Bioavailable Testosterone: Drugs That Distort This Test

What Bioavailable Testosterone Actually Measures

Bioavailable testosterone is the sum of free testosterone plus albumin-bound testosterone. It excludes the roughly 44-66% of circulating testosterone locked tightly to sex hormone-binding globulin (SHBG), which your tissues cannot use. Endocrine Society clinical practice guidelines published in 2010 position bioavailable testosterone as the preferred index of androgenic activity when total testosterone results are ambiguous, particularly in older men, pregnant women, or patients on drugs that shift SHBG.

Why Total Testosterone Is Not Enough

Total testosterone lumps together all three fractions: SHBG-bound, albumin-bound, and free. A patient on oral estrogen therapy may show a normal or even elevated total testosterone while bioavailable testosterone is substantially suppressed, because estrogen drives SHBG production up in the liver. The inverse occurs with exogenous androgens: total testosterone might look acceptable, but SHBG drops, pushing more testosterone into the bioavailable pool.

How the Test Is Performed

Laboratories calculate bioavailable testosterone from three inputs: total testosterone, SHBG, and albumin (assumed constant at 4.3 g/dL). The most validated method uses the Sodergard equation or ammonium sulfate precipitation. A 2011 study in the Journal of Clinical Endocrinology and Metabolism confirmed that calculated bioavailable testosterone correlates well with the precipitation method when assay-specific total testosterone and SHBG values are used.

Direct immunoassays for free testosterone are notoriously unreliable, which is why most high-volume labs default to the calculated approach. This matters for drug interference: any drug that changes SHBG or albumin will change the calculated bioavailable result, even if the underlying testosterone secretion is unchanged.

Normal Bioavailable Testosterone Ranges

Reference intervals vary by lab method, age, and sex. The figures below reflect ammonium sulfate precipitation data from large reference populations. The Endocrine Society's 2018 testosterone therapy guidelines recommend that clinicians use assay-specific reference ranges rather than universal cut-offs.

Men

| Age | Approximate Bioavailable T Range | |-----|----------------------------------| | 20-49 yr | 131-682 ng/dL | | 50-69 yr | 61-394 ng/dL | | 70+ yr | 40-244 ng/dL |

Bioavailable testosterone declines roughly 1-2% per year after age 40 in men, a trajectory documented in the Massachusetts Male Aging Study across a cohort of 1,709 men followed for nine years. That longitudinal analysis showed SHBG rising simultaneously, compressing the bioavailable fraction faster than total testosterone alone would suggest.

Women

Premenopausal women typically run 10-100 ng/dL bioavailable testosterone. Values shift across the menstrual cycle and drop substantially after oophorectomy. Postmenopausal women on oral estrogen therapy show further suppression because, as described below, oral estrogens are the single strongest SHBG-raising agent in clinical use.

Drugs That Raise SHBG and Suppress Bioavailable Testosterone

When SHBG rises, more testosterone gets bound and less remains bioavailable. The lab result drops, which can mimic hypogonadism even if the gonads are producing testosterone normally. A review in Clinical Endocrinology identified the following drug classes as the most clinically significant SHBG elevators.

Oral Estrogens

Oral estradiol and conjugated equine estrogens undergo first-pass hepatic metabolism, which directly upregulates SHBG synthesis. Transdermal estradiol bypasses the liver and raises SHBG far less. A woman switching from 1 mg oral estradiol to a 0.05 mg/24-hr transdermal patch may see bioavailable testosterone rise 20-40% with no change in ovarian output.

Oral contraceptives containing ethinyl estradiol are the most potent SHBG drivers in common use. A 2005 study in the Journal of Sexual Medicine (N=124) found that women on combined oral contraceptives had SHBG levels four to seven times higher than non-users, with corresponding reductions in free and bioavailable testosterone.

