Identification by Liquid Chromatography–Tandem Mass Spectrometry and Liquid Chromatography–Quadrupole Time-of-Flight Mass Spectrometry of the Contributor to the Thyroid Hormone Receptor Agonist Activity in Effluents from Sewage Treatment Plants

3,3′,5-Triiodothyroacetic acid (TRIAC) was identified as a major contributor to the activity of thyroid hormone receptor (TR) agonists in environmental water. TRIAC contributed 60–148% of the TR-agonist activity in effluents from sewage treatment plants (STPs). Meanwhile, the contributions of 3,5,3′-triiodothyronine (T3), 3,3′,5,5′-tetraiodothyronine (T4), and analogues were <1%. TRIAC concentrations in the range of 0.30–4.2 ng/L are likely enough to cause disruption of the thyroid system in living aquatic organisms. The origin of TRIAC in the STP effluents was investigated by analyzing both STP influents and effluents. Relatively high concentrations of T3 and T4 (2.5 and 6.3 ng/L, respectively) were found only in the influents. TRIAC was identified only in the effluents. These findings suggested that T3 and T4 in STP influents were potentially converted into TRIAC during activated sludge treatment or by other means. The evaluation of TRIAC at relevant environmental concentrations by in vivoassays and an appropriate treatment to reduce the TR activity in sewage are needed.