Testosterone is a steroid hormone from a group called androgen. Androgen affects many characteristics in our bodies, like the development of the male sex organs, the deepening of the voice during puberty, muscle and bone strength, and hair growth. Interestingly, the same circulating androgen, like testosterone, can increase hair growth in one area of our bodies and inhibit it in others. Thought to be gene dependent, these differences affect our appearance in several ways. Before we tackle why, it’s important to understand how testosterone is produced and how hair grows.

Testosterone is produced by specialized cells (Leydig cells) in the male testes and the female ovaries. They do this by receiving signals from your anterior pituitary gland. The pituitary senses the need for more testosterone and signals the Leydig cells with a hormone known as “luteinizing hormone”. Cholesterol (yes the ugly blood level doctors try to get us to reduce is actually very much needed in our bodies) is used to make steroid hormones known as progesterone and progenolone.  These are then transformed into testosterone. The luteinizing hormone, coming from your pituitary gland, increases the rate at which this happens.

Once testosterone is made, it begins to circulate around the body looking for receptor sites to attach too. Known as “androgen receptors” (in the case of hair), they reside in an area of your hair follicle called your dermal papilla. Once there, the testosterone (androgen) binds to androgen receptors and initiates any altered gene expression. This will either help the follicle or hurt it, depending on the gene expressed.

Hair, like cells in our bodies, have a growth cycle. There are three phases: Anagen- the active growth stage (80-85% of hair is in this phase); Catagen- this phase is also known as the transitional phase, when hair begins to stop growing; and telogen- this phase is when hair growth is completely shut down and the fibers fall out (10-15% of our hair is in this phase at any given time). After your hair goes through the Telogen phase, Anagen begins again and voila! More hair! This cycle repeats itself throughout our lifetimes.

Testosterone regulates hair growth by affecting the follicle and how it can produce the different types of hair, like facial, pubic and scalp hair. These follicle changes affect the growth phases of hair, producing different reactions. The result is changes in hair growth, or lack thereof, occurring over time, as the growth cycle of hair can take as long as 7 years to complete. What has baffled researchers for decades is how can a hormone like androgen cause some hair’s growth to be inhibited in one area of the body, while helping it in others? This paradox appears to be gene related.

There are two types of hair follicles within our bodies, Vellus and Terminal. Vellus hair follicles are the ones that create fine, virtually colorless hair, like the ones found on infants. They are usually tiny and don’t reside very deep within the skin. Terminal hair follicles are created from Vellus hair follicles by androgen. They are deeper and longer follicles that form thicker, longer and more pigmented hair. To change from Vellus to Terminal, or from Terminal to Vellus, the follicle must go through the hair growth cycle to regenerate as one type or another.

Normally, during puberty, our bodies begin to produce more androgen. This circulating androgen then turns our young Vellus follicles into more Terminal ones. The boy becomes a man! As time goes on, certain Terminal hair follicles (like the ones found on our scalp) can begin to regress back into Vellus follicles. This too is mediated by testosterone. The result is slowly dying hair follicles that don’t grow back. (See What Causes Baldness for more information on this.)

What researchers have found is that individual hair follicles have different expression of genes within the follicle. Each gene expression reacts differently to androgen. Some genes inhibit follicle health and some increase follicle health. Since each follicle is independent of one another, each gene expression is also independent of one another. This is why hair transplants work. The follicles on your head might be dying, but the ones in other areas of your body are not, so doctors can simply move them. The transplanted hair follicle will not die because the genes associated with that follicle are not negatively affected by androgen, no matter what area of the body they’re in.  Nothing like back hair on your head!  The process that allows a specific gene to be expressed in a certain follicle isn’t yet understood. What is known is that the programming occurs in the pattern processing during development.

If you liked this article, you might also like:

• Why Does Hair Only Grow to a Certain Length
• Why Does Chemotherapy Make Hair Fall Out
• Why Hair Turns Gray
• There is No Difference Between Fur and Hair
• Shaving Does Not Make Your Hair Grow Back Thicker or Faster

Bonus Facts:

• The higher the testosterone level is in vitro, the greater ability you will have to grow a beard. Interestingly, this has no effect on a non-balding scalp (the heads of people who don’t go bald).
• Certain body builders and other athletes love to take testosterone supplements or other steroids to increase muscle mass. While this works quite effectively, excessive testosterone can cause several problems. They include: heart muscle damage, acne, low sperm counts, shrinking of testicles, prostate enlargement, liver disease, high blood pressure, insomnia, and headaches. Steroid cocktail anyone! Should you take steroids during puberty, this will also stunt your growth.
• Low testosterone can also cause problems. In adulthood, this may decrease your sexual desire, cause erectile dysfunction, loss of hair, osteoporosis, and a decrease in your muscle mass. There are times, however, where it’s not such a bad thing. In the case of prostate cancer, testosterone is thought to stimulate the prostate gland and cause the cancer to grow. As a result, if you have prostate cancer, doctors sometimes prescribe medications to reduce your testosterone level like “Leuprolide”. In extreme cases, castration can also be performed. I think I’ll take the prostate cancer doc!