Testosterone is a steroid hormone that regulates reproductive development, muscle protein synthesis, bone maintenance, red blood cell production, metabolic signaling, libido, and secondary sexual characteristics. It is the primary androgen in males, although it is also present and biologically important in females. Testosterone supports development of reproductive tissues, maintenance of muscle mass, regulation of fat distribution, stimulation of erythropoiesis, and modulation of mood, energy, and cognitive function.
The hormone influences gene transcription through intracellular androgen receptors located in skeletal muscle, reproductive tissues, bone, liver, nervous tissue, skin, and cardiovascular tissues. Testosterone contributes to maintenance of lean tissue mass and influences amino acid utilization, mitochondrial metabolism, and cellular anabolic signaling. During puberty it supports voice deepening, increased muscle growth, facial and body hair development, and maturation of reproductive organs.
Testosterone is produced mainly by Leydig cells in the testes in males and in smaller amounts by ovaries and adrenal glands in females. The hormone is synthesized from cholesterol through sequential steroidogenic enzyme pathways involving CYP11A1, CYP17A1, 3-beta-hydroxysteroid dehydrogenase, and 17-beta-hydroxysteroid dehydrogenase.
After synthesis, testosterone enters circulation where much of it binds sex hormone-binding globulin and albumin, while a smaller free fraction remains biologically active. Some testosterone is converted into dihydrotestosterone through 5-alpha-reductase or into estradiol through aromatase depending on tissue type and enzyme expression. These conversions broaden the physiological influence of androgen signaling.
Testosterone production is regulated primarily by the hypothalamic-pituitary-gonadal axis. Gonadotropin-releasing hormone stimulates pituitary release of luteinizing hormone, which activates Leydig cell steroidogenesis. Follicle-stimulating hormone supports spermatogenic function within testes. Testosterone then exerts negative feedback on hypothalamic and pituitary signaling to regulate hormonal balance.
Production is influenced by circadian rhythm, nutritional status, sleep quality, stress hormones, adipose tissue activity, insulin signaling, inflammatory cytokines, and aging-related endocrine changes. Testosterone signaling activates androgen receptor pathways involved in protein synthesis, reproductive development, erythropoiesis, and tissue growth regulation. Through these integrated endocrine systems, testosterone coordinates reproductive physiology, musculoskeletal maintenance, metabolic adaptation, and anabolic tissue regulation.
Testosterone signals via AR to coordinate reproductive development, anabolic programs in muscle and bone, erythropoiesis, and aspects of metabolism.
