What is testosterone and what does it do?

Testosterone is the primary androgen in the male body. It is produced mostly in the Leydig cells of the testes (a small amount comes from the adrenal glands) under signaling from luteinizing hormone (LH), which is released by the pituitary in response to GnRH from the hypothalamus. The whole loop is called the HPTA — hypothalamic-pituitary-testicular axis.

What testosterone does

• Sexual: libido, erectile function, sperm production
• Musculoskeletal: muscle protein synthesis, bone density, joint integrity
• Hematological: red blood cell production via erythropoietin stimulation
• Metabolic: insulin sensitivity, fat distribution, basal metabolic rate
• Neurological: mood, motivation, spatial cognition, sleep quality
• Cardiovascular: vasodilation, lipid balance, vascular endothelial health

Normal levels and decline

Reference ranges vary by lab but typically run 264–916 ng/dL for total testosterone in adult men. Levels peak in the late teens to mid-20s and decline gradually — roughly 1% per year — after age 30. Hypogonadism is generally defined as confirmed levels under 264–300 ng/dL with symptoms.

Why some men become deficient

• Primary hypogonadism: testicular failure (injury, infection, mumps orchitis, chemotherapy, genetic causes)
• Secondary hypogonadism: pituitary or hypothalamic dysfunction (obesity, opioid use, prior steroid use, head trauma, pituitary tumors)
• Age-related: gradual HPTA aging often combined with metabolic decline