Low testosterone is also known as hypogonadism.
When it occurs in adult men (after puberty), it can result in a wide range of symptoms including infertility, decreased libido (sex drive), weakness, reduced muscle mass, enlarged male breasts, decreased facial or body hair growth, depression, mood disorders.
Hypogonadism can be related to a primary failure of the testes to produce testosterone, but may also be a result of poor stimulation of testosterone production from the brain, genetic diseases, infection, injury, systemic diseases (diseases affecting the whole body), or the effects of previous radiation or chemotherapy treatments for cancer.
A diagnosis may be suspected based on history and physical examination, and will likely require blood tests for confirmation. Treatment usually involves replacement of testosterone and in more complex cases may require a multi-team approach with urologists and endocrinologists (Hormone specialists).