Testosterone Testosterone

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Year of discovery: 1935; Drug category: Anabolic steroid hormone; Main uses: Treatment of male hypogonadism; Related drugs: Testosterone enanthate (Delatestryl), Testosterone cypionate (Depo-testosterone); Other brand names: Androderm, Androgel, Testim, Striant.

The steroid testosterone is the principal male sex hormone. The main approved therapeutic use of testosterone is the treatment of testosterone deficiency (hypogonadism). Although testosterone and other anabolic hormones enhance muscle development and improve athletic performance, risks are involved with their use.

Testosterone in males is produced in the testes. Testosterone secretion is stimulated by the release of the luteinizing hormone (LH) from the pituitary gland, which is regulated by the hypothalamic gonadotropin-releasing hormone (GnRH). Testosterone is transported in the body by specific plasma proteins. A fraction of testosterone is metabolized to active steroids, such as estradiol (by the enzyme complex aromatase), and dihydro-testosterone, which exhibits even greater androgenic activity (by the enzyme 5a-reductase).

Estradiol

Testosterone and its active metabolites have many physiological effects. Testosterone and dihydrotestosterone are responsible for the differentiation of internal and external genitalia in the fetus and maturation during puberty. Testosterone exhibits anabolic effects; it stimulates muscle protein synthesis that leads to larger muscle mass, increased muscle strength and resistance. Estradiol in the fetus is associated with the sexual differentiation of the brain. During puberty, estradiol accelerates the maturation of cartilage into bone leading to epiphysis and termination of bone growth.

Testosterone and dihydrotestosterone act via the androgen receptor, which belongs to the nuclear receptor superfamily.1 Binding of testosterone or dihydrotestosterone to the receptor initiates conformational changes that allow translocation of the complex into the nucleus, where it acts as a transcription factor and stimulates the expression of certain genes.

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Testosterone

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Binding of testosterone to the androgen receptor.

Testosterone deficiency in men can result from inadequate function either of the testes (primary hypogonadism), or of the hypothalamus and pituitary (secondary hypogonadism). Testosterone levels in men generally decrease with age. Although administration of testosterone to elderly men produces beneficial results, such as increased muscle mass, bone mineral density, and mental function, its use is limited due to the possible promotion of prostatic malignancy.2

Since the oral bioavailability of testosterone is low due to rapid metabolism in the liver, various alternative formulations have been developed including subcutaneous injection, transdermal skin patch (Androderm), topical gel (Androgel, Testim), and buccal tablet (Striant).3

1. Protein Sci. 2006, 15, 987-999 (2AM9); 2. Nat. Clin. Pract. Endoc. 2006, 2, 146-159; 3. Nat. Clin. Pract. Urol. 2006, 3. 653-665; Refs. p. 105

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