The Oxidoreductase Retinol Saturase in Thyroid Gland Is Regulated by Hypothyroidism and Iodide Overload and Its Deletion Impairs Metabolic Homeostasis in Mice

<b><i>Aims:</i></b> Thyroid hormones (TH) are major regulators of cell differentiation, growth, and metabolic rate. TH synthesis in the thyroid gland requires high amounts of H₂O₂ to oxidize iodide for the iodination of thyroglobulin (TG). Retinol Saturase (RetSat) is an oxidoreductase implicated in dihydroretinol formation and cellular sensitivity toward peroxides and ferroptosis. RetSat is highly expressed in metabolically active organs where it regulates lipid metabolism and the production of reactive oxygen species. Due to the high expression of RetSat in the thyroid gland and its role in peroxide sensitivity, we investigated the regulation and function of RetSat in the thyroid gland in appropriate mouse models. <b><i>Results:</i></b> RetSat is strongly expressed in thyrocytes, induced by hypothyroidism, and decreased by iodide overload in mice. Thyrocyte-specific deletion of <i>RetSat</i> increased circulating thyroid-stimulating hormone levels, altered thyroid morphology, and disturbed metabolic homeostasis in a diet- and sex-dependent manner without major effects on the concentrations of circulating TH. Moreover, deletion of <i>RetSat</i> increased TG protein levels but lowered TG iodination upon iodide overload. In cultured thyrocytes, acute RetSat depletion altered the expression of genes involved in TH biosynthesis and the response to endoplasmic reticulum stress. <b><i>Innovation:</i></b> This is the first report that specifically dissects the regulation and function of the oxidoreductase RetSat in the thyroid gland. <b><i>Conclusion:</i></b> Deletion of <i>RetSat</i> in thyrocytes induces compensatory feedback mechanisms to maintain TH homeostasis in mice. We conclude that RetSat in the thyroid gland is required for TH biosynthesis and secretion and metabolic homeostasis in mice. <i>Antioxid. Redox Signal.</i> 42, 463-479.