2024 Volume 73 Issue 1 Pages 174-179
We report a case of pseudohigh thyroid stimulating hormone (TSH) due to a nonspecific IgG response. The patient was an 88-year-old woman. At the time of initial examination, her blood sample showed TSH 19.66 μIU/mL, free triiodothyronine (FT3) 2.61 ng/dL, and free thyroxine (FT4) 1.76 pg/mL, indicating elevated levels of TSH only. Since no abnormality was found after a loading test and other tests, and only TSH remained elevated, a nonspecific reaction was considered. There was no significant difference in the measurement results of the three different reagents. No significant differences were observed between the three reagents and the human anti-mouse antibody absorption test. On the other hand, in the additive recovery test and polyethylene glycol treatment test, a decrease in recovery was observed for all three reagents, suggesting the influence of some interfering substance. Therefore, HPLC gel filtration analysis was performed, and it was confirmed that the interfering substance was IgG. In addition, the recovery rate of IgG and TSH standard purified from the fractionated aliquot of the case was decreased in the additive recovery test, suggesting that the false high value in this case was caused by measuring the TSH-IgG complex.
