Testosterone was first extracted from a male cow in 1935, and over the years, many different kinds of medicine have been developed. Currently in North America, testosterone is available in injectable forms (e.g., testosterone cyclopentopropionate and testosterone enanthate), oral, buccal, and skin penetrant packages, creams, and gels. The original use of testosterone was to treat hypogonadism, a condition in which there is insufficient autosecretion or no natural production of testosterone. The proper use of testosterone under hormone replacement therapy (HRT) can maintain the amount of testosterone in the blood at normal levels. Like other hormones, testosterone and other anabolic hormones can be used in addition to supplementation due to deficiency, but with greater side effects. Such as infertility, loss of sexual desire (sexual cold) or impotence, osteoporosis and so on. In the late 1940s, testosterone was marketed as an anti-aging drug, much like growth hormone in modern times. Taking advantage of the masculinization of testosterone, transgender men (that is, women who become men through sex change) are replaced with testosterone in order to maintain the testosterone levels of normal men. Similarly, transgender women use drugs to suppress testosterone production in order for estrogen to work.