Testosterone is a hormone, or a chemical that controls functions of the body. In men, testosterone produces many of the traits we associated with being male, including sperm production, building muscles and bone mass, and sex drive. However, as males age, their testosterone levels drops, which can cause a host of symptoms.

According to the National Institute of Health, low testosterone becomes a clinical issue when there is less than 300 nanograms of testosterone per deciliter of blood. Declining levels of testosterone associated with aging are called Andropause. However, if testosterone levels drop below this threshold, it can cause issues.

There are four main symptoms of low testosterone levels:

1. Declining Sexual Function

Testosterone regulates the male (and to an extent, the female) sex drive. So decreasing levels of testosterone tend to show up as decreases in sexual function. This can include poor sex drive and decreasing interest in sex. Dropping testosterone levels can also show up as erectile dysfunction, the inability to get or maintain an erection. While testosterone itself does not cause arousal directly, it primes certain parts of the brain to make it easier to achieve this state. Men with low testosterone may notice a decreased semen volume, or less sperm when they ejaculate.

2. Mood and Behavior Changes

Men suffering from low testosterone also suffer changes in their personality and temperament. Typically, they experience mood changes, which can manifest as irritability and depression. Low testosterone levels may also produce a lack of focus, energy and drive.

3. Hair Loss

Curiously, high levels of testosterone may produce male pattern baldness— at least earlier in life. However, men experiencing low testosterone levels may also suffer from suddenly thinning hair. This hair loss may affect body and facial hair, since testosterone promotes this secondary sexual characteristic.

4. Body Changes

Low testosterone can also cause larger-scale changes to a man’s body. Since testosterone helps develop muscle and bone mass, these processes tend to go awry when the hormone decreases. So a decrease in testosterone can mean decreased muscle and bone mass. Additionally, men suffering from low levels of testosterone may suffer from sudden weight gain.

If you’ve experienced these symptoms, you should consult a physician, since treatments do exist that can reverse these symptoms. Additionally, these symptoms could indicate a more serious problem requiring a physician’s expertise. In fact, the number of middle-aged men taking testosterone treatments (sometimes called androgen replacement therapies) has tripled in the past decade.

Testosterone Troubles

Unfortunately, one of the main treatments for low levels of testosterone have come under scrutiny due to concerns related to serious side effects. A peer-reviewed 2013 article in the Journal of the America Medical Association found that men with testosterone supplements faced an nearly 30% increase in their risk of heat attack and stroke. As a result, a class-action lawsuit investigation has been launched to explore the possibility of legal action against the manufacturers of various testosterone-boosting products.