Maternal thyroid hormones affect zebrafish embryo development.

Thyroid hormones (THs), including thyroxine (T4) and triiodothyronine (T3), regulate vertebrate development, influencing critical processes during embryogenesis and larval stages. However, their maternal transfer and developmental roles in fish remain poorly understood. This study investigates maternal THs' impact on zebrafish development, focusing on the somatotropic axis, a key regulator of growth and metabolism. Female zebrafish were fed diets with 12.5 or 25 μg T4/g body weight/day for five days, while controls received vehicle-only food. Plasma TH levels were measured, and females were bred with untreated males. Embryos and larvae were assessed from 0 to 144 h post-fertilization for survival, heartbeat, morphology, T3 content, and gene expression. Elevated maternal T4 increased embryonic T3, causing growth retardation, edema, spinal deformities, tachycardia, and mortality. Gene expression analysis revealed dysregulation of TH axis components and somatotropic genes, highlighting the maternal THs' critical role in developmental trajectories and normal growth.