As men age, testosterone levels drop very gradually, about 1% to 2% each year. More than a third of men over age 45 may have reduced levels of testosterone than might be considered normal. Symptoms of testosterone deficiency in adult men include reduced body and facial hair, loss of muscle mass, low libido, impotence, small testicles, reduced sperm count and infertility, irritability, poor concentration and depression, and brittle bones.
There are also a variety of conditions that may result in low testosterone other than normal aging, such as pituitary conditions, direct injury or damage to the testes, certain medications, and autoimmune conditions, among others.
