Everything You Need to Know About Testosterone

By Eric Trexler, PhD

Testosterone is receiving more attention than ever before. Between testosterone replacement therapy (TRT) clinics opening on every corner, online debates about "fake natties," and alarming headlines suggesting that male testosterone levels are in freefall, it can be hard to separate fact from fiction. Let's try to do exactly that.

What Are Normal Testosterone Levels?

For adult males, the commonly cited threshold for "low testosterone" (clinical hypogonadism) has historically been 300 ng/dL. But that number requires some nuance. The symptoms typically associated with low testosterone, such as reduced libido, fatigue, sexual dysfunction, irritability, and impaired mood or concentration, don't emerge at a universal threshold. Some men experience these symptoms at 320 ng/dL; others feel completely fine at 280 ng/dL. On top of that, total testosterone is also only part of the picture. Since sex hormone binding globulin and the ratio of free to bound testosterone influence how much biological "work" your testosterone is actually doing, total testosterone levels only give us a superficial glimpse at the total bioactivity of testosterone in your body.

Having addressed these nuances, we can still lean on some basic ranges to get our bearings. Average total testosterone levels in healthy adult males tend to land in the 400-800 ng/dL range, depending on their age (testosterone levels gradually drop throughout middle and late adulthood). Values below 300 ng/dL are generally considered low, and values above 1000 ng/dL are considered high (Figure 1). 

Figure 1. Total testosterone reference range for healthy adult males

Well, those are the thresholds we’ve used for the last several years, at least. If you’ve seen headlines saying that human testosterone levels are plummeting, you may be wondering (or worrying) that they’re all out of date by now.

Are Population-Level Testosterone Levels Falling?

Several studies have reported what are called "secular" declines in male testosterone, meaning today's 25-year-olds appear to have lower testosterone than 25-year-olds did several decades ago. This trend has generated a fair amount of alarm, with men scrambling to figure out what is tanking testosterone levels. But the explanation is almost certainly multifactorial – there’s no singular cause to blame or address.

First, assay-related measurement changes are a real contributor. When we get our blood tested, we tend to take the complexity of hormone measurement for granted. We assume that there is a singular “true” value, and our blood test will accurately identify that value without fail or ambiguity. But the reality of hormone estimation is messier. Two common methods for quantifying testosterone are immunoassays and liquid chromatography-tandem mass spectrometry (LC-MS/MS). Immunoassays use antibodies that bind to testosterone, like a chemical “lock and key,” and estimate how much is present based on that signal. In contrast, LC-MS/MS separates the components of the blood, then identifies testosterone based on its exact chemical structure. Because these approaches use different steps and detection processes, they can produce slightly different results from the same exact blood sample.

A 2025 paper by Arun and colleagues analyzed NHANES data spanning 2001 to 2016. These researchers found that a significant portion of the apparent testosterone drop over time coincided with a switch from immunoassays to LC-MS/MS. The two methods don't produce identical values (Figure 2), and the diagnostic threshold of 300 ng/dL was never recalibrated when labs made the switch. The Endocrine Society has since recommended a revised cutoff of 264 ng/dL for the modern LC-MS/MS measurement method. When applied, this new cutoff produces a similar prevalence of "low testosterone" to what was observed before the assay switch. In short: a meaningful chunk of the apparent decline in testosterone levels is a measurement artifact, and a lot of men who think they have low testosterone are simply being evaluated against an outdated reference range.

Figure 2. When the most common assay for measuring testosterone changed, a notable (but steady) drop in measured testosterone levels was observed. Figure credit: MASS Research Review (massresearchreview.com)

Second, lifestyle factors are clearly in the mix. When researchers statistically control for differences in BMI, physical activity, smoking, alcohol use, sleep, medications, and chronic health conditions, the magnitude of the secular decline in testosterone shrinks substantially, and even becomes non-significant in some studies. Excess fat mass, inactivity, sleep disruption, and chronic stress are hallmarks of modern life, and all of these factors impair testosterone production.

Third, environmental exposures may play a modest role. Endocrine-disrupting chemicals (such as phthalates, bisphenols, PFAS, and flame retardants) have become more prevalent in our environment over the last several decades. These chemicals can, in theory, interfere with hormone production, and there are plausible mechanisms for how they might contribute to testosterone reductions. But they're almost certainly not the whole story, and we still don’t know a ton about them. Given how preliminary the evidence is (and how little control we have over our environmental exposures), panicking about endocrine-disrupting chemicals isn't a particularly fruitful use of energy.

In summary, secular declines in male testosterone appear to be “real,” at least to some extent. However, the drop is relatively modest in magnitude, and the lion’s share of this drop seems related to measurement changes and modifiable health behaviors. If our species ends soon, it won’t be from insufficient testosterone levels.

