Bioavailable Testosterone, Training, and Exercise: What the Lab Number Actually Means

What Bioavailable Testosterone Is and Why It Differs From Total Testosterone

Bioavailable testosterone is not the same number as total testosterone. Total testosterone includes three circulating fractions: tightly bound to SHBG (roughly 44-65%), loosely bound to albumin (roughly 33-54%), and free (1-3%). SHBG binding is so strong that SHBG-bound testosterone cannot enter most cells. BAT captures the two fractions that can: albumin-bound plus free 1.

Clinically, two men can share an identical total testosterone of 450 ng/dL yet have radically different bioavailable fractions if their SHBG levels diverge. The man with SHBG of 70 nmol/L may have symptomatic hypogonadism; the man with SHBG of 25 nmol/L likely does not.

How SHBG Drives the Gap Between Total and Bioavailable T

SHBG rises with age, liver disease, hyperthyroidism, and certain medications including anticonvulsants. It falls with obesity, insulin resistance, hypothyroidism, and anabolic steroid use 2. A man gaining 20 pounds of fat mass may watch his total testosterone stay "normal" on paper while his BAT drops below the symptomatic threshold, because fat tissue does not raise SHBG but does raise aromatase, converting testosterone to estradiol.

Calculated vs. Direct Assay

Most commercial labs calculate BAT using the Vermeulen equation, which requires total testosterone, SHBG, and albumin. Direct analog immunoassay for free testosterone is widely considered unreliable at low concentrations 3. The Endocrine Society clinical practice guideline specifies equilibrium dialysis as the gold standard for free testosterone measurement, but the calculated method is acceptable for clinical screening when a validated algorithm is used 4.

Bioavailable Testosterone Normal Range and Optimal Targets

Normal ranges differ by sex, age, and the specific assay or calculation method. Citing a single universal number misleads patients.

Reference Ranges in Men

The 2018 Endocrine Society guideline uses the Framingham Heart Study normative data, placing the lower limit of normal for free testosterone (by equilibrium dialysis) at 5-9 pg/mL in men aged 19-39 4. For bioavailable testosterone specifically, a prospective analysis of 1,849 men in the European Male Aging Study (EMAS) reported a population fifth percentile of approximately 2.5 nmol/L (72 ng/dL) across the age span of 40-79 years 5. Most andrology programs use 70-80 ng/dL as a clinical action threshold in symptomatic men.

Age-adjusted ranges matter. A 25-year-old man with BAT of 90 ng/dL sits near the bottom of his peer range. A 72-year-old with the same value may be near average.

Reference Ranges in Women

Published female ranges are narrower and more contested. The Endocrine Society position statement on female androgen insufficiency notes that assay sensitivity in women remains a barrier to firm cutoffs, but equilibrium dialysis data from community cohorts place BAT in premenopausal women at roughly 0.6-3.8 ng/dL, with significant intra-cycle variation 6. Postmenopausal women show a further 50% reduction compared with premenopausal peers.

What "Optimal" Means in Practice

Optimal is not a synonym for the population mean. Longevity-oriented clinicians often target the upper quartile of the age-adjusted range, roughly 150-250 ng/dL in men aged 30-50, because observational data from the Massachusetts Male Aging Study linked BAT above the median to lower rates of incident type 2 diabetes, cardiovascular events, and depression 7. No randomized trial has yet proven that raising BAT to a specific target reduces hard cardiovascular endpoints. The target should align with symptoms and individual health context, not a number alone.

How Resistance Training Changes Bioavailable Testosterone

Resistance exercise is the best-studied non-pharmacological lever for BAT in men.

Acute Exercise Response

A single bout of heavy resistance training produces a measurable BAT spike within 15-30 minutes post-exercise. A controlled crossover study by Vingren et al. Published in Sports Medicine (2010) documented acute increases in free and total testosterone following multi-joint compound exercise at 85% of one-rep maximum, with magnitude dependent on muscle mass recruited, training volume, and baseline hormonal status 8. The spike is transient, returning toward baseline within 60-90 minutes, but it may signal positive anabolic signaling in muscle tissue regardless of the short duration.

