What is testosterone? Basic definition of what is Testosterone.


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Testosterone is the most abundant androgen. It is secreted by the testicular Leydig cells. In addition to its hormonal activity, testosterone is a prohormone that can be converted to dihydrotestosterone, a powerful androgen, and estradiol, an estrogen.

Testosterone secretion is dependent upon LH stimulation of the Leydig cells. Increasing levels of testosterone suppress secretion of LH and, conversely, decreasing levels of testosterone act to increase LH secretion. LH secretion from the pituitary is controlled by the hypothalamic gonadotropin-releasing hormone (GnRH).

Testosterone circulates in three major forms: unbound (free) testosterone, tightly-bound testosterone, and weakly-bound testosterone. The tightly-bound form is bound to sex hormone-binding globulin (SHBG), while the weakly-bound form is bound to albumin. Approximately two-thirds is tightly-bound, 30% to 32% is weakly-bound, and the remaining 0.5% to 3% is free. “Bioavailable” testosterone includes both unbound (free) and loosely-bound (to albumin) testosterone. Only bioavailable testosterone is able to bind to the androgen receptor.

Testosterone is metabolized by the 5-alpha-reductase enzyme to dihydrotestosterone, a biologically active androgen. In men, approximately 70% of dihydrotestosterone is derived from testosterone; in women, the primary prohormone for dihydrotestosterone is androstenedione. Androstenedione metabolism accounts for the majority of testosterone in women, but the ovaries and adrenal secrete small amounts.