Quick answer: Total testosterone below 300 ng/dL is diagnosed as hypogonadism, but testosterone levels have declined by approximately 1% per year in Western men since the 1980s — meaning a 40-year-old today has significantly lower testosterone than a 40-year-old in 1990, independently of aging. Elevated cortisol, visceral fat, vitamin D deficiency, poor sleep, and zinc deficiency are the four most common reversible drivers I see clinically. Testosterone replacement therapy is appropriate for some patients, but most men with testosterone in the 250–400 ng/dL range have correctable upstream causes they haven’t addressed.
The narrative around testosterone in men has become oddly binary: either you’re fine, or you need testosterone replacement therapy (TRT). The middle ground — where the majority of symptomatic men actually are — gets almost no attention. Men with total testosterone of 280–380 ng/dL are told they are “within normal range” while experiencing fatigue, reduced muscle mass, low libido, depression, and brain fog. Their labs are technically “normal” but their testosterone is low compared to where they functionally should be for their age.
The secular decline data is important context: a 2007 analysis in the Journal of Clinical Endocrinology & Metabolism documented a population-level decline in male testosterone of approximately 1.2% per year from 1987 to 2004 — a decline that is not explained by aging or obesity alone. A 40-year-old man in 2024 has testosterone levels comparable to a 55-year-old man in 1987. Something environmental and behavioral is driving this, and the evidence points to sleep, stress, endocrine disruptors, vitamin D deficiency, and physical inactivity as the primary culprits.
Here is what actually drives low testosterone, how to test it properly, and what the evidence-based optimization protocol looks like before TRT. Welcome to The Private Practice. I am Dr. Tom.
Understanding Your Testosterone: What the Tests Actually Mean
Most physicians order only total testosterone, but this is an incomplete picture. Total testosterone includes testosterone bound to sex hormone-binding globulin (SHBG) — which is biologically inactive — and testosterone bound loosely to albumin, plus the small free fraction. What your cells can actually use is free testosterone plus albumin-bound testosterone (collectively: bioavailable testosterone). You can have a total testosterone of 500 ng/dL but a low free testosterone if your SHBG is elevated — a common pattern in men over 50 and in those with liver dysfunction or hyperthyroidism.
The Tests to Request
The complete testosterone panel I recommend, as outlined in my guide to how to read blood test results:
- Total testosterone (morning draw, 7–10 AM): Testosterone peaks in the morning. A 3 PM draw can be 20–30% lower than a morning draw in the same person — this accounts for many “borderline” diagnoses. Two morning draws on separate days are required to diagnose hypogonadism.
- Free testosterone (by equilibrium dialysis, not calculated): The calculated free testosterone formula is notoriously inaccurate. Equilibrium dialysis is the gold standard. Optimal free testosterone: 15–25 pg/mL for adult men.
- SHBG: Elevated SHBG (above 50 nmol/L) reduces bioavailable testosterone even when total T is normal. Common causes: low-carbohydrate diets, elevated estrogen, liver disease, hypothyroidism.
- LH and FSH: These distinguish primary hypogonadism (testes failing — high LH/FSH) from secondary hypogonadism (pituitary or hypothalamic failure — low LH/FSH). Low testosterone with low LH suggests a reversible cause (stress, illness, weight gain, sleep deprivation) rather than primary testicular failure.
- Estradiol (E2): Men aromatize testosterone to estradiol in adipose tissue. Elevated estradiol with low testosterone is a common pattern in men with visceral fat. It worsens the feedback loop — elevated estradiol suppresses the hypothalamic GnRH pulse that drives LH and testosterone production.
Reference ranges from the lab tests over 35 guide: total testosterone optimal range is 550–900 ng/dL for adult men (not the lab standard of 300–1000), with free testosterone 15–25 pg/mL.
