Where is testosterone made?
Testosterone AKA “T” is made in the testicles. The testicles have essentially two jobs–to make testosterone and to make sperm.

How are testosterone levels regulated?
Testosterone is regulated through a feedback loop with the brain (the hypothalamus and pituitary gland, to be exact). This is often referred to as the HPG (hypothalamus-pituitary-gonad) axis. The hypothalamus makes a peptide hormone called GnRH (gonadotropin-releasing hormone). GnRH stimulates the pituitary gland to produce the gonadotropins. The word “gonadotropin” loosely means “affects the gonads.” The pituitary gonadotropins are LH (luteinizing hormone) and FSH (follicle-stimulating hormone). LH released into the bloodstream tells the testicles to make testosterone and FSH tells the testicles to make sperm.

What happens when testosterone levels go down?
In a properly functioning feedback loop, the hypothalamus should respond by making more GnRH, which in turn tells the pituitary to make more LH and FSH. As a result, the testicles should make more testosterone (and sperm).

What happens when testosterone levels go up?
The opposite of above. GnRH, LH, and FSH levels go down, reducing stimulation of the testicles.

Then how can a man ever have low T?
Unfortunately, this feedback loop does not always function as it should. The most common scenario with low T is what is called secondary hypogonadism. This means low testosterone is not the testicles’ fault. It is the brain’s fault because it does not respond properly to declining T by making more LH. Typically, labs will show low T but also low LH. Secondary hypogonadism is common with obesity, diabetes, and alcohol or drug use. There are also some benign pituitary tumors that can cause it.
Less commonly, a patient can have primary hypogonadism, caused by malfunction of the testicles. In this case labs will show high LH but low T. The brain is trying to stimulate the testicles but they aren’t responding.

Given above, any guess why PED abusers can get tiny nuts?
I’m not going to mention any names…Mark McGwire, Barry Bonds, Sammy Sosa, Roger Clemens et al. Anyway, what happens when somebody gets a lot of testosterone from external sources such as injections? In time, LH and FSH go way down–close to zero. The testicles see no signal to make anything and slowly just shrink up.

So, is it a good idea to give a 25 year-old (or any man who wants to have kids in the future) testosterone?
By now you should know the answer is no. Why? Because not only will LH decline, but FSH would too. Therefore sperm production would plummet. Sometimes it does not recover even after stopping testosterone. As a side note, it is possible to keep the testicles working even while on testosterone. This can be achieved by also taking clomiphene or HCG, but that is another blog.