Guide to Bioavailable Testosterone

WRITTEN BY

The Levels Team

Updated: 01/16/2026|16 min read

Summary

Bioavailable testosterone measures the portion of testosterone that is readily available for your body to use, including both free testosterone and testosterone loosely bound to albumin. This test provides a more complete picture of usable testosterone than free testosterone alone.

Why It Matters

Testosterone circulates in your blood in three forms: tightly bound to sex hormone-binding globulin (SHBG), loosely bound to albumin, and completely unbound (free). Understanding these distinctions is essential for accurately interpreting testosterone tests.

When most people think of testosterone testing, they're familiar with total testosterone, which measures all three forms combined. However, total testosterone doesn't tell you how much is actually available for your cells to use. The testosterone bound to SHBG is held so tightly that it can't easily enter cells to produce testosterone's effects.

Free testosterone—the completely unbound portion—represents only about 2-4 percent of total testosterone. While this is the most immediately active form, it's not the whole story of what's available to your body.

Bioavailable testosterone captures a more complete picture by measuring both free testosterone and the testosterone loosely bound to albumin. The albumin-bound testosterone can readily detach and enter cells when needed, making it functionally available even though it's not technically "free." Together, free and albumin-bound testosterone make up approximately 30-55 percent of total testosterone.

This distinction matters because someone might have normal total testosterone but low bioavailable testosterone if their SHBG levels are elevated, tying up too much testosterone in an unusable form. This can occur with aging, hyperthyroidism, liver disease, or certain medications. Conversely, someone might have low total testosterone but adequate bioavailable testosterone if their SHBG levels are low, as occurs with obesity, insulin resistance, or hypothyroidism.

Bioavailable testosterone testing is particularly valuable when symptoms of testosterone deficiency are present despite normal total testosterone, when SHBG levels are known to be abnormal, or when evaluating testosterone status in women, who have much lower levels that require more precise assessment of the available hormone.

By measuring what's actually usable rather than just what's present, bioavailable testosterone provides clinically relevant information that can guide treatment decisions and help explain symptoms that total testosterone alone might miss.

Associated Symptoms

Bioavailable testosterone levels themselves are laboratory findings rather than medical conditions. However, abnormal levels may be associated with certain symptoms that could indicate underlying health conditions.

In males, symptoms that may indicate low bioavailable testosterone include:

• Decreased sex drive and reduced sexual function (may indicate hypogonadism, age-related decline, or conditions affecting SHBG)
• Persistent fatigue and low energy (can occur in testosterone deficiency or other hormonal imbalances)
• Loss of muscle mass and increased body fat, particularly around the abdomen (may suggest long-term testosterone deficiency)
• Difficulty concentrating and memory issues (could indicate hormonal imbalances affecting cognitive function)
• Mood changes, including depression, irritability, or reduced motivation (may be associated with hypogonadism)
• Reduced body and facial hair growth (may be seen in more severe or prolonged testosterone deficiency)
• Decreased bone density leading to increased fracture risk (can result from long-term low testosterone)

In males, symptoms that may indicate high bioavailable testosterone include:

• Excessive sex drive and aggressive behavior (could suggest anabolic steroid use or, rarely, testosterone-producing tumors)
• Severe acne and very oily skin (can occur with steroid use or certain medical conditions)
• Early male pattern baldness in genetically predisposed individuals

In females, symptoms that may indicate low bioavailable testosterone include:

• Decreased sexual desire and reduced satisfaction (may suggest ovarian insufficiency or adrenal problems)
• Persistent fatigue and reduced energy (can occur in various hormonal conditions)
• Loss of muscle tone and strength (may be associated with aging, menopause, or hormonal imbalances)
• Reduced motivation and sense of well-being (could indicate overall hormonal changes)

In females, symptoms that may indicate high bioavailable testosterone include:

• Irregular or absent menstrual periods (potential sign of polycystic ovary syndrome or other hormonal imbalances)
• Excessive facial and body hair growth, particularly on the chin, upper lip, chest, or back (may suggest androgen excess from PCOS, adrenal disorders, or rarely tumors)
• Persistent acne, especially along the jawline and chin (can occur in PCOS and other conditions with androgen excess)
• Male-pattern hair loss or significant scalp hair thinning (may suggest substantial androgen excess)
• Deepening voice (can indicate more significant androgen excess requiring evaluation)
• Increased muscle mass and decreased breast size (may occur with significant testosterone elevation)

It's important to note that many of these symptoms can be caused by conditions unrelated to testosterone levels. The presence, severity, and combination of symptoms vary widely among individuals. A comprehensive evaluation, including hormone testing, physical examination, and consideration of other health factors, is necessary for an accurate diagnosis.

