What Is Testosterone? Where Is Testosterone Produced?
Testosterone is the principal male sex hormone and anabolic steroid. More than 95% of testosterone is manufactured by the Leydig’s cells in the testes at various amounts throughout a person’s life. Its actions are most pronounced during puberty. During puberty, it is responsible for causing the maturation of the male sex organs and other masculizing effects.
Yet, testosterone is far more than just a sex hormone. It affects the entire body. It is vitally important in the making of protein, which, in turn, forms muscle. It also plays a key role in the manufacture of bones.1-2 Testosterone also helps control blood sugar, regulate cholesterol, and maintain a healthy immune system. It even affects key functions of the brain.
Testosterone levels rise during puberty, peak during the late teen years, and then level off. In men, normal testosterone levels range between 300 and 1050 ng/dL. However, there is no absolute consensus on this normal range. Nor is there any normal range based on age.3-5 [Note: As a man ages, total levels decline. Additionally, free testosterone (testosterone not bound in the blood) levels decline even more rapidly than total levels.]
While most men will have normal levels throughout their lives, about one in five men above the age of 60 will have a total level that falls below the normal range.3-5 For men with low testosterone, testosterone replacement therapy (TRT) can be beneficial. Restoring testosterone levels to within the normal range by using TRT has shown to produce the following benefits:
- Increase lean body mass6,7
- Decrease fat mass6,7
- Increase bone mineral density8
- Improve sexual function
- Improve mood and well-being
It is important to understand that like any prescription-based therapy, TRT can cause side effects. Ultimately, treatment comes down to the individual’s personal problems with low testosterone.
What Is Free Testosterone?
Free testosterone is not bound in the blood. A very small amount of testosterone actually exists in a free state. As mentioned above, testosterone circulates through the blood. As it circulates through the bloodstream, it can either be bound or unbound. If it is bound, it is bound to sex hormone binding globulin (SHBG) or albumin. Most circulating testosterone is bound to sex hormone binding globulin or albumin. Typically, only about 2% exists in the free, unbound state. Studies have shown that total levels decrease by approximately 30% in healthy men between the ages of 25 and 75. Free levels decline even more significantly with decreases of approximately 50%.3-5 This larger decline in free testosterone occurs because the amount of SHBG increases with age.
Where Is Testosterone Produced?
In males, testosterone is largely produced (>95%) in the testes by Leydig cells. A small amount is produced from steroids secreted by the adrenal cortex, the outer layer of the adrenal gland.
Where is Testosterone Produced: Simplified Explanation:
The body controls the amount of testosterone produced through a feedback mechanism. When levels are low, chemicals signals released by the hypothalamus and pituitary gland stimulate the testes to produce testosterone. When levels are high, testosterone, itself, acts on the hypothalamus and pituitary gland by preventing these same chemicals signals.
Where is Testosterone Produced: Detailed Explanation:
The body controls the amount of testosterone produced through the hypothalamic-pituitary-testicular axis. Both the hypothalamus and pituitary gland are located in the brain. When levels are low, the hypothalamus releases gonadotropin releasing hormone (GnRH). Then, GnRH stimulates the pituitary gland to release follicle stimulating hormone (FSH) and luteinizing hormone (LH). Then, FSH and LH stimulate the testes to produce testosterone, which is made from cholesterol. Once testosterone is produced, testosterone acts on the hypothalamus and pituitary gland through a negative feedback loop. This negative feedback loop inhibits the release of GnRH, FSH, and LH.6
Every day, testosterone levels peak at 7 AM. They decline during the day to reach a low of around 60% of peak levels by early evening. Because of this variation, it is now recommended that levels are sampled in the early morning.
External Resources: Mayo Clinic: Testosterone, Total, Bioavailable, and Free, Serum
Updated May 5, 2015
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3. Moffat SD, Zonderman AB, Metter EJ, Blackman MR, Harman SM, Resnick SM. Longitudinal assessment of serum free T concentration predicts memory performance and cognitive status in elderly men. J Clin Endocrinol Metab. Nov 2002; 87 (11): 5001-5007.
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6. Page ST, Amory JK, Bowman FD, et al. Exogenous testosterone (T) alone or with finasteride increases physical performance, grip strength, and lean body mass in older men with low serum T. J Clin Endocrinol Metab. Mar 2005; 90 (3): 1502-1510.
7. Wittert GA, Chapman IM, Haren MT, Mackintosh S, Coates P, Morley JE. Oral T supplementation increases muscle and decreases fat mass in healthy elderly males with low-normal gonadal status. J Gerontol A Biol Sci Med Sci. Jul 2003; 58 (7): 618-625.
8. Snyder PJ, Peachey H, Hannoush P, et al. Effect of treatment on bone mineral density in men over 65 years of age. J Clin Endocrinol Metab. Jun 1999; 84 (6): 1966-1972.
9. Swerdloff RS, Wang C, Bhasin S (April 1992). “Developments in the control of testicular function”. Baillieres Clin. Endocrinol. Metab. 6 (2): 451–83.