Spurious T3 Thyrotoxicosis Unmasking Multiple Myeloma

Objective . To document a case of spurious T3 thyrotoxicosis in a 54-year-old woman. Methods . We present the diagnostic approach of a patient with euthyroid hypertri-iodothyronemia. Results . A 54-year-old, clinically euthyroid woman without personal or family history of thyroid disease referred to endocrinology for possible T3 thyrotoxicosis, after thyroid function tests revealed total T3 > 800 ng/dL (reference range 60–181), normal TSH, and T4. The laboratory data were not compatible with the clinical picture, so thyroid binding globulin abnormalities were suspected. Additional laboratory studies confirmed the diagnosis of multiple myeloma. Conclusion . Monoclonal gammopathy is characterized by the presence of a monoclonal immunoglobulin in the serum or urine, occurring in multiple myeloma, and can cause assay interference and spurious results. We identify a newly recognized cause of euthyroid hypertri-iodothyronemia, due to binding of T3 to monoclonal immunoglobulins in the setting of multiple myeloma. Our case is the only one to date suggesting that monoclonal immunoglobulins from multiple myeloma may exhibit binding to T3 only.