When someone brings up the word testosterone, the first thing that usually comes to mind is a gorilla-like creature with rippling muscles, covered in hair, and an aggressive attitude. This, of course, is the stereotype that society has presented us with. In reality, testosterone is a hormone that is present in both men and women, though men have significantly higher levels.
Testosterone also plays a vital role in men and is not just a medication used by bodybuilders to increase their muscle mass. Testosterone is essential for the proper development of male sexual characteristics. It also aids in fertility and sperm production, along with it helping maintain bone density and muscle mass.
When testosterone levels in men are low, it can lead to a condition known as hypogonadism. This is where the body does not produce enough testosterone, which can cause a range of symptoms.
Although the discovery of testosterone has been around for over 70 years, it has only been used therapeutically for the past few decades. In that time, testosterone replacement therapy (TRT) has become one of the most popular treatments for hypogonadism.
But is Testicular Atrophy on TRT a real thing?
What Is Testosterone Replacement Therapy?
TRT is a process where testosterone is artificially introduced into the body to reach normal levels. This can be done through a patch, gel, injection, or implant. Studies done on men receiving testosterone therapy have shown an increase in libido, more energy, a better mood, and less chance of developing osteoporosis (a decrease in bone density) .
Even though testosterone therapy has proved to be successful through years of research and the testimonies of thousands of men, side effects are possible (as with any medication). Some of which vary in frequency and intensity. One of the potential side effects that are not always discussed is testicular atrophy or testicular shrinkage.
Testicular Atrophy: What is It?
To understand how testosterone therapy can cause testicular atrophy, we must first understand how our bodies produce testosterone.
There is a region of the brain called the hypothalamic-pituitary-gonadal axis, a system in the body that helps to regulate testosterone levels.
The hypothalamus produces a hormone called gonadotropin-releasing hormone (GnRH), which stimulates the pituitary gland to produce two other hormones called follicle-stimulating hormone (FSH) and luteinizing hormone (LH).
These hormones then stimulate the testicles to produce testosterone.
Once testosterone is released into the bloodstream, it travels to various tissues in the body and binds to androgen receptors. This binding process triggers a cellular response that leads to the many effects of testosterone in the body, such as increased muscle mass and strength, increased libido, etc.
So, then how is it that TRT reduces testicular size? The answer is pretty simple. When exogenous testosterone (testosterone that is not produced by the body) is introduced into the body, the hypothalamus senses this and reduces the production of GnRH.
This, in turn, reduces the production of FSH and LH, which then leads to a decrease in the natural production of testosterone by the testicles. The result is smaller testicles due to the lack of testosterone production.
In other words, your brain is telling you that you don’t need to produce as much testosterone because there is already enough present, so it shuts down production. This is why testicular atrophy is a common side effect of TRT.
You should note that this is not a health threat and oftentimes may not be especially noticeable.
How To Prevent Testicular Atrophy
From the time we are born, testicular size gradually increases. For most men, testicular size rises rapidly during puberty and reaches a maximum around the age of thirty. At this point, testicular growth stops entirely until approximately age 60, when testicular atrophy begins to occur. 
Unfortunately, this can happen to some men due to testosterone therapy; however, no study has accurately depicted how severe testicular atrophy is while on TRT.
On the bright side, if you are on testosterone treatment and are concerned about testicular atrophy, there are ways to prevent this from happening.
Human Chorionic Gonadotropin (hCG)
hCG is a hormone that is very similar to LH. They only differ in a very small way, structurally.
It is most notably known as a way to determine pregnancy and determine the exact age of the fetus when a woman is pregnant. However, in men, it is well known for its use in fertility treatment and stimulation of testosterone production, just like LH.
When used with testosterone replacement therapy, hCG can help to keep the body producing testosterone which helps maintain testicular size and function, even when exogenous testosterone is present. It essentially tricks the brain into thinking that there is a need for more testosterone and prevents it from shutting down production.
This is particularly important since it has been shown that FSH cannot maintain testosterone production on its own in men with low levels of testosterone; LH is required for this. 
Note: hCG injections are administered intramuscularly (through the muscle), so they cannot be bought without a prescription. If you have concerns about testicular atrophy, speak with your physician to get more information and acquire a valid prescription.
Method of Administration
Another way to prevent testicular atrophy is to switch the methods of administration (i.e. from injection to gel).
A study discovered that those who received testosterone injections were more likely to experience testicular atrophy when compared with patients who applied testosterone ointment.  Therefore, if you have any concerns about testicular atrophy, it may be best to discuss other ways to administer testosterone.
Is Testosterone Replacement Therapy right for You?
Testosterone replacement therapy has become one of the most effective treatments for late-onset of hypogonadism (LOH), or andropause. LOH is a medical condition characterized by a gradual decline in testosterone levels as you age. It affects an estimated 38% of American men aged 40 and older .
When your body is running low on testosterone, it is very similar to what happens when your body is deficient in other hormones – things just don’t work as well as they should. Like a car running low on oil, your body begins to break down. This can manifest in a variety of ways, but the most common signs of “low T” are fatigue, low libido, loss of muscle mass, difficulty concentrating, and decreased semen volume. But, once you start testosterone replacement therapy, you can expect these symptoms to slowly fade away.
Ultimately, it is the decision of you and your doctor to understand the risks and benefits of therapy before starting. And, if you do start TRT and have concerns about testicular atrophy, be sure to discuss it with your doctor. This is a lifelong treatment and you want to make sure you are as comfortable and informed as possible.