Low testosterone: Abnormally low levels of the male sex hormone testosterone in the blood. Testosterone is produced by the testes and to a lesser extent by the adrenal glands. Testosterone encourages the development of male sexual characteristics at puberty, stimulates the activity of the male secondary sex characteristics, drives the production of sperm, and is involved in regulation of muscle mass and sex drive. Testosterone levels are highly regulated by a complex glandular system. Abnormally low testosterone levels can occur due to a number of conditions affecting the glands that regulate and produce this hormone. Symptoms of low testosterone include decreased libido, impotence or erectile dysfunction, breast enlargement, and a lowered sperm count. Low testosterone levels may be treated with hormone therapy.