What Can You Do to Support Healthy Testosterone Levels?

If you want to optimize your testosterone, the playbook is really boring, but also remarkably consistent with general health recommendations. Several pieces of common health advice can meaningfully support your testosterone levels.

Manage your body composition

Excess fat mass suppresses testosterone through two mechanisms: aromatase activity in fat tissue converts testosterone to estrogen, and chronically elevated leptin levels (a consequence of excess fat mass) interfere with testosterone production. However, being too lean is also problematic – just ask any natural bodybuilder what their testosterone levels are looking like a couple weeks before a big competition. For peak testosterone levels, you want to be lean enough, but not too lean.

Avoid excessive energy restriction and extreme macronutrient imbalances

Calorie restriction, fat restriction, and carbohydrate restriction have all been shown in meta-analyses to reduce testosterone levels. Low energy availability, which is often observed in athletes and avid exercisers who are underfueling, suppresses the entire hormone cascade responsible for testosterone production. For optimized testosterone levels, a moderate macronutrient distribution with sufficient overall energy intake is your best bet.

Get adequate sleep

Testosterone production is tightly coupled to sleep, so chronic sleep deprivation reliably suppresses testosterone. Whenever possible, aim for at least 7 hours per night to support testosterone levels.

Exercise regularly 

Physical activity is one of the most consistent lifestyle predictors of healthy testosterone levels. But you’re reading this article on RP’s website, so I won’t waste my time selling you on the idea of exercising.

Manage stress

Chronically elevated cortisol, which is a direct consequence of chronic stress, can suppress the hormone cascade responsible for testosterone production at every level. “Stress management” can take many forms – sometimes it involves avoiding stressors, but sometimes it involves using behavioral strategies to manage the way you react to stressors.

Optimize micronutrient status

Deficiencies in micronutrients like vitamin D, zinc, and magnesium can hinder testosterone production. If persistent dietary gaps are impacting your testosterone levels, correcting these deficiencies may restore levels toward normal.

Minimize alcohol

I like a good beer or whiskey as much as the next guy, but there’s no getting around it: alcohol isn’t doing your testosterone levels any favors. A few drinks a week won’t move the needle, but you probably don’t want to make alcohol a daily ritual, and you certainly don’t want to be having multiple drinks per day if testosterone optimization is high on your list of priorities.

What about supplements marketed as testosterone boosters? 

The history here is not encouraging. With few exceptions, herbal “test boosters” have generally cycled through a consistent arc: a small, preliminary study generates excitement, marketing hype follows, then replication attempts fail to confirm consistent, meaningful effects on testosterone levels. That said, if your testosterone is low because of a correctable deficiency, addressing that root cause may genuinely restore levels. This goes for both lifestyle factors (like sleep) or dietary factors (like energy intake or specific micronutrients). For example, zinc isn’t necessarily a “test booster,” but it can certainly play a role in restoring normal testosterone levels for someone who is chronically deficient.

How Much Would This Actually Impact Muscle Growth?

This is probably the biggest misconception about testosterone in the fitness world, so let’s dig into the evidence.

Testosterone does correlate with lean mass in men. A 2025 cross-sectional study analyzing data from over 2,300 men found that testosterone levels were positively associated with BMI-adjusted appendicular lean mass, and an approximately linear relationship was observed between these two variables. Men in the highest testosterone quartile had roughly 2.3 kg more lean mass than those in the lowest quartile. That’s not nothing, but it’s a small difference in lean mass from a large difference in testosterone levels.

Testosterone appears to have its largest influence on baseline fat-free mass levels – how much muscle you have at rest, all else equal – rather than how much muscle you gain in response to training. A study on prostate cancer patients undergoing androgen deprivation therapy powerfully demonstrates this point. Half of the participants had their testosterone levels totally depleted as part of their treatment for prostate cancer, and the other half did not. At rest, the untreated group (with normal testosterone levels) had higher rates of muscle protein synthesis (MPS) and larger increases in MPS when they consumed protein. But when both groups lifted weights, this gap in MPS essentially disappeared. In other words, testosterone has an impact on determining your baseline level of muscle, but resistance training provides a potent stimulus for protein synthesis and muscle growth – even when testosterone is quite low.

The best evidence to highlight the direct impact of varying testosterone levels is a study by Bhasin et al (2001). Participants were assigned weekly testosterone doses ranging from 25 to 600 mg, while endogenous production was fully suppressed by a pharmaceutical drug. As a result, blood testosterone levels were entirely determined by the assigned dose. Over 20 weeks, participants followed a standardized diet (1.2 g/kg/day protein) and avoided structured exercise. Body composition was assessed via underwater weighing and DXA (underwater values reported). As shown in Table 1, a massive jump in testosterone from the middle of the normal range (570 ng/dL) to well above the upper limit (1,345 ng/dL) was good for an extra 1.8kg of fat free mass. Bear in mind that only a portion of that extra mass is likely to be actual muscle tissue.

