Nanoplastics disrupt thyroid hormone homeostasis and neurodevelopment in zebrafish: a systems-level assessment.

Nanoplastics are increasingly detected in the environment, raising concerns about their endocrine disruption during vertebrate development. Here, zebrafish embryos were exposed from 0 to 120 hpf to environmentally informed concentrations of 30 nm polystyrene nanoplastics (PSNP; 0-3 mg/L) to assess internal burden, hypothalamus-pituitary-thyroid (HPT) axis disruption and neurodevelopmental outcomes in an intact early-life model. Time-lapse confocal imaging revealed rapid chorion-associated accumulation, progressive internalization and widespread distribution, with only partial clearance after depuration. Fluorescence/NTA-derived estimates confirmed concentration-dependent internal particle-equivalent burdens, with approximately 49% remaining after washout at 3 mg/L. PSNP exposure reduced whole-body TSH, T4, and T3 levels at 120 hpf, and altered thyroid-related transcription, including marked tshb downregulation and compensatory changes in genes involved in thyroid hormone synthesis, transport and metabolism. Neurodevelopmental genes related to oligodendrocyte lineage specification, myelination, synaptogenesis, and neuronal differentiation were also dysregulated, and embryos showed altered early motor output. At the organism level, PSNP induced multisystem developmental phenotypes, including reduced pigmentation, impaired eye and otolith development, swim bladder deflation, tachycardia, pericardial edema and stimulus-dependent locomotor and anxiety-like changes, some persisting after depuration. Effects were detected at the lowest tested concentration (LOAEL = 0.01 mg/L; 31,507 particle-equivalents/larva), with no NOAEL established. Eye-size reduction provided the most robust apical BMD fit (BMD 0.008 mg/L; BMDL 0.004 mg/L), corresponding to an exploratory internal bioactivity anchor of 25,206 and a conservative internal PoD of 12,603 particle-equivalents/larva. These findings link sustained internal PSNP exposure with multi-level HPT-axis disruption and endocrine-related developmental neurotoxicity, supporting their relevance for hazard and risk assessment of nanoplastics.