Muscle in Endocrinology: From Skeletal Muscle Hormone Regulation to Myokine Secretion and Its Implications in Endocrine–Metabolic Diseases

Skeletal muscle, traditionally recognized for its motor function, has emerged as a key endocrine organ involved in metabolic regulation and interorgan communication. This narrative review addresses the dual role of muscle as a target tissue for classical hormones—such as growth hormone (GH), insulin-like growth factor type 1 (IGF-1), thyroid hormones, and sex steroids—and as a source of myokines, bioactive peptides released in response to muscle contraction that exert autocrine, paracrine, and endocrine effects. Several relevant myokines are discussed, such as irisin and Metrnl-like myokines (Metrnl), which mediate exercise-associated metabolic benefits, including improved insulin sensitivity, induction of thermogenesis in adipose tissue, and immunometabolic modulations. It also examines how muscle endocrine dysfunction, caused by chronic inflammation, hormone resistance, or sedentary lifestyle, contributes to the development and progression of metabolic diseases such as obesity, type 2 diabetes, and sarcopenia, highlighting the importance of muscle mass in the prognosis of these pathologies. Finally, the therapeutic potential of interventions aimed at preserving or enhancing muscle function—through physical exercise, hormone therapy and anabolic agents—is highlighted, together with the growing research on myokines as biomarkers and pharmacological targets. This review expands the understanding of muscle in endocrinology, proposing an integrative approach that recognizes its central role in metabolic health and its potential to innovate the clinical management of endocrine–metabolic diseases.