Avoiding Misdiagnosis Due to Antibody Interference with Serum Free Thyroxin

INTRODUCTION: Interfering antibodies are capable of causing potentially misleading results in automated thyroid hormone immunoassays. CASE PRESENTATION: We report the case of a 46- year-old female patient with autoimmune hypothyroidism in chronic replacement treatment with levothyroxine who was presented 8 years after diagnosis with a thyroid function test showing an increased level of TSH and a very high level of FT4. Interference in the laboratory serum free thyroxin (FT4) test was suspected, due to the lack of symptoms of hyperthyroidism and a different immunoassay platform confirmed a low FT4 result. The discrepancy between the two results was explained by the presence of antiT4-autoantibodies. CONCLUSIONS: Antibody interference with serum free thyroxine must be considered when clinical findings and laboratory results show discrepancies.