Clinical Ranges

Lab Reference Ranges:

Female:

• 18-69 Years: 0.5-8.5 ng/dL
• 70-89 Years: 0.5-7.0 ng/dL

Male:

• 18-69 Years: 130-680 ng/dL
• 70-89 Years: 110-590 ng/dL

Lifestyle Factors That Can Impact It

Activities that may increase bioavailable testosterone include:

• Maintaining healthy body weight, particularly for overweight individuals (excess body fat increases SHBG and reduces bioavailable testosterone)
• Resistance training and strength exercises (can help maintain testosterone production and reduce SHBG)
• Adequate sleep, particularly 7-9 hours per night (sleep deprivation can lower testosterone production)
• Stress management techniques (chronic stress elevates cortisol, which can suppress testosterone)
• Consuming adequate dietary fats (very low-fat diets can reduce testosterone production)
• Foods rich in zinc, such as oysters, meat, legumes, and nuts (zinc supports testosterone synthesis)
• Adequate vitamin D through sunlight exposure or supplementation (vitamin D deficiency may be associated with lower testosterone)

Activities that may decrease bioavailable testosterone include:

• Chronic excessive alcohol consumption (can damage testosterone-producing cells and alter hormone metabolism)
• Severe caloric restriction or crash dieting (can suppress the hormonal axis controlling testosterone production)
• Very low-fat diets providing less than 15-20 percent of calories from fat (fats are needed for hormone production)
• Chronic endurance exercise without adequate recovery (excessive training can suppress testosterone)
• Chronic sleep deprivation (disrupts hormonal balance and testosterone production)
• Chronic psychological stress (elevates cortisol, which can suppress testosterone production)

Other Factors That Can Impact It

Medical Conditions

• Type 2 diabetes and insulin resistance: Lower bioavailable testosterone in males through multiple mechanisms, including reduced production and lower SHBG, though the net effect is typically decreased bioavailable testosterone despite lower SHBG
• Metabolic syndrome: Decreases bioavailable testosterone through insulin resistance, inflammation, and increased aromatization of testosterone to estrogen in fat tissue
• Polycystic ovary syndrome (PCOS): Increases bioavailable testosterone in females due to ovarian overproduction of androgens and lower SHBG levels from insulin resistance
• Hypothyroidism: Can reduce bioavailable testosterone by decreasing both testosterone production and SHBG levels, though the effect on SHBG may partially preserve bioavailable testosterone
• Hyperthyroidism: Increases SHBG levels, which can reduce bioavailable testosterone despite normal or elevated total testosterone
• Pituitary disorders affecting gonadotropin production: Decreases bioavailable testosterone by reducing the hormonal signals that stimulate testosterone production
• Klinefelter syndrome: Reduces bioavailable testosterone through impaired testicular development and function in males with an extra X chromosome
• Turner syndrome: Decreases testosterone production due to ovarian dysgenesis in females with a missing or incomplete X chromosome
• Premature menopause or primary ovarian insufficiency: Reduces bioavailable testosterone due to loss of ovarian function
• Chronic liver disease: Can alter sex hormone metabolism and increase SHBG production, potentially reducing bioavailable testosterone despite affecting total levels
• Chronic kidney disease: Lowers testosterone through disrupted hormonal signaling and increased inflammation
• HIV/AIDS: Can decrease testosterone through direct effects of the virus, opportunistic infections, or certain medications
• Obstructive sleep apnea: May reduce testosterone production through sleep disruption and decreased oxygen levels

