The Many Roles of Testosterone

By Michael Irwig, MD, FACE

You’ve heard people say, “You are what you eat.” Well, it turns out that your hormones also play a part in who you are. Many differences between men and women have to do with the different levels of testosterone and other sex hormones, such as estrogen.

Many people know that testosterone is important for a man’s sexual desire. Yet, testosterone has numerous other important functions throughout the body, several of which are not so obvious. For example, testosterone is important for strong bones. Low levels of testosterone can lead to osteoporosis and broken bones. Testosterone also causes men to have higher red blood cell counts than women. After puberty, testosterone in men causes them to develop more muscle mass and less body fat compared with women. And this is only the tip of the iceberg — testosterone may have other important effects relating to heart disease and even how our brains are wired from birth.


In men, nearly all testosterone is produced in the two testicles from the molecule cholesterol. Yes, this is the same molecule related to fatty food and heart health. High levels of testosterone are produced in boys going through adolescence and the levels remain high throughout a man’s life. On the other hand, after men reach about age 40, their testosterone levels begin to gradually decline. The levels of testosterone decline more rapidly in men with medical conditions. Men under age 50 tend to have higher levels of testosterone in the morning. Levels of testosterone don’t vary much with the time of day in men over age 50.

In a typical day, a man’s blood level of testosterone bounces up and down quite a bit. This is due to chemical signals from a gland below the brain called the pituitary. As you can imagine, measuring one level of testosterone can be tricky for your doctor to interpret because testosterone levels bounce around so much. It is therefore very important for a man with one low level of testosterone to have it rechecked to confirm that it is indeed really low. For a younger man, the blood test should be done between 7 a.m. and 10 a.m., when testosterone levels are often highest.

Speaking of low testosterone, this is a very controversial area within the medical world. Nobody agrees what number determines whether a guy has normal or low levels. Every study is somewhat different and each lab measures testosterone differently. The measurement of testosterone is also complex because men with certain conditions tend to have higher or lower levels of testosterone. For example, overweight and obese men often have lower testosterone levels than do thinner men. On the other hand, men with liver disease may have higher levels of testosterone.


If a man develops low testosterone, certain signs or symptoms may appear, such as

  • low sexual drive
  • erectile problems
  • fewer morning erections
  • less facial hair and shaving
  • osteoporosis/fractures
  • decreased strength
  • depression and depressed mood
  • tiredness
  • problems concentrating
  • problems with sleep
  • irritability
  • decreased interest in activities

You’re probably reading this list and thinking “I may have low testosterone because I have some of these symptoms!” Though low testosterone can cause these symptoms, many other conditions may also cause them. Some men have sleeping problems from being obese and having sleep apnea. Other men have difficulty achieving a firm erection due to aging, diabetes, heart disease, and other causes. Many of us are tired simply because of our busy and stressful lives! You therefore cannot assume a man with these symptoms has low testosterone. If you suspect that you might have low testosterone, speak with your doctor. He or she may want to measure your testosterone level.

There are other reasons why men may want to have their testosterone measured, such as

  • breast pain and enlargement
  • very small testicles
  • problems with fertility
  • low red blood cell count
  • hot flashes


Men with low levels of testosterone may have a better quality of life if they are treated with testosterone. Testosterone may help increase their sex drive and sexual ability. It may also increase their muscle mass and strength. It may increase bone density and prevent osteoporosis. Testosterone should not be used for men with normal testosterone levels for “anti-aging” purposes.

In the United States, there are two common ways to take testosterone. Neither of them is pill. The more traditional way is to receive injections of testosterone every two weeks into the muscle, typically in the thigh or buttocks. Men can be taught to give themselves these injections. Another way is for testosterone to be absorbed through the skin. Men can either place a patch or a small amount of liquid gel onto their chest or abdomen every day. Which type of testosterone to use depends on the man’s preference, the medication costs, and possible side effects. Like all medications, each one has its own pros and cons. The injectable testosterone is the less expensive form, but it causes more men to develop too high of a red blood cell count. Men who use the gel should be careful not to have skin-to-skin contact with a woman or child because the testosterone gel can be transferred to them if it is not dry.

Testosterone should not be given to men who have prostate or breast cancer, high red blood cell counts to start with, untreated obstructive sleep apnea, or severe heart failure.

For men who start testosterone replacement therapy, baseline bloodwork should be done for a red blood cell count (hematocrit) and for prostate specific antigen (PSA). Men over age 50 should also have a digital rectal exam to determine whether there are any prostate nodules, which could represent a prostate cancer. If nodules are present they should see a urologist for a possible prostate biopsy. The bloodwork is usually repeated three months after starting testosterone therapy and then yearly. Testosterone levels should also be rechecked to make sure that the levels are not too high and not too low. If the red blood cell count gets too high, testosterone may need to be stopped, the dose may need to be lowered, or the type of medication may need to be changed.


There still is a lot unknown when older men with low testosterone are given testosterone replacement therapy. A large and long-term clinical trial is needed to answer some key questions. For example, would treating men with low testosterone change their risk for heart attacks? Would treatment reduce the number of fractures? Would treatment affect the prostate in any way? Until a testosterone trial is started and completed, doctors will have to rely upon the best available evidence from smaller and shorter studies to guide them. As with any therapy, there are always benefits and risks and treatment needs to be tailored to each patient. There is no one-size-fits-all when it comes to a blood level of testosterone or number of signs and symptoms.

Dr. Michael S. Irwig is a board certified endocrinologist with a special interest in andrology. He completed medical school at Cornell University, an internal medicine residency at the University of Virginia and an endocrinology fellowship at the University of Washington. He currently practices at the Medical Faculty Associates/George Washington University where he is an Assistant Professor of Medicine and Director of Andrology. Dr. Irwig enjoys teaching and has lectured nationally and internationally on low testosterone and use and abuse of steroids. He serves on AACE’s Continuing Medical Education Committee and on the Board of the Mid-Atlantic chapter of AACE. He lives in Dupont Circle and has two kittens named Andro & Gen.