Primary hypothyroidism with paradoxically elevated plasma thyroxine due to thyroxine-binding antibody.

Investigation of an elderly patient with clinical hypothyroidism revealed a low serum triiodothyronine (T3) but markedly raised levels of total thyroxine (TT4) and free thyroxine (fT4) when measured by radio-immunoassay (RIA). Primary hypothyroidism was suggested by an elevated basal thyroid stimulating hormone (TSH) and confirmed by a low fT4 when measured by a commercial microencapsulated antibody method. The paradoxically elevated levels of fT4 and TT4 were due to the presence of an antibody which specifically bound T4 and grossly interfered with the RIA thus complicating the diagnosis and later the assessment of replacement therapy. The sensitive immunoradiometric TSH assay proved of value in resolving these problems.