Introduction: Plastic production has increased exponentially in recent decades. Macroplastics in the environment are degraded through mechanical, physical, and biological processes, ultimately producing nanoplastics (NPs). NPs have been found in air, water, and soil, and there is a growing concern regarding their effects on the environment and on human health. NPs can accumulate in tissues, affecting them and causing disruptions in metabolism, development, and fertility, although their mechanisms of action are still largely unknown. The zebrafish embryo (ZFE) poses an ideal model to investigate this due to it is high homology with humans. The present work’s main objective was to determine the impact of acute exposure to polystyrene (PS) NPs on the thyroid endocrine system in ZF larvae that had largely completed development.

Methods: Larvae were exposed to a concentration gradient of PSNP (0 -10 mg/L) at 120 hours post fertilization for 1h. After exposure, alterations in gene transcriptions of thyroid-related genes were examined via qPCR. In addition, behavioral assays were conducted to evaluate activity patterns in reaction to visual and auditory stimuli, and confocal microscopy was performed to assess NP incorporation using fluorescently labeled NPs. Results: NPs were able to enter ZF larvae within 1h of exposure. They altered the expression of different thyroid-related genes in a concentration-dependent or biphasic manner. In addition, acute NP exposure affected larvae locomotion, with both hypo- and hyperactivity observed depending on the type of stimuli and NP concentration. Conclusion: These findings underscore the urgent need for additional studies on the possible impacts of NPs on human health and offer more proof of NPs' possible mode of action as thyroid disruptors.