Lower than 4% of the total testosterone in the body circulates the blood as free testosterone. Free testosterone and testosterone bound to albumin can act on target tissues. This is called bioavailable testosterone.
The hormone is found in both males and females but is the primary sex hormone (androgen) in males. Women have a much lower level of testosterone level in comparison to men.
Testosterone is produced primarily by testicles in males, but it is also made by the adrenal glands of both females and males. Ovaries in women also produce minute quantities of testosterone. The hormone prompts the development of secondary sex characteristics like body hair growth, muscle development, deepening of the voice, and enlargement of the penis in males. Testosterone levels are high during puberty and adulthood in males as it controls sex drive and helps sustain muscle mass. Testosterone is changed to estradiol, a sex hormone in women.
The production of testosterone is regulated by the Luteinizing Hormone (LH), which is made by the pituitary gland. Testosterone works within a negative feedback loop. This means that lower levels of testosterone lead to high LH, which triggers the production of testosterone while an increase in testosterone levels leads to a decrease in the production of LH, which leads to a reduction in testosterone production.
Testosterone levels are diurnal. It is present in its highest concentration in the morning, between 4:00 to 8:00 am, and lowest concentration in the evening, between 4:00 to 8:00 pm. Testosterone levels lower with age and increase with exercise. Two-thirds of testosterone in the body is bound to the protein sex hormone-binding globulin (SHBG), while one-third is bound to the protein albumin, which is in high levels in the