Testosterone is a sex hormone primarily associated with male characteristics, but also present in females. In men, it plays a crucial role in developing and maintaining male traits, including sperm production, muscle and bone strength, and secondary sexual characteristics like body hair and a deeper voice.
The role of testosterone.
Sperm production
Testosterone plays a vital role in spermatogenesis — the process of producing sperm within the seminiferous tubules of the testes. It supports the function of Sertoli cells, which help nurture developing sperm, and works with other hormones like luteinising hormone (LH) to regulate this reproductive process. Low testosterone levels can lead to reduced sperm count and fertility issues.
Muscle and bone mass
Testosterone promotes muscle growth by enhancing protein synthesis in muscle cells, which increases muscle size and strength. It also plays a key role in maintaining bone density by stimulating bone-forming cells (osteoblasts) and regulating the balance between bone formation and breakdown. As testosterone levels decline with age, men may experience muscle loss (sarcopenia) and weaker bones (osteoporosis).
Secondary sexual characteristics
During puberty, testosterone triggers the development of male physical traits, including the growth of facial, chest, and pubic hair, enlargement of the larynx (leading to a deeper voice), and increased oil production in the skin. It also contributes to the broader shoulders, narrower hips, and overall body shape typically associated with male puberty and maturity.
Red blood cell production
Testosterone stimulates the bone marrow to produce erythropoietin, which in turn boosts red blood cell production. Adequate red blood cells are essential for carrying oxygen throughout the body. This function helps support stamina and physical performance. Abnormally low testosterone can contribute to anaemia, while excessive levels may increase the risk of thickened blood or clotting.
Sex drive and mood
Testosterone is a key driver of libido (sexual desire) in men. It helps regulate sexual arousal and erectile function. It also affects mood, mental clarity, and emotional well-being. Low testosterone is often linked to symptoms such as fatigue, irritability, depression, and reduced motivation, while healthy levels are associated with vitality and confidence.
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Fat distribution
Testosterone influences where and how fat is stored in the body. Higher levels are typically associated with leaner body mass and fat distribution around the abdomen. When levels drop, fat may accumulate more easily, particularly around the midsection, and body composition may shift, with more fat and less muscle.
Further reading
- Low testosterone symptoms »
- How to increase low testosterone »
- The benefits of testosterone replacement therapy (TRT) »