Watch the whiteboard explainer (Part 1, the Causes):

Want to know the science? Keep reading below.

As a doctor, the fastest growing thing I see in men’s health is a guy who gets handed a testosterone prescription after one blood test. Nobody asked about his sleep, his weight, or his stress. Testosterone therapy is booming right now, and most of the men starting it were never really checked out first.

So we built a three part series to fix that. It’s long and detailed on purpose, because the honest version of this topic doesn’t fit in a 60 second video. Part 1 (this one) is the causes, or what’s actually pulling testosterone down. Part 2 is the workup, meaning the right lab tests, read the right way, and when a brain scan is worth it. Part 3 is the options, or who actually needs testosterone and what the full menu of treatments looks like.

Here’s the one idea behind all three: for most men, low testosterone is a symptom of something else, not the real problem.

Low testosterone is real. But in most men, it’s the result of something else going on, like extra weight, bad sleep, high stress, untreated diabetes, certain painkillers, or eating way too little. Doctors call this functional low testosterone: the testicles work fine, but the brain is telling them to slow down (Wu et al., N Engl J Med 2010).

The smart move, backed by the evidence, is to fix that upstream cause first. The most common move in men’s health right now is to skip that step completely.

Here’s the uncomfortable part: most of the stuff people blame on “low T” doesn’t actually predict low T.

Symptoms fall into two groups. The specific ones are about sex: lower sex drive, weaker erections, and losing your automatic morning erections. The vague ones that almost every tired 45 year old has, like low energy, brain fog, low mood, less muscle, and more belly fat, have many possible causes.

A big European study looked at thousands of men and was blunt about it: only the sex related symptoms consistently lined up with low testosterone. The vague stuff was basically background noise and can overlap with depression, sleep problems, thyroid issues, and a stressful life (Wu et al., N Engl J Med 2010). So if your only complaint is “I’m tired and want to feel better at the gym,” that’s a completely normal thing to want. But on its own, it is not a diagnosis.

Sleep moves testosterone more than almost any supplement on the shelf.

A national survey of American men found every hour of sleep lost was tied to lower testosterone, no matter their age or weight (Patel et al., World J Urol 2019). Cut healthy young men down to 5 hours a night for just one week, and their testosterone dropped 10–15% (Leproult & Van Cauter, JAMA 2011).

Why? Two reasons. First, most of your testosterone is made while you sleep, in little bursts of hormones from the brain. Less sleep means fewer bursts. Second, missing sleep raises cortisol, a stress hormone, and cortisol is one of the strongest testosterone suppressors.

Body weight is the single biggest thing you can change that affects testosterone. Most of the time the extra weight is causing the low testosterone, not the other way around (Eriksson et al., PLoS One 2017). Your body has a feedback system that keeps hormones from getting too low or too high, and obesity throws that system off.

Extra fat lowers testosterone in a few ways at once (Fui, Dupuis & Grossmann, Asian J Androl 2014). It changes a carrier protein in the blood called SHBG, so your total testosterone number looks low even when your usable testosterone is okay. This is called pseudo-hypogonadism which is basically normal biologically active testosterone with borderline low total testosterone levels. Up in the brain, leptin resistance (which comes with obesity) and low grade inflammation from fat cells reduce the brain’s release of an important hormone called GnRH, the signal that tells the testicles to make testosterone. And an enzyme in fat tissue called aromatase actively turns testosterone into estrogen, and that extra estrogen tricks the brain to dial testosterone production down even further.

Guidelines tell doctors to talk to every man with low testosterone about losing weight, and note you usually need to lose at least 5–10% of your body weight to see a real change (Mulhall et al., AUA Guideline, J Urol 2018). On average, dieting your way to weight loss raised testosterone by roughly 80 ng/dL, and weight loss surgery raised it far more, sometimes fixing the problem entirely (Corona et al., Eur J Endocrinol 2013). There’s usually nothing permanently broken here. It’s a weight problem pretending to be a hormone problem.

One honest catch: the gains match how much weight you actually keep off.

Stress lowers testosterone the same basic way, whether it’s mental stress, hard physical stress, or the stress of not eating enough. It turns down the brain’s signal. It doesn’t damage the testicles.

The clearest proof comes from the military. During Army Ranger training, with brutal physical demands on very little food, testosterone falls about 70–80% over eight weeks, all the way into the low range (Friedl et al., Med Sci Sports Exerc 2007). Then it comes roaring back once the men eat and rest, usually within weeks, sometimes even higher than where they started. The key detail: it wasn’t the exercise that crashed it, it was the food shortage that came with it. Train hard but eat enough, and your testosterone is fine.

Like we saw with sleep, chronic stress also raises cortisol, and cortisol is a powerful suppressor of testosterone production. You can even prove the testicles are still working fine. Give a stressed man a shot that stimulates the brain’s signal, and the testicles pump out testosterone normally. The problem is upstream, in the brain. The real fix is food, sleep, and stability, not an outside hormone.

This is exactly why the testing matters, and it’s the whole point of Part 2. Testosterone should be checked on two separate mornings, and a “low” number is often just that carrier protein (SHBG) playing tricks, which is why the usable (free) testosterone number is the one that really settles it. But that’s next week’s issue.

You almost certainly know a guy, a dad, a husband, a friend, who got put on testosterone after one blood test, and nobody ever asked about his sleep, his weight, or his stress. Please forward this to him. It’s exactly what he needs to walk into that appointment asking the right questions.

Next up, I’ll cover the workup: the exact labs to run, why a single low number can lie to you, and the one test almost no clinic orders correctly.

Wu et al., N Engl J Med 2010 · Patel et al., World J Urol 2019 · Leproult & Van Cauter, JAMA 2011 · Eriksson et al., PLoS One 2017 · Fui, Dupuis & Grossmann, Asian J Androl 2014 · Corona et al., Eur J Endocrinol 2013 · Mulhall et al., AUA Guideline, J Urol 2018 · Friedl et al., Med Sci Sports Exerc 2007 · Jayasena et al., Clin Endocrinol (Oxf) 2022