Testosterone Explained: Everything You Should Know

Testosterone Explained: Everything You Should Know


We have all heard of it, we know that it’s linked to building muscle and athletic performance and that people sometimes supplement it; but what does it really do? Where does it come from? What other effects does it have? This video is designed for folks without a major scientific background who want to learn more about testosterone.

One quick point before I get started. Although testosterone does have effects on the human body from conception all the way through puberty, I’m going to focus specifically on the effects of testosterone in adults (post-puberty).

What is testosterone?

Testosterone is an androgenous steroid hormone. The term androgen or androgenous refers to any compound, natural or synthetic, that binds to receptors that promote or control the development and maintenance of male characteristics. Steroid refers refers to a broad class of compounds with a very specific structure (composed of 4 6-carbon rings), of which testosterone is just one example. The term hormone refers to any signaling molecule created by the body that exerts its influence on other organs of the body.

How and where is testosterone synthesized?

Testosterone synthesis is derived from cholesterol, like all steroid hormones. This is why you will hear some folks advocating for certain fats (namely MUFA, PUFA and saturated) in your diet to promote hormone synthesis. For example, a decrease in dietary saturated fat is associated with a decrease in serum testosterone. The individual steps of synthesis are beyond the scope of this video.

In men, testosterone is primarily synthesized in the testicles (or testes). Specifically, a class of cells called ‘leydig cells’ are responsible for their synthesis. Women also produce testosterone as well, but at lower amounts, and this occurs primarily in the ovaries in cells called thecal cells. Both genders also produce small amounts of testosterone in the adrenal cortex (which sits above the kidneys) and from our skin.

Testosterone is transported around the body primarily bound to sex hormone binding globulin (which is also responsible for carrying other androgen and estrogen hormones).

How is testosterone synthesis regulated?

Regulation of testosterone synthesis is somewhat complicated but I will briefly illustrate it to make an important point.

  1. Hypothalamus (a gland in the brain) detects low testosterone and releases a hormone called ‘gonadotropin releasing hormone (GnRH)’.
  2. GnRH acts on another gland in the brain called the pituitary gland which releases two hormones: luteinizing hormone (LH) and follicle stimulating hormone (FSH).
  3. These two hormones travel to the leydig cells in the testicles and tell it to produce more testosterone.

Here is where it can get confusing. If your body’s testosterone level goes down, all of those hormones will go up in an attempt to stimulate more testosterone production (increased GnRH, FSH and LH). If your testosterone level goes up, however, all of those pro-testosterone hormones will decrease because they are detecting elevated testosterone. This is termed a negative feedback loop and is how many of the hormones in the body are regulated.

The important point here is that if you supplement testosterone for an extended period of time, you can suppress these regulatory hormones to the point where they do not get released at adequate levels and hinder your body’s ability to synthesize testosterone naturally.

What effects does testosterone have on the body?

Testosterone acts on two clusters of bodily function: androgenic and anabolic activity.

Anabolic (promote building)

  • Promote Muscle mass, strength
  • Body composition, decrease body fat
  • Increase bone Density
  • Increase Red Blood Cell production (via kidneys)
  • Prostate: Testosterone can be converted to dihydrotestosterone (DHT) which promotes prostate growth (and why our prostates tend to get larger as we age)

Androgenic (Male function)

  • Aids in erectile function
  • Libido and sex drive
  • Normal sperm development: Sustains sperm development and fertility


  • Estradiol: In adipose tissue, testosterone is converted to estradiol which is the primary female sex hormone. Men do require some estradiol for healthy function, however obese men can significantly increase their estradiol levels (which can cause development of female features and also suppress testosterone synthesis).
  • Psych:
    • Helps reduce risk of depression, improves mood
    • May stave off dementia, improve cognitive function

When does testosterone tend to decline?

In women, they go through something called menopause, which is the cessation of the ovarian and uterine cycle and loss of fertility. Typically this occurs around the age of 51 and happens rather abruptly (measured in months). There is no such event that occurs in men.

Our serum testosterone level peaks around age 30, and decreases about 1% per year as we age. In otherwise healthy men, there is no specific point at which our testosterone levels suddenly drop off. This slow and steady decline in testosterone is referred to as andropause.

Signs of low testosterone

Decrease: bone density, cognitive function, erectile function and libido, red blood cells, muscle mass and strength, sense of well being and cognitive decline

Increase: body fat

Side effects of exogenous steroid supplementation

Let me be clear. Under no circumstances should you be ingesting or injecting steroids without the supervision of a physician. The long term consequences can be very severe and include:

  • Liver dysfunction
  • Coronary artery disease (increased risk of heart attack)
  • Decreased sperm count
  • Decreased production of hormones that promote testosterone (GnRH, LH, FSH)
  • Decreased production of natural testosterone
  • Too many red blood cells (polycythemia)
  • Altered lipid profile (increase in LDL and decrease in HDL cholesterol)
  • Sleep apnea
  • Enlarged prostate
  • Behavioral changes including irritability, aggressiveness, nervousness
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