The pathogenesis of Euthyroid Sick Syndrome (ESS) in critically ill patients is a complex process characterized by the suppression of hypothalamic thyrotropin-releasing hormone, persistent reduction in thyroid-stimulating hormone secretion despite low plasma thyroid hormone levels, altered expression and activity of thyroid hormone metabolism enzymes (D1, D2, and D3), increase of D3、decreased of D2, reduced thyroid hormone transporter levels, specific changes in thyroid hormone receptors, and the influence of inflammatory cytokines. Here, TRH is linked to euthyroid sick syndrome.