Teleost fish metamorphosis under the influence of estrogenic hormones: targeting the thyroid axis - a literature review.

Estrogens, whether naturally excreted by humans and livestock or introduced through pharmaceuticals, are endocrine-disrupting chemicals (EDCs) with strong estrogenic activity. While their effects on the reproductive functions of teleosts are well-documented, their influence on other vital biological functions such as metamorphosis remains understudied. Teleost metamorphosis represents a critical life stage involving profound morphological, physiological, and behavioral transformations essential for survival. This process is tightly regulated by multiple endocrine factors, with the thyroid axis playing a central role. This review synthesizes current knowledge on the impact of estrogens on teleost metamorphosis, focusing on the thyroid hormonal pathway. Several studies have examined estrogen effects on specific aspects of metamorphosis, such as skeletal development, salinity adaptation, and migratory behaviours. However, these investigations often remain fragmented and lack an integrative analysis of metamorphosis as a whole. Furthermore, few studies directly link estrogen-induced disruptions to thyroid axis alterations, despite findings of such links in other aquatic species, such as amphibians, and with other EDCs. While the impact of estrogens on the thyroid cascade of teleosts has been documented, observations are sometimes contradictory due to experimental and methodological differences, complicating data interpretation and hindering mechanistic understanding. Nonetheless, available studies suggest that estrogens are potent modulators of thyroid function at multiple levels of the hormonal cascade. Such disruptions may significantly affect thyroid-regulated functions, including metamorphosis, whose long-term ecological implications warrant greater attention. Given the environmental threat posed by estrogenic pollution, driven by human activities such as hormonal contraceptive use and agricultural runoff, further research in this field is necessary.