Chronic exposure to sodium fluoride or to tebuconazole disrupts the thyroid hormone and immune systems during early development of rainbow trout (Oncorhynchus mykiss).

Over the past 20 years, there has been a growing interest in endocrine-disrupting chemicals (EDCs) found in both terrestrial and aquatic environments. These substances have garnered attention from scientists, the public and regulatory bodies, with many being classified as substances of very high health concern. Only recently, disruption of the thyroid hormone system (THS) by EDCs has also attracted significant attention: In vertebrates, the THS interacts with various physiological systems, including the immune system (IS), and plays a crucial role in development. It can also influence the microbial diversity in certain tissues. The main objective of this study was to investigate the potential impact of sodium fluoride (NaF) on the THS and IS in rainbow trout (Oncorhynchus mykiss), a species of both economic and environmental importance. For this end, trout were exposed from the embryonic stage (48 h post-fertilisation) to the juvenile stage for 7 months. Fish were subjected to 1.5 mg F-/L or 5 mg F-/L and 20 µg/L tebuconazole (TBZ; positive control) monitored daily and sampled after 3 and 7 months of exposure to assess potential disruption to the thyroid hormone and immune systems. Results document that chronic exposure to NaF or TBZ during early life stages of rainbow trout induces a broad range of sublethal effects, including physiological condition, immune competence, endocrine regulation and organ morphology. Although mortality was reduced, most probably due to the antifungal activity of TBZ, the consistent alterations observed in thyroid hormone dynamics and immune parameters clearly indicate a deterioration of overall health.