Waterborne exposure of zebrafish embryos to micromole concentrations of ioxynil and diethylstilbestrol disrupts thyrocyte development

•Micromolar concentrations of waterborne IOX and DES disrupt thyrocyte embryonic development in zebrafish.•IOX and DES do not seem to directly interact with components of thyroid hormone genomic and non-genomic cellular pathways.•IOX but most notably DES disrupt thyrocyte development indirectly via altering heart development. The herbicide ioxynil (IOX) and synthetic estrogen diethylstilbestrol (DES) are common aquatic contaminants with an endocrine disrupting action. In juvenile teleost fish IOX and DES disrupt the hypothalamic–pituitary–thyroid (HPT) axis. To assess how IOX and DES influence the developing HPT axis prior to establishment of central regulation of thyroid hormones, zebrafish embryos were exposed to low concentrations of the chemicals in water. IOX and DES (1 and 0.1μM) exposure failed to modify hypothalamic development but had a negative effect on thyrocyte development. Specifically, IOX and DES caused a significant (p