SAT-614 False-Positive in Measurement of Thyroglobulin in a Patient with Mixed Connective Tissue Disease

Background: Serum thyroglobulin (sTg) evaluation is mandatory on the follow-up of differentiated thyroid cancer (DTC), as it estimates response totherapy and predicts clinical outcomes. The risk of persistent/recurrent disease positively correlates to sTg, commonly measured by immunoassays.Biochemical incomplete response is defined as a nonstimulated sTg level >1ng/mL and negative image in the absence of Tg antibody (TgAb). False negative sTg on immunoassay tests could occur on the presence ofTgAb. On the contrary, false positive sTg is a very rare condition and it may be suspected when sTg increases despite of non-identified disease. Case report: A 39 years-old female patient with DTC was submitted to total thyroidectomy and neck dissection in 2016. sTg on levothyroxine (LT4) was 0.3 ng/mL and raised to 7.5 ng/mL even after radioiodine ablation. There was no evidence of structural disease on post-ablation whole-body scan, neck ultrasound and (18)FDG-PET/CT scanning, defining biochemical incomplete response. sTg on LT4 fluctuated from 3.0 to 22 ng/mL, always with negative TgAb. In the meanwhile, she presented polyarthralgia, puffy hands, proximal muscle weakness and alopecia. Esophagopathy and interstitial pneumopathy were also detected. Mixed connective tissue disease was diagnosed with high levels of anti-U1 ribonucleoprotein (RNP) and antinuclear antibody. Immunosuppressive treatment with corticotherapy and azathioprine was introduced and suprisely sTg decreased from 22.7 to 1.6 ng/mL. It was also noteworthy that sTg did not increase on stimulated TSH (sTg 1.6 ng/mL with TSH 35 UI/mL). Heterophilic antibodies interference was suspected, thereafter the same sample, was measured in two different commercial immunoassays, by ECLIA-Beckman was 4.5 ng/mL and undetectable (<0.1 ng/mL) by ECLIA-Roche, both in the absence of TgAb. Conclusion: Lower levels during immunosuppressive therapy and discrepancy between two immunoassays alert the hypothesis of serum antibodies causing false positive sTg results. False positive sTg is a very rare condition but must be considered to avoid misdiagnosis, excessive image exams and thereafter overtreatment.