Testosterone testing is used to diagnose several conditions in both men and women, as well as detecting low levels or deficiencies in men. Testosterone is the main sex hormone in men and is responsible for male physical characteristics. Typically testosterone is considered to be a male sex hormone, but it is present in the blood of females as well.
This testosterone blood test measures the total amount of testosterone that is bound to proteins in the blood as well as testosterone that is not bound (free testosterone). Approximately two-thirds of testosterone circulates in the blood bound to SHBG and less than one-third is bound to albumin. A small percent (less than 4 percent) circulates in the blood as free testosterone.
Testosterone, like other steroid hormones, is derived from cholesterol with production regulated by a classic negative feedback loop called the hypothalamic-pituitary-gonadal (HPG) axis. When testosterone levels are low, the hypothalamus secretes gonadotropin-releasing hormone (GnRH) to stimulate the anterior pituitary to produce follicle-stimulating hormone (FSH) and luteinizing hormone (LH). In men, FSH stimulates spermatogenesis, while LH stimulates testicular production of testosterone.
Testosterone secretion occurs in a circadian pattern with the highest levels occurring at 6 AM and the lowest levels at 6 PM. Testosterone is both an androgen and a prohormone which can be converted into an even more potent androgen, dihydrotestosterone (DHT), as well as an estrogenic hormone, estradiol. In males, testosterone and DHT are responsible for the development of external genitalia and secondary sexual characteristics; in females, the main role of testosterone is as an estrogen precursor (Matsumoto 2008).
Testosterone circulates in the blood as a free, active form (less than 5%); weakly bound to albumin (approximately 30%); or tightly bound to sex hormone binding globulin (SHBG, approximately 65%) (Manni 1985). Free and albumin-bound testosterone readily enter cells, and their levels better reflect the testosterone available for use by the body. Together free testosterone and albumin-bound testosterone are called bioavailable testosterone.
Bioavailable testosterone measurements are appropriate in the evaluation of females and children, as well as in males where the total testosterone levels are in the low-normal range or where abnormal SHBG levels are suspected (Ohl 2006).
Testosterone testing may be used to help evaluate the following conditions:
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