Тестостерон свободный (Free Testosterone)

The Free Androgen Index (FAI), also referred to as the testosterone:SHBG ratio, estimates the physiologically active (bioavailable) component of circulating testosterone. This measurement assists in evaluating androgen excess in women (including polycystic ovary syndrome and other causes of virilization), assessing disorders of puberty in boys, and investigating male sexual dysfunction, infertility, or suspected hypogonadism. It is also used alongside therapy monitoring in prostate cancer treated with gonadotropin-releasing hormone analogs or antiandrogens and may be incorporated into evaluations of acne, hypothalamic–pituitary dysfunction, and ambiguous genitalia in newborns.

Testosterone is the principal androgen. In men it is produced mainly by Leydig cells in the testes with a minor adrenal contribution, and in women it derives from both the ovaries and adrenal glands. Androgen action supports development of the male reproductive tract, increases skeletal muscle mass, stimulates spermatogenesis, and promotes hair growth on the face, axillae, and pubic regions. Serum concentrations show a diurnal pattern with a peak between 4–8 AM and a nadir between 16–20 PM, rise through early adulthood to about 30 years of age, and gradually decline after 50 years. Excess androgen production in women can lead to virilization, hirsutism, voice deepening, and clitoral hypertrophy, and is commonly associated with acne due to sebaceous gland stimulation. Insufficient testosterone in men presents with hypogonadism, reduced development of skeletal musculature, underdevelopment of external genitalia, infertility, gynecomastia, and increased adiposity; in older men, low androgen levels raise the risk of osteoporosis. In circulation, total testosterone comprises three fractions: free (approximately 1–4%), sex hormone–binding globulin (SHBG)–bound (about 60–70%), and albumin-bound (roughly 25–40%). The free and albumin-bound fractions constitute bioavailable testosterone, while SHBG-bound hormone is not biologically active. The Free Androgen Index provides an estimate of physiologically active testosterone by considering total testosterone together with SHBG. Because SHBG varies with clinical states—elevated in hyperthyroidism, cirrhosis, use of oral contraceptives or antiepileptics, and hyperestrogenic conditions including pregnancy; and reduced in hypothyroidism, obesity, androgen excess, and nephrotic syndrome—assessments that account for SHBG can be more informative than total testosterone alone when SHBG is altered.