FACTBOX: The male sex hormone testosterone

(Reuters) - Factbox on the banned male sex hormone testosterone after 2006 Tour de France winner Floyd Landis of the United States was found guilty of doping by a U.S. arbitration panel:

* Testosterone is a hormone which occurs naturally in both men and women. From puberty in men the testes release 20 to 40 times the amount found in childhood which leads to changes including increased muscle and bone density.

* Anabolic steroids were first developed in the 1930s to replicate the effects of testosterone and increase muscle and body mass. They work for both men and women. Testosterone can also be manufactured artificially and is listed with anabolic steroids on the World Anti-Doping Agency's (WADA) list of prohibited substances in sport.

* A urine test, developed more than two decades ago, measures the ratio with epitestosterone, a hormone produced along with testosterone but without its effects. The ratio in most people is 1:1.

* Until 2005, a ratio of 6:1 was considered to be evidence that testosterone had been administered artificially. The ratio now used by the International Olympic Committee (IOC) is 4:1