Although they may still act like boys, most males start to become men in their early teens.
The physical changes a boy undergoes during puberty are caused by the release of hormones called androgens. The most influential androgen is called testosterone.
During puberty, testosterone and other androgens cause a variety of changes in boys, such as:
<ul>
 <li>A deeper voice.</li>
 <li>Growth of facial and body hair.</li>
 <li>Bigger, stronger muscles.</li>
 <li>The beginnings of a sex drive and fertility.</li>
</ul>
When testosterone levels are too low
When a man's body doesn't produce enough testosterone, the condition is called hypogonadism.
According to the <a href="http://www.netreturns.biz/healthinfo/bouncelink.aspx?shortcut=hormone" target="_blank"> Endocrine Society</a>, a lack of testosterone may cause:
<ul>
 <li>Reduced body hair. </li>
 <li>Less muscle mass and strength. </li>
 <li>Increased fat. </li>
 <li>Sexual troubles, such as low sex drive, impotence and infertility. </li>
 <li>Osteoporosis. </li>
 <li>Mood changes. </li>
 <li>Enlarged breasts. </li>
</ul>
Several problems may cause low testosterone levels, including:

Testicle problems. Boys with undescended testicles or absent testicles can't produce testosterone or sperm. Injury or infection of the testicles can affect testosterone too. 
Tumors and cancer treatments. Some tumors, such as those in the pituitary gland, and chemotherapy and radiation for cancer may affect testosterone production.
Aging. Men may experience a drop in hormone levels as they age. 
Chronic diseases. HIV infection, AIDS, type 2 diabetes, liver and kidney disease, and obesity are some of the diseases that may cause a drop in testosterone levels. 
Klinefelter syndrome. Boys born with this problem have at least one extra X chromosome. They don't produce normal amounts of sperm. Men with the condition may have an increased risk of health problems such as breast cancer, according to the <a href="http://www.netreturns.biz/healthinfo/bouncelink.aspx?shortcut=cancersoc" target="_blank">American Cancer Society</a>.
A hormone boost
Treatment of an underlying condition that is causing low testosterone levels might resolve the problem. If not, then testosterone replacement therapy might be an option. The therapy can be used as a supplement when the body doesn't produce enough testosterone or to help jump-start puberty in boys who are late in starting naturally. 
Testosterone therapy comes in several forms, including injections, patches, tablets and a gel. 
These therapies may work well for some men who have a deficiency. 
Possible benefits of testosterone replacement therapy include: 
<ul>
 <li>Increased growth of body and facial hair. </li>
 <li>Increased interest in sex. </li>
 <li>Bigger and stronger muscles. </li>
 <li>Better mood and energy levels. </li>
 <li>Stronger bones. </li>
</ul>
Replacement risks
 
Certain men taking testosterone treatments may have a heightened risk of heart attack or stroke. 
Prostate cancer is another serious possible risk, which you and your doctor should discuss. Men with known or suspected prostate cancer or with breast cancer should not receive testosterone treatment. &nbsp;
 Other possible health risks from testosterone therapy include: 
<ul>
 <li>An increase in red blood cells. </li>
 <li>Acne. </li>
 <li>Breast enlargement. </li>
 <li>Sleep apnea. </li>
 <li>Fluid buildup in ankles, feet and legs. </li>
</ul>
The <a href="https://www.acponline.org/" target="_blank">American College of Physicians</a> (ACP) says men with age-related low testosterone should be prescribed testosterone only to treat sexual dysfunction, as evidence shows men may experience slight improvements in sexual function. On the other hand, there's little to no evidence that such treatment improves energy, vitality or other common aging concerns, according to the ACP.&nbsp;&nbsp;
Reviewed 11/8/2023

Men's health: Understanding low testosterone levels

A deficiency in testosterone can cause a variety of problems for men. But while treatment can help many men, it's not for everyone.