The Four Most Common Reversible Causes of Low Testosterone
1. Elevated Cortisol and HPA Axis Stress
Cortisol and testosterone are physiological antagonists. Both are steroid hormones synthesized from the same precursor (pregnenolone), and under conditions of chronic stress, the metabolic pathway preferentially shifts toward cortisol production — a phenomenon sometimes called “pregnenolone steal,” though the mechanism is more complex than simple substrate competition. The more established mechanism is direct: cortisol suppresses GnRH pulse frequency from the hypothalamus, reducing LH secretion and downstream testosterone synthesis. Elevated cortisol also increases 5-alpha reductase activity, accelerating testosterone conversion to dihydrotestosterone (DHT), while simultaneously upregulating aromatase, increasing conversion to estradiol.
The sleep connection is particularly damaging: testosterone production is predominantly nocturnal, occurring during deep slow-wave sleep. The 2011 study in JAMA showing that one week of sleep restricted to 5 hours per night reduced testosterone by an average of 10–15% in healthy young men. Men who sleep less than 6 hours consistently have testosterone 15–25% lower than those sleeping 7–9 hours. The mechanism — cortisol suppressing nocturnal GnRH pulsation — is detailed in why cortisol prevents you from sleeping. Fixing sleep is not a soft recommendation — it is the highest-leverage single intervention for testosterone that does not involve a needle or prescription.
2. Vitamin D Deficiency
Vitamin D receptors are expressed in Leydig cells (the testosterone-producing cells in the testes), the pituitary, and the hypothalamus. A 2011 RCT in Hormone and Metabolic Research found that vitamin D supplementation (3,332 IU daily for 12 months) increased total testosterone by an average of 25% in men who were vitamin D deficient at baseline. This is one of the most reproducible and clinically significant supplement effects on testosterone in the literature. Observational data consistently shows that serum 25(OH)D correlates positively with total testosterone, with the association strongest below 20 ng/mL and plateauing around 50 ng/mL. Read my post on what your vitamin D level actually means — the 30 ng/mL threshold is inadequate, and most men are not supplementing enough to reach 50–80 ng/mL.
3. Zinc Deficiency
Zinc is a direct cofactor for the 17-beta-hydroxysteroid dehydrogenase enzyme that converts androstenedione to testosterone in Leydig cells. A 1996 study in Nutrition found that zinc restriction reduced testosterone by 75% in healthy young men over 20 weeks, and zinc supplementation in marginally deficient older men raised serum testosterone by 93% over 6 months. Zinc also inhibits aromatase — the enzyme that converts testosterone to estradiol — providing a second mechanism for testosterone optimization. Optimal zinc intake: 25–40 mg elemental zinc daily (as zinc picolinate or zinc citrate). Excessive zinc (above 40 mg daily long-term) depletes copper and should be balanced with 1–2 mg copper daily. Athletes, vegetarians, and frequent alcohol consumers are the highest-risk groups for zinc deficiency.
4. Visceral Fat and Aromatase Upregulation
Visceral adipose tissue is rich in aromatase — the enzyme that converts testosterone to estradiol. The more visceral fat you carry, the faster this conversion occurs. This creates a vicious cycle: low testosterone promotes visceral fat deposition (testosterone normally supports fat oxidation and muscle maintenance), visceral fat converts more testosterone to estradiol, estradiol further suppresses LH and testosterone production. The clinical pattern: man in his 40s with growing abdominal fat, fatigue, and low libido has total testosterone of 290 ng/dL, estradiol of 42 pg/mL (elevated for a male), and LH of 2.1 mIU/mL (suppressed). This is reversible with visceral fat reduction — as little as 10% body weight loss can restore testosterone by 50–100 ng/dL. The insulin resistance connection is relevant here: visceral fat drives insulin resistance, insulin resistance promotes visceral fat, and both suppress testosterone through overlapping mechanisms. See insulin resistance: why 40% of adults have it and don’t know it.
The Evidence-Based Testosterone Optimization Protocol
Before considering TRT, the following interventions have clear RCT evidence for improving testosterone. Each addresses a distinct mechanism and the effects are additive:
Sleep: 7–9 Hours of Quality Sleep
Testosterone production occurs predominantly during sleep, specifically during slow-wave sleep stages 3 and 4. Seven to 9 hours of consolidated, cortisol-suppressed sleep is the foundation. If sleep quality is poor — frequent awakenings, shallow sleep, high cortisol — fixing this is step one. See the full cortisol and sleep protocol for implementation. Magnesium glycinate (300–400 mg 1 hour before bed) is the highest-leverage single supplement for improving sleep quality specifically in the context of testosterone optimization.