Medications

• Opioid pain medications: Significantly decrease bioavailable testosterone through suppression of the hypothalamic-pituitary-gonadal axis, even at therapeutic doses
• Glucocorticoids such as prednisone: Reduce bioavailable testosterone through suppression of pituitary hormone production that signals testosterone synthesis
• Hormonal contraceptives: Decrease bioavailable testosterone in females by suppressing ovarian production and significantly increasing SHBG levels
• Chemotherapy: Can reduce bioavailable testosterone through direct damage to testicular or ovarian tissue
• Anabolic steroids: Initially increase total and bioavailable testosterone from the exogenous source, but suppress natural production, often leading to very low levels after discontinuation
• Spironolactone: Decreases bioavailable testosterone by blocking androgen receptors and inhibiting testosterone synthesis
• 5-alpha reductase inhibitors like finasteride: Don't directly lower bioavailable testosterone but may affect its downstream conversion to more potent androgens
• Metformin: May help normalize bioavailable testosterone in females with PCOS by improving insulin sensitivity and reducing ovarian androgen production
• Antiepileptic medications: Certain anticonvulsants can affect sex hormone metabolism and SHBG levels

Supplements

You should always consult with a doctor or healthcare professional before using a supplement.

• Vitamin D: May support healthy testosterone levels in deficient individuals, though effects are modest
• Zinc: Could help maintain testosterone production in people with zinc deficiency
• Magnesium: May support testosterone production, particularly in people who exercise regularly
• D-aspartic acid: Some research suggests potential effects on testosterone, though evidence is mixed
• Fenugreek: May influence testosterone levels through various mechanisms, though more research is needed
• Ashwagandha: Could help reduce stress-related decreases in testosterone

Individual Factors

• Age: Bioavailable testosterone naturally declines with age in both males and females, though the rate varies considerably among individuals
• Body composition: Higher body fat percentage, particularly visceral fat, can decrease bioavailable testosterone through multiple mechanisms including increased conversion to estrogen and altered hormone metabolism
• Race and ethnicity: Some studies suggest variations in average testosterone levels across different populations, though individual variation is substantial
• Genetic factors: Inherited variations in genes affecting hormone production, metabolism, and receptor sensitivity can influence bioavailable testosterone levels

Testing Accuracy and Stability

Bioavailable testosterone testing provides valuable information but can be affected by various factors that are important to consider when interpreting results.

Factors That Can Affect the Accuracy of Your Test

• Testing method significantly impacts results. Bioavailable testosterone can be measured directly through a specialized technique called ammonium sulfate precipitation, or calculated from total testosterone, SHBG, and albumin levels. Calculated bioavailable testosterone is more commonly available but slightly less precise than direct measurement.
• Time of day dramatically affects results in males, with levels typically highest in the early morning (around 8 AM) and lowest in the evening. This diurnal variation is less pronounced in females and older males.
• Recent illness or acute stress can temporarily decrease testosterone production through stress hormone effects on the hypothalamic-pituitary-gonadal axis.
• Recent vigorous exercise within 24 hours of testing can either increase levels temporarily (short, intense exercise) or decrease levels (prolonged endurance exercise).
• Fasting status may affect results slightly, though the effect is generally less than with some other hormones.
• For females, testing during certain phases of the menstrual cycle could affect results, though bioavailable testosterone is relatively stable throughout the cycle compared to other hormones.
• Laboratory variation can occur between different testing facilities and methods, making it important to use the same laboratory for serial measurements when monitoring changes over time.

How It Relates to Other Markers

Other tests can provide insights about health status when viewed alongside bioavailable testosterone results. These tests may include:

• Total testosterone: Provides the complete picture of all testosterone in circulation. Comparing bioavailable testosterone to total testosterone helps identify whether binding proteins are affecting testosterone availability.
• Free testosterone: Measures only the unbound portion. Bioavailable testosterone includes free testosterone plus albumin-bound testosterone, providing a more complete picture of usable hormone.
• SHBG: Directly affects how much testosterone is bioavailable. High SHBG reduces bioavailable testosterone despite normal total testosterone, while low SHBG increases bioavailable testosterone.
• Albumin: This secondary binding protein affects the calculated bioavailable testosterone. Significant albumin abnormalities can affect interpretation.
• Luteinizing hormone (LH): Helps distinguish primary gonadal problems from pituitary or hypothalamic issues affecting testosterone production.
• Follicle-stimulating hormone (FSH): Important for evaluating reproductive function and distinguishing causes of testosterone abnormalities.
• Estradiol: The balance between testosterone and estrogen affects many bodily functions. In males, excessive conversion of testosterone to estrogen can occur with obesity.
• Prolactin: Elevated prolactin can suppress testosterone production and cause symptoms similar to low testosterone.
• Thyroid function tests (TSH, free T4): Thyroid disorders significantly affect SHBG and can alter the relationship between total and bioavailable testosterone.
• Hemoglobin A1c and fasting glucose: Insulin resistance and diabetes significantly affect testosterone levels and SHBG, particularly in males, and can elevate bioavailable testosterone in females.
• Liver function tests: Liver disease can affect sex hormone metabolism and SHBG production.

