THU667 Severe Hypothyroidism With Paradoxically Elevated Direct Free T4 Due To Anti-thyroxine Antibodies

Disclosure: O. Magvanjav: None. M.J. McPhaul: None. I. Choucair: None. E. Holt: None. Introduction Laboratory interference may cause abnormal thyroid function test (TFT) results, leading to misdiagnosis and inappropriate patient care. There are several major types of interference. Here, we describe a hypothyroid patient with an elevated thyroid stimulating hormone (TSH) level with a paradoxically elevated direct free thyroxine (FT4) level caused by interference from anti-thyroxine autoantibodies (THAAB). Clinical Case Our patient is a 38-year-old man who was found to have markedly elevated TSH and FT4 levels during an outpatient workup for Lyme disease. The patient was overall well-appearing but reported some symptoms suggestive of hypothyroidism including excessive fatigue, non-pitting edema, and dry skin. Physical exam was notable elevated blood pressure (systolic blood pressure of 150 mmHg and diastolic blood pressure of 95 mmHg), normal temperature, normal heart rate (72 bpm), regular heart rhythm, and normal body mass index. He had non-tender thyromegaly without nodules, non-pitting edema of bilateral hands and legs, decreased leg hair over bilateral shins, and coarse, dry skin of his hands. Initial TSH level was >150 mIU/ml, with a paradoxically elevated direct FT4 of 10 ng/dl (normal range 0.80-1.70 ng/dL). The FT4 level by equilibrium dialysis was <0.2 ng/dl. Outside laboratory testing to eliminate the effect of heterophile antibodies using two different assays again returned with elevated FT4 levels. Screening for macro TSH confirmed the markedly elevated TSH level. Further investigations at a second outside laboratory revealed that he had THAAB. A repeat FT4 level by dialysis confirmed the markedly low FT4 levels. He was empirically started on low dose levothyroxine and after increasing the levothyroxine dose, he showed improvement in clinical symptoms and the expected decline in TSH. Conclusions Here we present a rare case of THAAB that caused laboratory interference during thyroid function testing, leading to falsely elevated thyroid hormone levels on immunoassays. This case shows the importance of suspecting interference when there is a discrepancy between clinical symptoms and laboratory results or between results obtained from different testing methods. Our patient likely had a genetic predilection for autoimmunity, and Lyme disease is a known trigger for various autoimmune conditions. Our case also emphasizes the importance of taking a systematic approach to investigate the cause of the laboratory interference. This will help prevent misdiagnosis of thyroid disorders that may lead to additional unnecessary testing and treatment. Presentation: Thursday, June 15, 2023