SAT482 A Case of Falsely Elevated Free T4 and Free T3 Levels Caused by Human Anti Mouse Antibodies.

Disclosure: I.J. Romao: None. M.H. Shanik: None. Introduction: When there are clinical and biochemical discrepancies with thyroid function tests immunoassay interference should be considered (1). Human anti mouse antibodies (HAMA) are antibodies found in humans that react with immunoglobulins in mice and are used as diagnostic reagents for laboratory measurements. The presence of HAMA most commonly causes a false elevation of thyroid stimulating hormone but can also increase Free T4 and Free T3 levels. Case presentation: A 31 year old woman presented for evaluation of elevated Free T4 and Free T3 levels. For several weeks she complained of feeling neck soreness and palpitations. She had a recent infection and low-grade fever. She also felt pressure behind her eyes. She had a history of anxiety and took Xanax as needed, multivitamin and probiotic. Cousin had thyroid disease. On exam her blood pressure was 138/86, heart rate 80, respiratory rate 12, weight 149 lbs., height 5’5”. Her thyroid was firm and not tender. She had a thyroid ultrasound that showed a homogenous gland with no discrete nodules. TSH was 1.3 uIU/mL (0.27-4.2), Total T4 10.4 ug/dL (4.5 – 12), Free T4 4.6 ng/dL (0.9-1.7), Total T3 1.6 ng/mL (0.8-2.0), Free T3 10.1 pg/mL (2.0-4.4). C reactive protein, erythrocyte sedimentation and interleukin 6 levels were in the normal range. Thyroid peroxidase antibodies and thyroglobulin antibodies were elevated. The differential diagnosis included thyroiditis, pituitary tumor producing thyroid stimulating hormone and thyroid hormone resistance. She did not have a thyroid uptake and scan because she had small children at home. To rule out a pituitary adenoma she had a pituitary MRI that showed a normal small pituitary gland. The test for resistance to thyroid hormone mutation analysis was negative. Over a few months she had some clinical improvement, and her thyroid function tests remained the same which would not be consistent with thyroiditis. HAMA level was checked to evaluate for immunoassay interference with thyroid function tests. The HAMA level was elevated at greater than 600 ng/mL (0-188). Free T4 by equilibrium dialysis was normal at 1.6 ng/dL (1.1 – 2.4). Conclusion: HAMA can cause falsely elevated Free T4 and Free T3 levels. It is important to suspect immunoassay interference when there are inconsistencies in the clinical presentation and thyroid function tests. Identifying the presence of elevated HAMA can prevent misdiagnosis and inappropriate patient management. 1. Favresse J, Burlacu MC, Maiter D, Gruson D. Interferences with thyroid immunoassays: Clinical implications and detection algorithm. Endocrine Reviews. 2018. Pages 830-850. Presentation Date: Saturday, June 17, 2023