Low Testosterone

Testosterone Replacement Therapy and Muscle Gain


Shaun Ward BSc, MSc
June 7, 2024
Take-home points
  • TRT causes small to moderate increases in muscle mass, even without exercising.
  • Men with low testosterone can expect higher than average muscle growth from TRT.
  • Combining TRT with resistance exercise leads to more muscle growth than TRT alone.
  • Larger amounts of muscle gain are expected from intramuscular versus transdermal TRT.

Testosterone is a powerful hormone that we produce naturally. Sharp rises in testosterone during adolescence spur the body into an anabolic state. Shoulder girdles widen, bones become dense, and muscles grow, sometimes rapidly. These effects are typically maintained throughout early adulthood, when testosterone levels stabilise. Then, at the age of around 40, testosterone levels slowly begin to decline year by year, and losses in muscle mass often follow, more than some men are happy with. Not only is muscle mass linked with masculinity but from a health perspective, severe losses in muscle can lead to sarcopenia, reduced physical function and poor quality of life.1 Therefore, many men of older age and/or with low testosterone levels may consider using testosterone replacement therapy (TRT) to prevent muscle loss and, with a bit of luck, turn back the years to regain lost muscle tissue. But does TRT really help with this?

Importantly, this article is for the purposes of hypogonadal patients only. Without a clinical reason, we strictly do not support any man commencing TRT for muscle growth. We are observing a rising trend in ‘underground’ testosterone usage, which can increase the risk of serious health issues in men who are not hypogonadal (confirmed low testosterone with symptoms).

What is testosterone replacement therapy (TRT)?

TRT is a commonly prescribed medical treatment aimed at restoring testosterone levels in people (usually older men) with biologically low testosterone and symptoms of low testosterone. It involves administering testosterone through various methods such as injections, patches, gels, implants or oral tablets. For hypogonadism, TRT has been FDA-approved and is recommended by several major organisations such as the Endocrine Society, the British Society of Sexual Medicine, and the European Association of Urology.2,3,4

Read our article on hypogonadism (Low Testosterone) Treatment to learn more about the types of TRT and its various benefits.

Does testosterone replacement therapy cause muscle gain?

The class of drugs related to testosterone (called anabolic steroids) are the most commonly banned drugs in sport for a good reason: at the right dose, they dramatically increase muscle mass. In just 10 weeks, men taking high doses of testosterone (600 mg/week) can gain roughly 3kg of fat-free mass (including muscle mass) without even lifting a weight or exercising.5 With weight lifting added, results can double to around 6kg.5 That means in just over a month, high dose testosterone supplementation achieves more than what “natural” men can expect with years of hard work and dedication in the gym.

Rates of muscle protein synthesis in the body, pre and post testosterone treatment. Taken from Brodsky et al. 1996.

TRT, which is technically a low-dose anabolic steroid, is also banned in competitive sport unless athletes are granted a Therapeutic Use Exemption (TUE) for medical purposes. Similar to anabolic steroids, TRT also increases the rate of synthesising muscle proteins and reduces the rate of muscle breakdown, creating a favourable environment for muscle growth.6,7 But since TRT aims to achieve normal testosterone levels (300–1000 ng/dL) with relatively low dose testosterone—not close to the “supraphysiological” levels of most anabolic steroid users—results are not the same. In three large analyses (studies of studies) of high-quality studies, the average increase in lean body mass from 3+ months of TRT ranged from 1.5–3.5 kg.8,9,10 How much is that, you ask? Well, not enough to look like Arnold Schwarzenegger, but more than enough to notice in the mirror—pick up a few kg of chicken breast and imagine it attached to your body! A portion of these kg’s will be water retention (a common side effect of TRT),11 no doubt, but increases in muscle fibre size are inevitable (few studies have confirmed this using modern technologies12).

Will I gain more muscle on testosterone replacement therapy if I have low testosterone?

One of the limitations of many studies of TRT and muscle gain is that the participants do not always have low testosterone to begin with. Therefore, TRT might only be increasing their testosterone levels within the normal range—from the lower end to the higher end—which would not be expected to lead to considerable muscle gain. In studies investigating the effects of TRT in men with hypogonadism (low testosterone), increases in lean body mass are typically higher than common averages and some people gain in the range of 5.0–8.0 kg.13,14,15 Therefore, if you have been diagnosed with low testosterone and decided to try TRT, expect more noticeable muscle gain than someone with normal or high testosterone levels.

Does the type of testosterone replacement therapy matter for muscle gain?

Importantly, not all TRT leads to the same amount of muscle gain. When studies have looked at how the route of TRT administration affects results, they have reported that transdermal injections may cause 3–5 times greater increases in lean body mass compared with transdermal formulations (gels and patches).10 For example, one analysis of studies reported that intramuscular TRT was associated with a 5.7% increase in fat-free mass, while transdermal TRT was associated with only a 1.7% increase in fat-free mass.10 One theory to explain this difference is that the large, temporary spikes in blood testosterone from intramuscular TRT are more anabolic than the gradual, steady increases in blood testosterone from transdermal TRT.

How to build muscle on testosterone replacement therapy

If a hypogonadal patient starts to use TRT for medical purposes, muscle gain will likely occur, especially if the patient has weakened muscles. However, results will not be optimal unless some other fundamentals are in place. When building muscle on TRT (for medical purposes), the same principles apply as when growing muscle naturally. Here are some recommendations to maximize your results:

  1. Introduce or continue resistance training: maximizing muscle gain requires a frequent and progressive muscle stimulus, which most people achieve by lifting weights and trying to get stronger with time on compound exercises that utilise multiple muscle groups (e.g. squats, deadlifts, pullups, dips, overhead pressing, bench press).
  2. Consume a high amount of protein: Protein recommendations for maximising muscle growth are higher than general recommended daily amounts. A good rule of thumb is to consume 1.6 grams of protein per kilogram of your body weight per day.16 So for a person of 80 kg, that is roughly 130 grams of protein per day.
  3. Consider creatine supplementation: Creatine is a top-tier supplement that has been heavily researched. It has a large body of evidence showing that it can increase muscle mass and strength (which often go hand-in-hand).17
  4. Rest up: Ever heard the saying “you build muscle while you sleep?”. It holds some truth. Sleep duration and quality are important predictors of muscle gain.18 Also, given that there is a maximal amount of muscle that can be gained in any period of time, we recommend exercising smart and not spending endless hours in the gym until you feel burnt out.19

Do not use testosterone replacement therapy recreationally

TRT is indicated for male hypogonadal patients.1 If you are looking to use TRT for strictly muscle-building purposes outside of a clinical need (recreationally), you may be putting your health at serious risk. High doses of TRT can lead to supraphysiological levels in your body, increasing the risk of severe side effects such as cardiovascular complications, or even death.20

If you are experiencing a loss of muscle mass and symptoms of low testosterone, then we highly recommend speaking with a healthcare professional. Muscle gain is one of many potential benefits of TRT for people with diagnosed hypogonadism.

Learn more about hypogonadism, including signs and symptoms.

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