Table 1. Testosterone levels and fat-free mass changes reported by Bhasin et al (2021). Table credit: MASS Research Review (massresearchreview.com).

The practical implication: moving from the lower end to the upper end of the normal testosterone range is associated with a pretty small increase in lean mass – typically only 1-3 kg or so, depending on the specific study you look at. This type of increase in testosterone levels can be meaningful if you're experiencing symptoms of low testosterone, but it won't transform your physique.

The Pros and Cons of TRT

TRT has become a major topic of discussion in recent years. Some people dramatically exaggerate the benefits, while others catastrophize the risks. The truth, as is typically the case, rests somewhere in the middle.

What TRT can reasonably do

Properly administered TRT moves testosterone from a below-normal level into the normal physiological range. This can attenuate or reverse the symptoms of hypogonadism, such as low libido, sexual dysfunction, fatigue, mood disturbances, and poor sleep. For someone who is genuinely hypogonadal, this can meaningfully improve quality of life. In terms of one’s physique or muscularity, this can restore a person with clinically low testosterone to roughly the same "baseline muscularity" of someone in the median testosterone range. But that’s typically only a few kilograms of lean mass at most. People's physiques don't transform when they go on legit, medically supervised TRT that keeps them squarely within the normal testosterone range. In most cases, the impact on their physique or muscularity is virtually undetectable to the naked eye.

What about safety? 

If you read chatter about TRT on social media, you’ll see people talking about all sorts of catastrophic side effects like wrecked blood lipids, cardiac hypertrophy, and "roid rage.” For legitimate TRT doses aimed at restoring normal physiological levels, these worries are largely unfounded. TRT is generally safe when properly prescribed and monitored by a qualified medical professional. Risks and side effects that are genuinely worth monitoring include elevated hematocrit, potential acceleration of prostate issues in those with existing conditions, excessive aromatization to estradiol, and the acceleration of male-pattern baldness in genetically predisposed individuals. A competent clinician prescribing TRT will monitor signs, symptoms, and biomarkers relevant to these concerns and maintain continuous discussion about them throughout treatment. These issues can generally be managed or mitigated by medications, dosage adjustments, or cessation of TRT treatment (as appropriate).

A major caveat

Much of what gets called "TRT" online isn’t actually TRT. True TRT uses conservative doses to keep testosterone squarely within the physiological range. If you’re wondering how to spot a TRT user on social media, you really shouldn’t be able to tell, particularly throughout young or middle adulthood. That’s kind of the point – restoring normal testosterone levels should have you looking pretty typical (relative to your training and diet habits, of course). High-dose testosterone administration that drives levels above the normal range is steroid use, not TRT. To be clear, I have no personal issue with steroid use, provided that it’s being implemented safely, ethically, and with competent medical supervision by someone who understands the potential risks. My point is that we shouldn’t conflate moderate-dose steroid use with TRT, as they’re fundamentally different interventions.

Putting It Together

The behaviors that support healthy testosterone production (adequate sleep, regular exercise, moderate body composition, sufficient energy and micronutrient intake, stress management, and minimal alcohol intake) are also the behaviors associated with optimal health across almost every other domain you could measure. So, you don't need to frame these choices as "testosterone optimization strategies" to justify making them. Testosterone optimization is often an unintended “side effect” of engaging in health-promoting behaviors.

Well-formulated testosterone support supplements may move the needle a little bit for some people with low testosterone, but this entirely depends on why testosterone was low in the first place. If the problem is a correctable deficiency, addressing that deficiency (via supplementation or other changes to one’s diet or lifestyle) can restore normal levels. The effect size, however, will typically be small. You might go from below the clinical "low T" threshold to above it, which could meaningfully resolve symptoms like low energy levels or low libido. But you should not expect a dramatic change in your physique, training performance, or body composition as a result.

The decision to consider TRT is a medical choice that should be individualized based on the underlying reason for low testosterone, a thorough assessment of one’s health history and risk factors, and a clear understanding of what the intervention can and can't accomplish. Medically supervised TRT, when it's genuinely warranted, is generally safe and can meaningfully improve quality of life for someone who is symptomatic. But the benefits are far more modest than the hype would suggest, which is probably why some studies show that only about 15% of hypogonadal patients are still taking their TRT medication one year after initiating treatment. If your testosterone wasn't clinically low to begin with, the returns are even smaller.

In summary, the testosterone research gives us a very clear roadmap to testosterone optimization. Sleep well, train hard, eat a balanced diet, maintain a healthy body weight, and see your doctor if you're symptomatic. That’s about all there is to it.

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Eric Trexler, PhD
IG: @trexlerfitness