Testosterone hormone. Polarised light micrograph (PLM) of crystals of the male sex hormone testosterone. Testosterone is the main human androgen hormone, the class of steroid hormones responsible for stimulating the development of the secondary sex characteristics. Testosterone causes the growth of the beard and pubic hair, the change in voice and the development of the penis. It is mainly produced in the Leydig cells of the testes by the action of a series of enzymes on pregnenolone, a steroid derived from cholesterol. Magnification: x29 at 6x7cm size. magnification: x50 at 4x5 inch size.