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.