Diagnostic Dilemma In Discordant Thyroid Function Tests Due To Thyroid Hormone Autoantibodies

ABSTRACT Objective: Assay interference could be the cause of abnormal thyroid function tests. Early recognition prevents inappropriate patient management. The objective of this report is to present a case with discordant thyroid function tests in different thyroid assay platforms due to thyroid autoantibodies. Methods: We present a case along with family history, laboratory data, and methods that investigate immunoassay interference. Results: A 21-year-old woman with autoimmune thyroid disease was treated for hypothyroidism with levothyroxine and noted to have elevated total and free thyroxine and free triiodothyronine but normal thyroid-stimulating hormone. Repeat thyroid function tests using different platforms revealed discrepant results. Further investigation showed that the patient had positive thyroid hormone autoantibodies (THAAbs). Conclusion: We demonstrated abnormal thyroid function tests caused by THAAbs. The latter were the cause of interference with assays resulting in discrepant test results inconsistent with the clinical presentation. Early recognition would prevent inappropriate patient management. Abbreviations: AITD = autoimmune thyroid disease; FT4 = free thyroxine; HAMAb = human anti-mouse antibody; RTH = resistance to thyroid hormone; T3 = triiodothyronine; THAAb = thyroid hormone autoantibody, THRB = thyroid hormone receptor–beta; TSH = thyroid-stimulating hormone; TT4 = total thyroxine