Factitious hyperthyroxinemia due to a monoclonal IgA in a case of multiple myeloma

A clinically euthyroid 53-year-old woman with an IgA-lambda-secreting multiple myeloma presented with increased serum concentrations of thyroid hormones. Laboratory studies revealed increased total thyroxine (T4) and triiodothyronine (T3) concentrations, a high-normal free T4 concentration, and a normal basal thyrotropin (TSH) concentration with a normal response to thyroliberin (TRH). Her serum concentration of IgA was 11,040 mg/L (normal range 900-4500 mg/L) and immunoelectrophoresis revealed it to be monoclonal. This monoclonal IgA bound both T4 and T3, as determined by serum immunoelectrophoresis and direct binding studies. Immunoelectrophoresis in the presence of [125I]T4 or [125I]T3 localized the radiolabeled iodothyronines to a band corresponding exactly to the precipitin arc of the monoclonal IgA. We performed direct binding studies with IgA purified by affinity chromatography with the lectin jacalin. Purified IgA (50 micrograms) bound both [125I]T4 (12.3%) and [125I]T3 (2.7%) specifically and in a dose-dependent manner. Scatchard analysis of competitive-binding data utilizing [125I]T4 and unlabeled T4 revealed a Kd of 2.2 x 10(-7) mol/L. The binding capacity for T4 was approximately 7 mumol/L. Thus, in this case of IgA-secreting myeloma, the monoclonal IgA acts as an additional thyroid hormone-binding protein in serum that interferes in the T4 and T3 RIAs. This is the first report of a monoclonal IgA producing an apparent euthyroid hyperthyroxinemia.