In the testes testosterone is produced by the Leydig cells. Due to dual function of the male gonad testosterone directly influences spermatogenesis. Transport of testosterone is performed by specific sex hormone binding globulin (SHBG) that is found in the plasma. Then in the target tissues transformation of testosterone into 2.5 times more potent form 5α-dihydrotestosterone is carried out. The sequence of actions is needed for the hormone to exert its physiological role. First it binds to steroid hormone receptors located in the cytosol and the nucleus. These receptors are ligand-activated and in effect they undergo structural change that renders them capable of binding to specific nucleotide sequences in the DNA called hormone response elements (HREs). While bound they influence transcriptional activity of certain genes.
Testosterone was first isolated from a bull in 1935. Pharmaceutical products derived from testosterone have been used for a rather diverse range of indications: male infertility, athletic enhancement, libido problems, erectile dysfunction, prevention of osteoporosis caused by a lack of androgens. Real testosterone deficiency is rare and the indications for use must be chosen prudently.
Male fetuses need testosterone for the development of male external genitalia, and at puberty in males an increase in testosterone level causes further growth of male genitalia and the male secondary sex characteristics such as facial hair and voice changes. An excess of testosterone in women causes male-type physical development (masculinisation\). Testosterone also stimulates protein synthesis and causes the greater muscular development of the male.
Its abuse by athletes to enhance muscular development is illegal.