Mixture Effects Investigation of a Thyroid Hormone System-Specific Interaction with Azole Fungicides

This study investigated the thyroid effects of azole fungicide mixtures in male rats, addressing the gap in regulatory assessments that typically focus on single chemicals rather than combined exposures. Using a 28-day rat model, binary and ternary mixtures of azole fungicides were tested at high doses, with thyroid hormone levels, liver enzyme activity, and thyroid tissue changes as the key endpoints. The mixtures caused elevated TSH, reduced T4, increased UGT enzyme activity, and thyroid tissue alterations, including follicular cell hypertrophy and hyperplasia. Interestingly, the mechanism observed with single-compound exposures (hepatic metabolism driving secondary TSH elevation) appeared less prominent in the mixture groups. Whether these responses reflect additive, less-than-additive, or more-than-additive interactions could not be determined without formal dose-response modeling. As the tested dose (NOAELx10) is substantially higher than the human realistic exposure levels permitted under the current regulation, direct extrapolation of the present findings to realistic human exposure conditions is limited. However, the current regulatory evaluation with respect to the effect dose was confirmed. While existing approaches appear adequate, a deeper understanding of thyroid-specific endpoints is needed to ensure that mixture risk assessments remain robust and appropriately protective.