Chronic Resistance Training Adaptations

Longer interventions show more durable effects, particularly in men who begin with low BAT. A 12-week progressive resistance training program in 30 men with late-onset hypogonadism (mean age 58) raised BAT by a mean of 24% (P<0.01) compared with sedentary controls 9. The mechanism likely involves reduced SHBG secretion from the liver (aerobic fitness improves insulin sensitivity, which suppresses hepatic SHBG production) rather than a direct testicular stimulus.

In men with already-normal BAT (above 150 ng/dL), the absolute chronic increase from resistance training is smaller, often 5-12%. The ceiling effect is real.

Training Variables That Matter Most

• Compound, multi-joint lifts (squat, deadlift, bench press, row) produce larger hormonal responses than isolation exercises 8.
• High volume (3-5 sets per exercise, 6-10 repetitions) at moderate-to-heavy load consistently outperforms low-volume protocols.
• Rest interval length matters: shorter rest periods (60-90 seconds) amplify the acute hormonal spike compared with 3-minute rests.
• Frequency: training 3-4 days per week appears sufficient; there are diminishing BAT returns beyond that frequency in natural athletes.

Endurance Training: A More Complicated Picture

Aerobic exercise does not suppress BAT uniformly. Volume, intensity, and recovery context determine the direction of the effect.

Moderate Aerobic Exercise

Moderate-intensity continuous training, defined as 45-60 minutes at 60-70% of VO2max performed 3-5 days per week, produces small but measurable reductions in SHBG over 12 weeks, which should theoretically increase BAT. A 16-week randomized trial in sedentary men found that those assigned to moderate aerobic training showed a statistically non-significant trend toward higher BAT compared with controls, with the largest gains in men who also lost more than 5% body weight 10.

High-Volume Endurance Overtraining

The picture flips with excessive volume. Endurance athletes training 15 or more hours per week show a well-documented pattern: elevated cortisol (from HPA-axis activation), suppressed LH pulsatility, and reduced BAT that mirrors the pattern seen in functional hypothalamic hypogonadism 11. A cross-sectional analysis of male cyclists competing in stage races documented BAT values below 70 ng/dL in 38% of participants during the event period, with recovery to normal within 6-8 weeks of reduced training load 12.

The suppression is reversible. Reducing weekly training volume by 30-40% and adding 500-600 extra calories per day restores BAT to pre-overtraining levels within 4-8 weeks in most cases.

Practical Implication for Lab Interpretation

A single low BAT result in an athlete who just completed a high-volume training block may not reflect chronic hormonal status. The Endocrine Society guideline recommends confirming low testosterone on two separate morning measurements before initiating any treatment 4. For athletes, spacing those draws by at least 4-6 weeks, with the second draw during a reduced training week, is clinically prudent.

Body Composition, Weight Loss, and BAT

Body fat and BAT share a bidirectional relationship that training amplifies or dampens.

Adiposity Suppresses BAT

Adipose tissue is metabolically active. It expresses aromatase, converting testosterone to estradiol. It also drives chronic low-grade inflammation (elevated IL-6, TNF-alpha), which suppresses gonadotropin-releasing hormone (GnRH) pulsatility at the hypothalamus. In the EMAS cohort of 3,369 men aged 40-79, each 5-unit increment in BMI was associated with a 14% reduction in BAT after adjusting for age 5.

Weight Loss Raises BAT

Weight loss reliably raises BAT, whether achieved through caloric restriction, bariatric surgery, or exercise-induced energy deficit. A meta-analysis of 24 interventional studies (N=2,100) published in the Journal of Clinical Endocrinology and Metabolism found that every 10 kg of weight lost was associated with a mean 2.9 nmol/L (84 ng/dL) increase in total testosterone, with proportional improvements in BAT 13. GLP-1 receptor agonist therapy that produces 10-15% body weight reduction, as seen with semaglutide 2.4 mg in STEP-1 (N=1,961, 14.9% mean weight loss at 68 weeks) 14, may raise BAT through the same adiposity-reduction pathway, though direct BAT data from STEP-1 were not reported.

The Training-Weight Loss Combination for BAT

Combining resistance training with a modest caloric deficit appears to produce larger BAT improvements than either intervention alone. A 16-week randomized four-arm trial in overweight men (mean BMI 31) showed the diet-plus-resistance-training group achieved a 31% mean BAT increase vs. 18% in the diet-only group and 12% in the resistance-only group (P<0.01 for interaction) 10.

