Testosterone

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Updated: September 7 2013

What Is It?
Testosterone is a steroid hormone from the androgen group. It is primarily secreted in the testicles of males and the ovaries of females, although small amounts are also secreted by the adrenal glands. It is the principal male sex hormone and an anabolic steroid.

Synthesis
The adrenal cortex, ovaries, testes, and placenta produce steroid hormones from cholesterol. 
The relative amount of the the hormoes depends on special enzymes and the trophic hormones (LH, FSH, and adrenocorticotropic hormone) stimulating specific cells within the organ. The Leydig cells in the testes produced testosterone under the inlfuence of LH.

Function
1. Testosterone and other androgens promote protein synthesis and growth of those tissues with androgen receptors:

• 1. Anabolic effects: growth of muscle mass and strength, increased bone density and strength, and stimulation of linear growth and bone maturation.
• 2. Androgenic effects: maturation of the sex organs, particularly the penis and the formation of the scrotum in the fetus, and after birth (usually at puberty) a deepening of the voice, growth of the beard and axillary hair. Many of these fall into the category of male secondary sex characteristics.

2. Testosterone functions as a prohormone and is converted to the biologically active derivatives DHT by 5a-reductase or estradiol by aromatase. 

3. Skin, hair follicles, and most of the male reproductive tract contain an active 5a-reductase. DHT has a high binding affinity for the androgen receptor and is 2-3 times more potent than testosterone.

4. Aromatization of some androgens to estrogens occurs in fat, liver, skin, and brain cells.

5. Men are protected from feminization as long as production of androgens and and tissue responsiveness to androgens are normal.

6. High doses of aromatizable testosterone analogs (or testosterone), use of anabolic steroids, abnormal reductions in testosterone secretion, and tissue insensitivity to androgens can lead to gynecomastia (breast enlargement). All of these conditions are characterized by a decrease in the testosterone/estradiol ratio.

Regulation

1. The number of Leydig cells is regulated by LH and FSH. 

2. The amount of testosterone synthesized is regulated by the hypothalamic-pituitary-testicular axis (see figure to the right).When testosterone levels are low, GnRH is released by the hypothalamus which in turn stimulates the pituitary gland to release FSH and LH. These latter two hormones stimulate the testis to synthesize testosterone. Finally, increasing levels of testosterone through a negative feedback loop act on the hypothalamus and pituitary to inhibit the release of GnRH and FSH/LH, respectively.

3. There are a lot of studies made about the testosterone levels and how to increase the level of testosterone. But to build an extensive amount of muscle one has to have supraphysiological levels of testosterone in their body. Small increases of testosterone will not do anything significant, this because of the negative feedback. In other words: elevation of testosterone cannot be done by intake of different supplements (because of the negative feedback). To get more muscles (by increasing testosterone and its benefits) one has to have supraphysiological levels of testosterone and overcome the negative feedback.

Research About Factors Affecting Testosterone Levels
First of all read about the testosterone regulation (see Regulation headline above), most of the supplements/herbs/roots etc. will not increased your testosterone levels significantly for you to build any noticeable amount of muscle; 

• Weight loss may result in an increase in testosterone levels. Fat cells synthesize aromatase (see functions above) which converts testosterone into estradiol (female sex hormone).
• Vitamin D in levels of 400–1000 IU (10–25 mcg) raise testosterone level.
• Zinc deficiency lowers testosterone levels but over supplementation has no effect on serum testosterone.
• Magnesium raise free testosterone in individuals that have lower levels of magnesium. 
• Sleep (REM dream) increases nocturnal testosterone levels.
• Resistance training increases testosterone levels
• Licorice (active ingredient in licorice root), glycyrrhizinic acid has been linked a decreases in testosterone levels
• Spearmint tea reduce testosterone levels 
• Aromatase inhibitors and 5-alpha reductase inhibitors significantly affectes the percent body fat, total testosterone, and bioavailable testosterone - Link
• Ethanolic extract of Paederia foetida Linn. leaves may increase testosterone: Link

References
Does weight loss improve semen quality and reproductive hormones? results from a cohort of severely obese men
Effect of vitamin D supplementation on testosterone levels in men
Zinc status and serum testosterone levels of healthy adults

The effects of testosterone on sleep and sleep-disordered breathing in men: its bidirectional interaction with erectile function
One Session of Resistance Training May Increase Serum Testosterone and Triiodetironine in Young Men?

Liquorice consumption and salivary testosterone concentrations

Licorice reduces serum testosterone in healthy women

Effect of spearmint (Mentha spicata Labiatae) teas on androgen levels in women with hirsutism.
Spearmint induced hypothalamic oxidative stress and testicular anti-androgenicity in male rats - altered levels of gene expression, enzymes and hormones
Spearmint herbal tea has significant anti-androgen effects in polycystic ovarian syndrome. A randomized controlled trial.
Effect of ethanolic extract of Paederia foetida Linn. leaves on sexual behavior and spermatogenesis in male rats

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