Groundwater for human consumption induces thyroid histological and molecular changes that disrupt Xenopus laevis morphogenesis.

Groundwater is a vital source of fresh water for human consumption, cooking, and irrigation. In populations with endemic goiter, high nitrate concentrations have been reported in groundwater, raising concerns about its impact on thyroid function. This study evaluated the effects of chronic exposure to groundwater on Xenopus laevis larvae throughout the entire metamorphic process, focusing on thyroid function. Larvae were divided into three groups: control (C, drinking water), groundwater-exposed (G), and positive control (PC, drinking water plus potassium perchlorate). Metamorphic progression, thyroid histology, and sodium-iodide symporter (NIS) protein expression were assessed. Metamorphosis was completed in 100% of C, 37.7% of G, and 0% of PC larvae. At NF stage 60, G showed reduced wet weight and snout-to-tail length compared to C and PC, whereas hind limb length did not differ significantly. At NF stage 62, G had lower wet weight than C, and PC exceeded C; hind limb length was greater in G and PC than in C. Thyroid histology revealed hyperplasia, epithelial hypertrophy, and colloid depletion in G and PC at both stages. NIS protein expression increased at NF stage 60 in G and PC relative to C but was suppressed at NF stage 62. Groundwater contained nitrates (24-83 mg/L). These results indicate that nitrate-contaminated groundwater disrupts thyroid function in Xenopus laevis, highlighting potential environmental and regulatory concerns.