Falsely increased free triiodothyronine values in a woman affected by thyroiditis and multinodular goiter

We describe a case of a 63-years old woman affected by Hashimoto's thyroiditis and multinodular goiter. Her laboratory results showed elevated free triiodothyronine (FT3) concentrations (18.5 ng/L), with free thyroxine (FT4) and thyrotropin (TSH) within the physiologic range. On the basis of these results, she started methimazole therapy, with persistence of inappropriately elevated FT3 concentrations. The therapy was thus stopped and blood examination was repeated after one month in our laboratory: concentrations of thyroid tests were within the physiologic range, including FT3 (2.8 ng/L). The difference between this result and that previously obtained raised the suspicion of the presence of an interference in the first result. In fact, in our laboratory a competitive electrochemiluminescence immunoassay with labeled antibody is used, while the first laboratory employed a competitive chemiluminescence immunoassay with labeled analogue, which has a more important risk of interference. After treating sample by polyethylene glycol, FT3 resulted indeed normal also by the immunoassay used by the first laboratory.