How Testosterone Is Made and Regulated

Like thyroid hormone, testosterone doesn't operate on its own — it's part of a feedback loop involving the brain and the glands that produce it. This system is called the hypothalamic-pituitary-gonadal axis (HPG axis), and it works the same way as the thyroid feedback loop described in Set 1: the brain monitors hormone levels and adjusts its signaling to keep them within a target range.

The HPG Axis — Testosterone Regulation
Hypothalamus releases GnRH (gonadotropin-releasing hormone)
signals ↓
Pituitary Gland releases LH and FSH
signals ↓
Testes produce testosterone and sperm
Rising testosterone feeds back to suppress GnRH and LH  ·  Falling testosterone releases that suppression, driving production back up

In men, testosterone is produced primarily in the testes, with a small contribution from the adrenal glands. The pituitary releases two hormones to control this: luteinizing hormone (LH), which directly stimulates testosterone production, and follicle-stimulating hormone (FSH), which supports sperm production. When testosterone levels rise, the brain dials back both signals. When levels fall, the signals increase.

This feedback system is why giving testosterone from outside the body suppresses the testes' own production — and why men on testosterone therapy typically have smaller testes and produce little or no sperm. The testes haven't failed; they've simply stopped receiving the signal to work because the brain sees enough testosterone in circulation and has turned off its request. This is important to understand before starting therapy, particularly for men who may want biological children in the future.

What Testosterone Actually Does

Testosterone has genuine, well-documented effects across multiple body systems. These aren't disputed — the question in clinical practice is whether any given patient has a deficiency significant enough to produce them, and whether replacing testosterone would reverse them.

Sexual function

Drives libido, supports morning erections and erectile function, and is required for normal sexual interest. This is the most testosterone-specific effect and the most reliably treated by replacement.

Muscle and strength

Stimulates muscle protein synthesis and lean body mass. Clinically significant deficiency reduces muscle mass and strength; replacement partially restores both.

Bone density

Maintains bone mineral density; men with very low testosterone have higher fracture risk. Replacement improves bone density in confirmed hypogonadism.

Red blood cell production

Stimulates the bone marrow to produce red blood cells. This is why testosterone raises hematocrit — a useful effect in anemia, a risk factor when hematocrit climbs too high.

Fat distribution

Influences where fat is stored. Low testosterone tends to shift fat toward the abdomen; replacement modestly improves body composition in true deficiency.

Mood and cognition

Has some influence on mood and energy, but the relationship is weak and nonspecific — many other factors overlap. Does not reliably improve cognition.

Total vs. Free Testosterone — and Why It Matters

Like thyroid hormone, most testosterone in the bloodstream is bound to proteins and biologically inactive. About 98% is attached to sex hormone-binding globulin (SHBG) or albumin; only about 2% is "free" and available to enter cells and do anything.

Total testosterone — what most standard lab tests measure — includes both bound and free. This is usually sufficient for diagnosis. But in certain situations, total testosterone can be misleading: obesity, diabetes, and hypothyroidism all lower SHBG, which lowers total testosterone while free testosterone remains normal. Conversely, high SHBG — seen with aging, liver disease, and hyperthyroidism — can make total testosterone look normal while free testosterone is actually low.

⚠ Timing Matters for Testosterone Testing Testosterone follows a daily rhythm — levels are highest in the early morning and drop significantly as the day progresses. The AUA requires at least two fasting, early-morning measurements before making a diagnosis of testosterone deficiency. A single afternoon level drawn casually can look low in men with perfectly normal testosterone, which is one of the ways unnecessary treatment gets started.[1]

How Testosterone Changes With Age

Testosterone levels do decline gradually with age — roughly 1–2% per year after age 30 in men. This is real and measurable. What's less clear is how much of this decline is clinically meaningful for most men. And the numbers here are striking.

19%
of men in their 60s have hypogonadal testosterone levels — yet most are asymptomatic [3]
28%
of men in their 70s have hypogonadal levels — most still asymptomatic [3]
49%
of men in their 80s have hypogonadal levels — and still, most remain asymptomatic [3]

These numbers make an important point: a low testosterone level is not the same as hypogonadism (testosterone deficiency syndrome). The diagnosis requires both a consistently low level and symptoms that are plausibly related to that deficiency. Age-related testosterone decline is part of normal aging, and most men with low-normal or even frankly low testosterone never develop the specific symptom complex that responds to treatment.

The Symptom Overlap Problem — Again

The symptoms most commonly attributed to low testosterone — fatigue, low energy, poor concentration, low mood, weight gain — are the same symptoms we discussed in the thyroid section, and for the same reason: they are nonspecific and driven by many more common causes. The evidence on testosterone is remarkably consistent on this point.

📊 The Evidence

The European Male Aging Study (EMAS) — a landmark study of over 3,000 middle-aged and older men — found that after adjusting for age, only three symptoms had a reliable syndromic relationship with low testosterone: poor morning erections, decreased libido, and erectile dysfunction.[2] Fatigue, low energy, depressed mood, and poor concentration showed weak or absent relationships with testosterone levels.[2]

The probability of experiencing fatigue was greater than 25% even in men with testosterone levels that were unequivocally normal.[4] Age-related testosterone decline frequently produces no symptoms at all — nearly half of men in their 80s have testosterone levels that meet the technical threshold for hypogonadism, yet most feel fine.[3]

Functional Hypogonadism: When Low T Isn't the Real Problem

A significant and often overlooked cause of low testosterone is not a problem with the testes or pituitary at all — it's something else entirely suppressing the system. Obesity, type 2 diabetes, sleep apnea, chronic illness, opioid use, and glucocorticoid medications can all lower testosterone levels, sometimes dramatically. This is called functional hypogonadism (or secondary hypogonadism due to systemic illness), and it's important to recognize because the right treatment is addressing the underlying cause — not prescribing testosterone.

📊 The Evidence Obesity and diabetes can lower total testosterone while free testosterone remains normal, because both conditions reduce SHBG. Testosterone levels often normalize with meaningful weight loss or treatment of sleep apnea — without any testosterone therapy.[2] Much of the dramatic increase in testosterone prescribing in the United States over the past two decades has been driven by men with nonspecific symptoms and low or low-normal testosterone attributable to obesity or other comorbidities rather than primary hypogonadism.[5]
My Synthesis The conversation I have most often about testosterone goes something like this: a man in his 40s or 50s comes in tired, not feeling like himself, maybe carrying more weight than he used to, and has read that low testosterone could explain it. Sometimes his levels are low-normal. Sometimes they're actually low. What I'm asking myself before reaching for a prescription is: why is testosterone low? If the answer is obesity, poor sleep, or untreated diabetes, then testosterone replacement is treating a symptom of the real problem rather than the problem itself — and it comes with its own consequences, including suppressing fertility and committing to an indefinite therapy. The specific symptoms that make me think low testosterone is genuinely causal are the sexual ones — reduced libido, poor morning erections, erectile changes. Fatigue alone, without those, almost never turns out to be a testosterone story.