Testosterone physiology in men
How testosterone is made and what it does
Key takeaway for patientsTestosterone affects muscle, bone, mood and sexual function, and it is regulated by the brain. Levels vary through the day and with health, which is why testing is timed and repeated.
Testosterone is made by the Leydig cells of the testes, under the control of luteinizing hormone (LH) from the pituitary. It is released in a daily rhythm, highest in the morning. In the blood most of it is bound to proteins, chiefly sex hormone-binding globulin (SHBG); only the free fraction is active.
It acts on muscle, bone, the production of red blood cells, mood, libido and sperm production, partly after conversion to dihydrotestosterone or estradiol. Levels decline gradually with age, and more with obesity and chronic illness.
Key steps in testosterone physiology
- The signal from the brain
- GnRH from the hypothalamus prompts the pituitary to release LH.
- Production
- Leydig cells in the testes make testosterone.
- Transport
- It is carried in the blood bound to SHBG and albumin.
- Action and feedback
- It acts on tissues and signals the brain to adjust production.
Factors that lower testosterone
Many of these are reversible:
- Age
- A gradual decline through adult life.
- Obesity and insulin resistance
- Lower SHBG and total testosterone.
- Poor sleep
- Including sleep apnoea.
- Chronic illness, some medicines and alcohol
- Suppress the signal from the brain or production in the testes.
Clinical management and physician guidance
One result is never enough; testing is timed and repeated.
- Hormonal assessment
- Morning total testosterone, repeated, with SHBG, LH and FSH.
- Physician-guided care
- Weight, sleep and underlying conditions come first; any treatment is a clinical decision.
- Monitoring and follow-up
- Symptoms and markers are reviewed over time, and the plan is adjusted to them.
Research focus in androgen physiology
| Core focus area | Biological target | Clinical relevance |
|---|---|---|
| Production | Leydig cells and LH | Synthesis of testosterone |
| Transport | Sex hormone-binding globulin | How much hormone is free |
| Tissue action | The androgen receptor | Effects on muscle, bone and brain |
| Aging | The hypothalamic-pituitary-testicular axis | Decline with age and illness |
Frequently asked questions
- Why is testosterone tested in the morning?
- Levels are highest then and fall through the day.
- Does testosterone fall with age?
- Gradually, and more with weight gain and illness.
- Does a low result mean I need treatment?
- Not necessarily. A diagnosis needs symptoms and repeated low results, and what follows is your physician’s decision.
Academic references and literature
- Kaufman JM, Vermeulen A (2005). The decline of androgen levels in elderly men and its clinical and therapeutic implications. Endocr Rev, 26(6), 833-76. PMID 15901667.
- Feldman HA et al. (2002). Age trends in the level of serum testosterone and other hormones in middle-aged men: longitudinal results from the Massachusetts male aging study. J Clin Endocrinol Metab, 87(2), 589-98. PMID 11836290.
- Wu FC et al. (2008). Hypothalamic-pituitary-testicular axis disruptions in older men are differentially linked to age and modifiable risk factors: the European Male Aging Study. J Clin Endocrinol Metab, 93(7), 2737-45. PMID 18270261.
This guide belongs to the Sexual Health program