• Low Energy and Fatigue - Men may experience reduced levels of energy as they age. Some of this can be attributed to the natural aging process, but it's also a common symptom of low testosterone levels.
• Low Sex Drive - Testosterone plays an essential role in a man's libido. If low testosterone is an issue, a man would experience much greater reduction in sex drive than they normally should.
• Difficulty Building and Maintaining Muscle Mass - A man's testosterone level is the key element to the body's ability to recover from exercise and build muscle. If testosterone levels are too low your body cannot recover properly, which can even lead to injury.
• Mood Issues - Hormones play a vital role in a person's mood. Testosterone is just one of many hormones involved in the process that can change how you feel. If a man has low testosterone they can feel anxious, depressed, and less confident. 

There are some natural methods of increasing testosterone that have been proven to be effective.

• Diet - The basic idea is to eat good balance of macronutrients. Protein, Fat, and Carbohydrates. From there you can fine tune your nutrient intake. Foods like fish, eggs, leafy greens, berries, and avocados all contain nutrients that aid on the processes of creating testosterone, and your body's ability to effectively use testosterone. Vitamin D, Zinc, Copper, Lecithin, and Omega-3 fatty acids are some of the many reasons to eat whole and healthy foods.
• Exercise - About 15 minutes to 1 hour after exercise the body's testosterone levels rise. Large compound lifts, like squats and deadlifts, can work the best to increase testosterone because they move the most blood around the body.
• Sleep - Natural testosterone levels peak during sleep. This allows the body to maximize recovery time. When a person doesn't get enough sleep they are potentially lowering their testosterone. Not only does sleep effect testosterone levels, sleep is an essential factor in the body's ability to effectively make use of its testosterone in recovery, muscle building, mental ability, and other qualities. Melatonin supplementation and other natural products (such as passiflora extract) can improve sleep quality.

TRT is implemented through injections of exogenous testosterone. These injections are intended to bring a person with low testosterone into their healthy range. The bodies natural testosterone levels then shuts down (unless you use HCG) and the testosterone injections take over all of the body's testosterone needs. 

Users should see improvements in ability to gain muscle mass, sex drive, mood, and more within 1-2 months of beginning TRT.

• Strength and Muscle - Increasing testosterone levels allows you to experience an elevated ability to build muscle mass. Heavier lifting is more beneficial, and will result in more efficient strength gains.
• Mood - TRT users report less anxiety and nervousness. Having a more confident attitude is a common effect of TRT.
• Libido - One of the most common symptoms of low testosterone is low libido. Raising testosterone levels increases your sex drive.
• Energy - Many TRT users experience increased energy levels. One reason for this is higher levels of testosterone allow for quicker recovery from sports and exercise, allowing the user to feel more energized when they may have previously felt burnt out.

• Reproductive System Shutdown - Since your body is receiving exogenous Testosterone, the body shuts down its own testosterone production methods including the testicles.  This can be considered a positive for some men, as the shutdown of your ability to produce sperm can mean a form of (not necessarily effective) birth control. There is a fix, however, in the form of the hormone human chorionic gonadotropin, or HCG. HCG allows the body to continue its natural production of testosterone, and keeps your testicles full sized and producing semen and sperm. 
• Hair Loss - If you are genetically predisposed to male pattern baldness, or MPB, using elevated levels of testosterone could assist in hair loss. For people that don't have a family history of early baldness, TRT will likely not effect them.
• Mood Changes - Though most users report an elevated mood, more confidence, and less anxiety, some people report mood issues. Some include feeling tense, angry, and sad. Some of these symptoms can be related to an abnormal estrogen level. This is one of many reasons why it's important to get regular blood work done while on TRT.

1. https://www.usrf.org/news/TRT/history.html

2. https://www.healthline.com/health/low-testosterone/warning-signs

3. https://www.healthline.com/nutrition/testosterone-boosting-food

4. https://www.webmd.com/men/features/exercise-and-testosterone