Resistance Training
Resistance training acutely elevates testosterone for 15–30 minutes post-session and, with consistent training over months, increases resting testosterone by 15–25% in previously untrained men. The mechanism involves increased LH sensitivity in Leydig cells and reduced SHBG. The key variables: compound movements (squat, deadlift, bench press, rows), heavy loads (75–85% of 1-rep max), and sufficient volume (3–5 sets per movement). High-volume resistance training combined with Zone 2 aerobic training produces the best overall hormonal environment — resistance training for testosterone, Zone 2 for cortisol reduction and insulin sensitivity.
Supplement Stack
Vitamin D3 + K2: 4,000–6,000 IU D3 with 100 mcg K2 MK-7 daily. Target 25(OH)D of 60–80 ng/mL. This is the most evidence-backed supplement for testosterone in men with vitamin D deficiency (which is the majority). The K2 ensures calcium is directed to bone rather than arteries at high D3 doses.
Zinc picolinate: 25–30 mg with dinner. Take with food to reduce nausea. If supplementing for more than 8 weeks, add copper glycinate 1 mg to prevent zinc-induced copper depletion. Oysters remain the most zinc-dense food source by a substantial margin (74 mg per 3 oz), followed by red meat and pumpkin seeds.
Magnesium glycinate: 300–400 mg nightly. Magnesium reduces SHBG in an RCT published in Biological Trace Element Research, directly increasing free testosterone even when total testosterone remains unchanged. Combined with its sleep benefits, this is one of the highest-leverage single supplements for male hormonal optimization. See magnesium deficiency symptoms for the full protocol.
Ashwagandha KSM-66 extract: 300–600 mg twice daily. Multiple RCTs show ashwagandha increases serum testosterone by 10–22% and reduces cortisol by 20–30% in men with mild-to-moderate HPA axis stress. A 2019 Medicine study found that ashwagandha supplementation for 8 weeks increased testosterone by 14.7% and improved muscle strength outcomes compared to placebo. The cortisol-reducing effect is the likely primary mechanism — consistent with the cortisol-testosterone antagonism described above.
Boron: 10 mg daily. Boron reduces SHBG (freeing bound testosterone) and reduces estradiol by 39% in an RCT from Environmental Health Perspectives. Often overlooked, inexpensive, and consistently effective in the small but clean literature. Boron appears to work via inhibition of the SHBG-binding affinity for testosterone and competition at the aromatase binding site.
Dietary Framework
The dietary factors with the most consistent evidence for testosterone: adequate dietary fat (testosterone synthesis requires cholesterol — the extreme low-fat diet of the 1990s was the worst possible testosterone environment), adequate total calories (caloric restriction suppresses LH and testosterone within days, even in the absence of body weight change), and adequate protein for muscle maintenance (0.7–1 g/lb body weight). Ultra-processed food consumption is independently associated with lower testosterone in prospective studies, likely through endocrine-disrupting compounds, microbiome disruption, and the visceral fat-aromatase pathway.
When TRT Is Appropriate
Testosterone replacement therapy is appropriate when: total testosterone is consistently below 300 ng/dL on two morning draws, symptoms of hypogonadism are present and quality-of-life is meaningfully impaired, and reversible causes (sleep, stress, vitamin D, zinc, weight) have been properly addressed. TRT in the setting of untreated primary drivers is ineffective long-term — the same upstream problems that suppressed endogenous testosterone will blunt the response to exogenous testosterone and create new complications (erythrocytosis, testicular atrophy, infertility).
The forms with the best evidence: topical testosterone gel (daily application, smooth physiological levels), testosterone cypionate or enanthate injections (weekly to biweekly, requires monitoring for hematocrit and estradiol), and subcutaneous pellets (3-6 month implants, convenience but less adjustability). I do not recommend testosterone pellets as a first-line option because dose adjustment after implantation is not possible — getting the dose right requires more precision than pellets allow.
