subjects, and this band was not compatible with any of 4 albumin bands seen in normals. These data suggest that only two simple measurements including the level of serum T4 and the single immunoprecipitation for [125I] T4-bound albumin can distinguish FDH from other causes of hyperthyroxinemia.