Obesity: Recent studies have shown that obese men are more likely to have low testosterone than non-obese men.
Testicle injury: Trauma or injury to testicle may result in low testosterone level. Testicles produce testosterone. Injury to tissues of testicles can impair production of testosterone.
Diseases: Infectious diseases like mumps infection during puberty can affect testicles causing inflammation. Other infectious diseases like tuberculosis (TB) , HIV may also affect production of testosterone. Certain inflammatory diseases like like arthritis and autoimmune diseases can cause low testosterone.
Increased iron in blood (hemochromatosis): Increased iron in blood may damage testes and pituitary gland.
Aging: For most men, the level of testosterone start declining after the age of 30. It continue to decrease about 1% each year. By age 70, testosterone levels can decline by about 28%.
Chemo and Radiation therapy: Chemicals and Rays used in these therapies can damage testicular cells and diminish testosterone production. Sometimes these cells recover to function normally. Sometimes the damage may be permanent.
Performance boosting drugs: Performance boosting drugs like anabolic steroids, beta-2 agonists, selective androgen receptor modulators (SARMs), and various human hormones, etc can lower testosterone level.
Smoking: Cigarette smoking considerably alters concentrations of the sex hormone testosterone, in men. Male cigarette smokers have low testosterone levels than male nonsmokers. This leads to lower sex drive and lower sperm counts than those with normal or high concentrations of testosterone hormone
Alcohol: Studies have shown that alcohol lowers testosterone levels about 40%. Chronic alcohol abuse leads to lowered sperm counts, sperm motility and free testosterone.
Marijuana: Marijuana smoking leads to low testosterone level, erectile dysfunction and male infertility.