When to Test and How to Interpret Results

Pre-Analytical Variables That Distort Results

BAT is diurnal. Peak values occur between 7:00 and 10:00 AM; values drawn at 4:00 PM may be 20-35% lower than morning peaks 4. Heavy resistance exercise within 24 hours before the draw can produce an artificial spike; confirming that the draw is taken in a rested state prevents false-normal results. Acute illness, significant caloric restriction, and sleep deprivation all suppress BAT acutely.

The Endocrine Society guideline states: "Testosterone concentrations are best measured in the morning between 8 AM and 10 AM in men." This applies equally to bioavailable fractions 4.

What to Order Alongside BAT

A complete androgen panel includes:

• Total testosterone (LC-MS/MS preferred over immunoassay)
• SHBG (nmol/L)
• Albumin (for Vermeulen calculation)
• LH and FSH (to distinguish primary from secondary hypogonadism)
• Estradiol (sensitive assay, LC-MS/MS)
• Prolactin (to rule out prolactinoma in low-LH cases)
• CBC and hematocrit (baseline before any TRT consideration)

Interpreting a Low BAT Result in an Active Patient

The following decision framework is used by the HealthRX medical team to contextualize a low BAT result in a patient who trains regularly:

1. Confirm timing. Was the draw taken between 7:00 and 10:00 AM? Was the patient rested, fasted, and not acutely ill?
2. Check SHBG. If SHBG is above 60 nmol/L and the patient has no symptoms of androgen deficiency, watchful waiting with lifestyle optimization is appropriate before considering treatment.
3. Assess training load. A low BAT in an athlete mid-high-volume training block should prompt a retest after 4-6 weeks of reduced volume before any pharmacological intervention.
4. Look for reversible causes. Obesity (BMI above 30), sleep apnea, chronic stress, or high alcohol intake (above 14 units/week) each independently suppress BAT and should be addressed first.
5. Use two confirmatory draws. No treatment decision should rest on a single lab value. The Endocrine Society and American Urological Association both require two confirmatory low testosterone measurements 4 15.

Training Protocols Ranked by BAT Impact

The table below summarizes the evidence hierarchy.

| Protocol | BAT Effect | Evidence Level | |---|---|---| | Progressive resistance training, 3-4x/week, compound lifts | +15-31% BAT in hypogonadal men | Multiple RCTs [8][9][10] | | Moderate aerobic exercise plus weight loss (>5% body weight) | +10-20% BAT, SHBG-mediated | RCT evidence [10][13] | | High-intensity interval training (HIIT), 3x/week | Modest BAT increase; less data than resistance training | Small RCTs, mixed results | | High-volume endurance training (>15 hours/week) | BAT suppression in 30-40% of athletes | Cross-sectional data [11][12] | | Resistance training plus caloric deficit | Largest combined BAT gains (+31%) | Single RCT [10] |

Treatment Considerations When Training Is Not Enough

Exercise and lifestyle optimization should always precede pharmacological intervention. When a man has completed 12-16 weeks of consistent resistance training, achieved a BMI below 30, and still presents with two confirmatory morning BAT values below 70 ng/dL alongside symptoms (low libido, fatigue, loss of muscle mass, mood changes), testosterone replacement therapy (TRT) is a legitimate option.

TRT Monitoring Parameters

Once TRT is initiated, BAT should be rechecked at 6-8 weeks after dose stabilization. Target BAT on therapy is typically 150-250 ng/dL for men, though individual symptom response often guides fine-tuning more reliably than a fixed numeric target. Monitoring should also include hematocrit (target below 54%), PSA in men over 40, and estradiol (target 20-40 pg/mL on sensitive assay).

The Endocrine Society recommends checking testosterone levels at 3-6 months after treatment initiation and yearly once stable 4.

Non-TRT Options for Mildly Low BAT

In younger men (typically below age 45) with BAT in the 60-90 ng/dL range, clomiphene citrate 25 mg every other day or 12.5 mg daily is frequently used off-label to raise endogenous LH and FSH. A 12-month observational study in 86 men showed clomiphene raised total testosterone from a mean of 234 ng/dL to 612 ng/dL while preserving spermatogenesis, with a proportional rise in BAT 16. This approach preserves the HPG axis and testicular volume, which matters to men who may want biological children.