Testosterone is crucial for building muscle and fueling your sex drive.

But receptors for the hormone actually exist throughout your body, from your brain to your bones to your blood vessels.

So if you’re low on T, the health consequences could extend far beyond the gym and the bedroom, says University of Washington endocrinologist Bradley Anawalt, M.D., a spokesperson for the Endocrine Society.

Related: 5 Easy Ways to Increase Your Testosterone

The eight symptoms that follow aren’t proof of low testosterone on their own. You’ll need two blood tests showing low levels—usually around 300 nanograms per declileter (ng/dL) or lower, depending on the lab—before your doctor makes an official low testosterone diagnosis, Dr. Anawalt says.

The good news is, if low T is truly to blame, many of the health side effects of low testosterone can be reversed, or at least improved, with testosterone therapy

Oh, and how does your sex drive work? Like this:

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1. Low testosterone symptom: Your sex drive disappears.

Perhaps the best-known, quickest, and most common effect of low T is low libido, says urologist Philip Werthman, M.D., director of the Center for Male Reproductive Medicine and Vasectomy Reversal in Los Angeles.

In fact, nearly every patient who comes to his office with known or suspected low T complains of a lack of sexual appetite. Besides wanting less sex, men with low T may also masturbate less and report fewer fantasies and erotic dreams.

Related: 7 Crazy Things That Happen When You Sleep

Brain areas involved with sexual desire, including the amygdala, are packed with testosterone receptors, says S. Adam Ramin, M.D., urologic surgeon and medical director of Urology Cancer Specialists in Los Angeles. 

The hormone fits inside them like a lock inside a key, lighting them up to arouse you. Without it, you’re missing a critical step in the turn-on process.

This lack of desire to have sex can cause problems with erections, though low T doesn’t directly affect the plumbing involved in getting or staying hard, Dr. Ramin says.

Related: The Men’s Health Guide To Erectile Dysfunction: What Causes It, How to Fix It, and How to Avoid It

2. Low testosterone symptom: Your muscles shrink.

Ample testosterone puts your body in an anabolic, or muscle-building, state by helping your body produce and assemble proteins that form the building blocks of lean mass.

When your testosterone levels drop, your body turns catabolic instead, breaking down muscle tissue instead of building it up, Dr. Werthman says. 

At first, you might notice that it’s tougher to push as much weight at the gym or build muscle, he notes. 

Related: What’s the Most Muscle You Can Gain?

And after a few weeks of low T, you can expect to lose muscle mass, Dr. Anawalt says.

In fact, in one Japanese study, men with low free testosterone levels—a measure of the amount of hormone available to bind to receptors—had double to triple the risk of muscle loss with aging as those with normal levels.

3. Low testosterone symptom: Your penis may get smaller, too.

Without a steady flow of testosterone, the tissues in your penis, scrotum, and testicles can atrophy, or shrivel, says Dr. Ramin.

As a result, your penis might lose length and girth. You may notice your balls shrink, too—they often shrivel to half the size and turn squishy instead of firm, he says. 

Related: Why Your Penis May Shorten 50 Percent In the Winter

Though testosterone replacement therapy won’t bring back your testicular volume, when it comes to your penis, the treatment “has a good chance of restoring its glory,” Dr. Ramin says. (In fact, testosterone therapy in boys with a micropenis can increase their size by up to an inch and a half, according to a study in the Indian Journal of Urology.)

4. Low testosterone symptom: Your belly grows.

Even as you lose size where it counts, you gain it where it hurts, Dr. Anawalt says. 

In one Australian study, men with prostate cancer gained 14 percent more body fat and 22 percent more visceral fat after one year of androgen deprivation therapy, a treatment which turns off testosterone’s effects.

Visceral fat is the deep abdominal fat that forms around your organs and increases your risk for diabetes and heart disease.

Related: METASHRED EXTREME—the Ultimate Fitness Program From Men’s Health Designed to Shred Body Fat and Max Out Muscle Growth At the Same Time

In men, low testosterone may increase the activity of an enzyme called lipoprotein lipase, the study authors suspect. This compound drives circulating building blocks called lipids into visceral fat cells, plumping them up.

5. Low testosterone symptom: Your memory falters.

Trouble with thinking and memory often occurs in men with low T, Dr. Werthman says. 

In 2015 study from Australia, men whose testosterone levels declined over 5 years also experienced a drop in scores on tests of their mental function and memory.

Related: 6 Easy Fixes You Can Do Today to Improve Your Memory

Besides the amygdala, areas of the brain important for memory and attention—such as the cerebrum—also have testosterone receptors. 

When there’s not enough of the hormone pumping in to those receptors, your brain cells may not be able to function as well, the study authors note.

6. Low testosterone symptom: Your mood tanks.

Some of the side effects of low testosterone—such as sexual dysfunction and weight gain—can bring on the blues. But there’s also evidence of a more direct effect of low testosterone on mood. 

According to research in the Endocrine Journal, 23 percent of young men with newly diagnosed low testosterone met the criteria for depression, compared to only 5 percent of young guys with normal levels of the hormone.

Related: Road Rage, Daydreaming, and 7 Other Strange Predictors Of Depression 

Empty testosterone receptors in brain areas linked to mood are likely responsible for your depressed state, Dr. Ramin says. 

What’s more, mood disorders like depression or anxiety can kick off a vicious cycle, he notes—depression can suppress your testicles’ ability to produce testosterone, worsening the problem.

7. Low testosterone symptom: Your bones weaken.

Bone is actually living tissue, constantly broken down and rebuilt, Dr. Ramin says.

When testosterone levels fall, your bone breaks down faster than your body can build it back up. 

Related: How Running and Lifting Make Your Bones Stronger

As a result, you’re at a higher risk of low bone density, osteoporosis, and fractures, Dr. Anawalt says. 

8. Low testosterone symptom: Your heart might be at risk.

The effect of testosterone levels on the risk of heart problems has stoked controversy among experts, Dr. Anawalt says. 

On one side, low levels of testosterone may be linked to heart problems. In fact, one study from the U.K. found men with low T had a greater risk of dying from heart disease than men with normal levels. 

This may be because testosterone can help open up blood vessels to the heart, allowing blood to flow more freely. 

But on the flip side, some studies have suggested that testosterone therapy—especially in older men or those with existing heart conditions—might increase the risk of heart attack or stroke. Experts think it may thicken the blood, making a clot more likely. 

Related: 30 Easy Ways to Save Your Heart Today

It seems that the extra testosterone might be raising levels too high, Dr. Anawalt says. 

So if you’re a candidate for testosterone therapy, make sure you talk to your doctor about the benefits and the risks, Dr. Werthman says. 

Your doctor may test your testosterone levels after you first start therapy or change dosages—or sometimes between shots, if you’re receiving treatment by injection—to make sure your levels aren’t surging too high, Dr. Anawalt says.