Male sex hormone. Polarised light micrograph of crystals of testosterone. Testosterone is the main human androgen, the class of steroid hormones responsible for the development of male reproductive organs and secondary sex characteristics. It is mainly produced in the testes by the action of a series of enzymes on pregnenolone, a steroid derived from cholesterol. Testosterone may be used in hormone replacement therapy to treat delayed puberty in boys, hypogonadism and some types of impotence. In women, testosterone may be effective in the treatment of breast cancer. Magnification: x12 at 35mm size.