Testosterone
Testosterone is a hormone primarily responsible for male characteristics and is produced in the testicles of men, and to a much lesser extent, in the ovaries of women. It regulates sex drive and plays a crucial role in controlling bone mass, fat distribution, muscle mass, strength, as well as the production of red blood cells and sperm. Testosterone levels in men naturally decline after the age of 30, but lower than normal levels can occur at any age, causing symptoms such as low libido, erectile dysfunction, difficulty in gaining and maintaining muscle mass, and lack of energy. Women have much lower levels of testosterone than men, but it is equally important for regulating libido, muscle and fat distribution, and red blood cell formation. The normal reference range for testosterone levels varies slightly depending on the laboratory and population tested, but the British Society for Sexual Medicine recommends that low testosterone can be diagnosed when testosterone is consistently below the reference range, or when levels are below 12 nmol/L, especially in men who report symptoms of low testosterone or have low levels of free testosterone.