What Results May Mean in the Context of Other Markers

• Low bioavailable testosterone with low total testosterone and high LH/FSH: Could suggest primary testicular or ovarian failure—the gonads aren't producing adequate testosterone despite hormonal stimulation.
• Low bioavailable testosterone with low total testosterone and low/normal LH/FSH: May indicate secondary hypogonadism—a pituitary or hypothalamic problem failing to signal the gonads.
• Low bioavailable testosterone with normal total testosterone and high SHBG: May suggest functional testosterone deficiency where too much hormone is bound and unavailable, which can occur with hyperthyroidism, aging, or certain medications.
• Low bioavailable testosterone with normal total and free testosterone: Rare, but could indicate an albumin-related issue affecting the calculation, or suggest that the small free testosterone fraction alone isn't sufficient.
• High bioavailable testosterone with high total testosterone in females: May indicate PCOS, congenital adrenal hyperplasia, or rarely, testosterone-producing tumors.
• High bioavailable testosterone with normal total testosterone and low SHBG: May suggest functional testosterone excess due to increased availability, which can occur with insulin resistance, obesity, or hypothyroidism.
• Declining bioavailable testosterone with age despite stable total testosterone: May indicate age-related increases in SHBG, reducing testosterone availability.
• Low bioavailable testosterone with low SHBG and low-normal total testosterone: Could suggest that even though more is bioavailable percentage-wise, the total amount available is still insufficient, as can occur in severe obesity or insulin resistance.

Follow-up Considerations

If your bioavailable testosterone levels are abnormal, your healthcare provider may work with you on a plan to address any levels that are out of range. Here are some recommendations they might make. You should always speak to your doctor if you have medical questions or before making medical decisions.

When Re-Testing May be Appropriate

• Initial abnormal result: Often repeated to confirm findings, typically 2-4 weeks apart, with morning testing for males
• After starting testosterone therapy: Usually 2-3 months after initiation or dose change, then every 6-12 months once stable
• After implementing significant lifestyle changes: 3-6 months to assess impact
• During substantial weight loss: Every 3-6 months to monitor changes in both testosterone production and SHBG levels
• Monitoring known conditions: As directed by your provider, typically every 6-12 months
• With changing symptoms: As needed to reassess hormonal status

Additional Testing Your Doctor May Consider

• Complete thyroid panel including TSH, free T4, and possibly thyroid antibodies (thyroid function significantly affects SHBG)
• Hemoglobin A1c, fasting glucose, and insulin (to evaluate insulin resistance, which affects testosterone and SHBG)
• Comprehensive metabolic panel for liver and kidney function (these organs affect hormone metabolism)
• Lipid panel (testosterone levels correlate with metabolic health)
• Complete blood count (testosterone affects red blood cell production; anemia can affect testosterone)
• DHEA-S and 17-hydroxyprogesterone if adrenal disorders are suspected (particularly in females with elevated testosterone)
• Bone density scan if testosterone has been chronically low (testosterone is important for bone health)
• Semen analysis for males with fertility concerns
• Pelvic ultrasound for females with signs of PCOS or other ovarian abnormalities
• Sleep study if sleep apnea is suspected (can lower testosterone)
• Pituitary imaging if secondary hypogonadism is suspected

When Additional Care May be Warranted

• Bioavailable testosterone significantly outside reference range (more than 50 percent below or above normal)
• Symptoms of significant testosterone deficiency despite borderline laboratory values
• Infertility concerns; delayed or precocious puberty in adolescents
• Rapidly progressing symptoms of hormone imbalance
• In females, signs of virilization such as voice deepening or clitoral enlargement (requires prompt evaluation)
• Extremely high testosterone in females with rapid onset (possible tumor requiring urgent evaluation)
• Concerns about bone density or osteoporosis with low testosterone
• Cardiovascular risk factors combined with abnormal testosterone
• Development of adverse effects from testosterone therapy
• Suspected medication-induced hormonal changes requiring adjustment

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