Frequently Asked Questions
What is a normal testosterone level for a man in his 40s?
Lab reference ranges (typically 300–1000 ng/dL) are population-derived and include many men with lifestyle-driven low testosterone. A more clinically meaningful target is 550–900 ng/dL total testosterone with free testosterone 15–25 pg/mL for a man in his 40s who is optimizing for health and function. Symptoms matter more than absolute numbers — a man with total T of 420 ng/dL who has no symptoms, good energy, normal libido, and normal body composition is not undertreated. A man with T of 380 ng/dL and significant symptoms who has correctable causes should correct them before considering TRT.
Does masturbation or sexual activity affect testosterone?
Transiently, yes — abstinence for 7 days is associated with a 45% spike in LH receptor expression and a small increase in testosterone in one study. But there is no evidence that normal sexual activity in either direction meaningfully affects long-term testosterone levels. The “nofap” claim that abstinence dramatically and permanently raises testosterone is not supported by the controlled literature. Focus on the reversible causes above — sleep, stress, vitamin D, zinc — rather than sexual activity modification.
How does testosterone relate to muscle loss and sarcopenia?
Testosterone is anabolic in skeletal muscle through androgen receptor-mediated protein synthesis upregulation and satellite cell activation. Low testosterone is one of the most significant drivers of sarcopenia (age-related muscle loss), alongside vitamin D deficiency, physical inactivity, and inadequate protein intake. Men who decline into the hypogonadal range (below 300 ng/dL) lose muscle at a substantially accelerated rate and respond poorly to resistance training. The connection between testosterone, muscle preservation, and aging is covered in why sarcopenia starts at 30 and how to stop it.
Can intermittent fasting lower testosterone?
Prolonged caloric restriction and aggressive fasting protocols can suppress LH and testosterone by reducing the metabolic signal to the hypothalamus. Short-term intermittent fasting (16:8 time-restricted eating without caloric deficit) does not appear to suppress testosterone and may modestly improve it by reducing visceral fat and improving insulin sensitivity. The caveat: if your fasting protocol is producing significant caloric restriction and you are losing weight rapidly, testosterone will fall. The full context is in intermittent fasting: what the science actually shows.
Is testosterone replacement therapy safe?
The TRAVERSE trial (2023), the largest RCT of TRT in men with hypogonadism and cardiovascular risk, found no significant difference in major cardiovascular events between TRT and placebo at 3 years — resolving the longstanding debate about cardiovascular safety. TRT does increase erythrocytosis (elevated red blood cell count) and suppresses spermatogenesis (relevant if fertility is desired). In appropriately selected men with true hypogonadism and reversible causes ruled out, TRT is safe and effective for improving quality of life, body composition, and bone density.
The Bottom Line
Testosterone levels have declined population-wide by roughly 1% per year for 40 years. Most men in the symptomatic low-normal range (280–400 ng/dL) have correctable upstream causes: poor sleep, chronic stress, vitamin D deficiency, zinc insufficiency, and visceral fat. Addressing these produces measurable testosterone improvement within 8–12 weeks — without the cost, inconvenience, or fertility implications of TRT.
The optimization protocol: fix sleep first (7–9 hours, address cortisol), add resistance training 3 times per week, supplement vitamin D3 to 60–80 ng/mL serum, zinc picolinate 25–30 mg with dinner, magnesium glycinate 300–400 mg nightly, and ashwagandha KSM-66 for HPA stress. If testosterone remains below 350 ng/dL after 12 weeks of this protocol, a complete hypogonadism workup with an endocrinologist or men’s health specialist is warranted.
I test all of this on myself first. That is the honest truth.
For personalized hormonal health evaluation and protocol design, reach out at health-consultation or browse the course library at health-courses.
Dive Deeper
- Low Testosterone in Men: Causes, Testing, and the Natural Optimization Protocol
- Testosterone Optimization in Men: Natural Protocol, Lab Targets, and TRT Guide
- Testosterone Optimization Naturally: Sleep, Training, Nutrition, and Supplements
- Low Testosterone in Men: Symptoms, Testing, and Natural Protocol
- Zone 2 Training: The Science-Backed Exercise for Longevity