Testosterone is an “androgen”, which means that it is a compound that has male hormonal activity. Men need high levels of testosterone within their testicles in order for normal sperm production to take place. Consequently, low levels of testosterone are a common cause of decreased sperm production (a condition referred to as “hypogonadism”). So if a man has a low sperm count and low testosterone levels, it seems like an easy fix to just give him testosterone and thereby solve both problems at once, right? Wrong.
When it comes to hypogonadism and fertility, the way that testosterone levels are increased in the bloodstream matters greatly. There are two different ways to increase a man’s blood testosterone levels:
#1) Give the man testosterone (called giving “exogenous” androgens) or
#2) Have the man make more testosterone himself (called increasing “endogenous” androgen production)
The key to understanding the effect that both of these approaches have upon sperm production lies in the complex feedback mechanisms that control sperm production within the testicles. The pituitary gland (which sits at the base of the brain) is the hormonal control center of the body. This gland produces luteinizing hormone (LH), which tells the testicles to make testosterone, and follicle stimulating hormone (FSH), which tells the testicles to make sperm. The brain monitors the end-products of its stimulation (testosterone for LH, and sperm production for FSH), so that if production of either drops off, it can produce more LH and/or more FSH to ramp up testicular sperm and testosterone production.
For FSH and sperm production, this feedback mechanism works quite well. If sperm production starts to drop, FSH levels rise, which drives the testicles to make more sperm. However, for testosterone regulation, Mother Nature did not anticipate humans being able to isolate and bioengineer androgens to administer to themselves. If evolution could have foreseen this development, it may have provided for a mechanism in which the pituitary gland monitored testosterone levels within the testicles (in order to protect the environment for sperm production).
Instead, the pituitary gland unfortunately monitors testosterone levels only in the blood stream. If a man takes exogenous androgens (in the form of testosterone replacement, anabolic steroids for weight lifting, etc.), then the blood levels of androgens rise. The pituitary gland senses these high levels of androgens in the blood and in turn reduces its hormonal stimulation of the testicles. LH and FSH levels fall and the high testosterone levels within the testicles plummet, despite high levels of testosterone being maintained within the bloodstream.
The combination of very low testicular testosterone levels and decreased FSH stimulation results in a significant decrease in sperm production in most men, with the majority actually becoming completely azoospermic (having no sperm in the ejaculate) if the dose of exogenous androgens is high enough. Occasionally I will come across a man who maintains good sperm production despite being on exogenous androgens, but these men are the exception to the rule. These observations fit with the findings of scientific trials that have been performed to see if testosterone injections could be used as an effective form of male contraception.
These studies found that in most men sperm counts fell to a point where conception would not be expected naturally, but enough men did not respond to make the failure rate for contraceptive unacceptably high compared to standard hormonal birth control methods from the female side.
The bottom line is that men who want to have children should definitely stay away from the use of exogenous androgens. The negative effect of testosterone replacement on male fertility is well-established in the scientific literature. Unfortunately, large numbers of primary care doctors still are not well trained in this and continue to prescribe testosterone to their patients who want to have children.
A recent survey of general urologists even found that 25% of these doctors would consider prescribing testosterone to men who desire to have a child- that is a surprising finding in a group of doctors who are supposed to be specialists in men’s health!
If a man has low testosterone and wishes to have children, there exist a range of medications which can be used to boost endogenous testosterone production, such as clomiphene, anastrazole, and human chorionic gonadotropin. These medications can actually help to improve sperm production by increasing testosterone levels within the testicles (as well as in the bloodstream).