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. 1. Introduction Many conditions may interfere with the measurement of total T4 and T3, and may also cause small changes in free T3, and T4 levels. These conditions in the past were a diagnostic challenge, and patients may have been falsely treated for thyroid disease. Nowadays, TSH alone is considered a sufficient screening tool to rule out thyroid dysfunction, without being followed with a T3, T4 measurement . But if T3 and T4 are measured and the patient is found to have abnormal levels of thyroid hormones, closer evaluation is needed. 2. Case Report Our patient is a 54-year-old woman, who was referred from primary care physician for elevated T3 and possible T3 thyrotoxicosis. For the past few months, she had been complaining from fatigue and insomnia. She denied history of weight loss, hyperdefecation, heat intolerance, skin or hair changes, tremors, visual changes, and palpitations. She has no known personal or family history of thyroid disease. Physical exam revealed no palpable goiter, and the patient was clinically euthyroid. Previous thyroid function tests were all within normal limits. Patient’s past medical history includes hypertension and total abdominal hysterectomy with bilateral salpingoophorectomy in 2006 for benign disease. Home medications include amlodipine 5？mg, irbesartan 150？mg, and hydrochlorothiazide 12.5？mg. After complaining of fatigue, her primary care physician ordered thyroid function tests,
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