Antiepileptic Drugs

Phenytoin and carbamazepine induce hepatic CYP enzymes and also directly stimulate SHBG production. Valproate, by contrast, tends to lower SHBG. A prospective study in Epilepsia documented significantly higher SHBG in men on phenytoin compared with untreated controls, with reduced free testosterone despite normal total testosterone values. Clinicians managing men with epilepsy should order bioavailable testosterone specifically rather than relying on total testosterone alone.

Thyroid Hormone at Supratherapeutic Doses

Levothyroxine at replacement doses has a modest effect on SHBG. Supratherapeutic dosing, or untreated hyperthyroidism, can double SHBG concentrations. A study published in Thyroid showed SHBG rising in parallel with free T4, returning toward baseline after dose reduction. Always check thyroid function alongside a bioavailable testosterone panel if clinical suspicion exists.

HIV Protease Inhibitors

Certain antiretrovirals, particularly ritonavir-boosted regimens, alter hepatic enzyme activity and shift SHBG. This makes bioavailable testosterone interpretation especially difficult in men living with HIV, who already face a higher prevalence of hypogonadism. An observational study in AIDS (N=280) found that total testosterone and free testosterone frequently diverged, recommending the free or bioavailable fraction for this population.

Drugs That Lower SHBG and Inflate Bioavailable Testosterone

SHBG-lowering agents push the equilibrium the other direction: more testosterone stays free and albumin-bound, raising bioavailable testosterone on the lab report. This can mask true hypogonadism or make androgen excess harder to detect.

Exogenous Androgens and Anabolic Steroids

Any administered androgen, including testosterone cypionate, testosterone enanthate, or oral anabolic steroids such as stanozolol, suppresses SHBG by 30-60% within weeks. A controlled crossover study in JCEM confirmed dose-dependent SHBG suppression with testosterone enanthate injections at 100 mg/week. Bioavailable testosterone therefore climbs steeply with exogenous androgen use, partly from the added testosterone itself and partly from the SHBG drop.

Glucocorticoids

Prednisone, dexamethasone, and other glucocorticoids suppress SHBG. A study in the Journal of Clinical Endocrinology and Metabolism showed a significant reduction in SHBG after just four days of high-dose dexamethasone. Patients on chronic steroid therapy for asthma, rheumatoid arthritis, or inflammatory bowel disease may show falsely elevated bioavailable testosterone. Paradoxically, the same glucocorticoids also suppress the hypothalamic-pituitary-gonadal axis, so true testosterone production may be low even as calculated bioavailable testosterone looks normal.

Insulin and Insulin-Sensitizing Agents

Hyperinsulinemia lowers SHBG. Both endogenous insulin excess (as in type 2 diabetes and metabolic syndrome) and exogenous insulin therapy suppress hepatic SHBG production. The Nurses Health Study data linked higher fasting insulin to lower SHBG across a large female cohort. Metformin, by improving insulin sensitivity, may modestly raise SHBG and therefore lower calculated bioavailable testosterone. In women with polycystic ovary syndrome (PCOS), metformin-induced SHBG changes can complicate serial bioavailable testosterone monitoring.

Progestins

Synthetic progestins with androgenic activity (norethindrone, levonorgestrel) lower SHBG. Less androgenic progestins such as drospirenone or micronized progesterone have a smaller effect. Clinicians titrating progestin therapy in transgender women or in postmenopausal HRT should note that switching progestin type may shift bioavailable testosterone without any change in testosterone production.

Danazol

Danazol, used for endometriosis and hereditary angioedema, drops SHBG substantially and has mild androgenic activity of its own. Bioavailable testosterone can appear strikingly high in women on danazol, confusing androgen-excess workups.

Assay-Level Interference: Drugs and Supplements That Corrupt the Measurement Itself

Some substances do not alter SHBG but interfere with the immunoassay or mass spectrometry steps used to measure total testosterone or SHBG.

Biotin (Vitamin B7)

Biotin at doses above 5 mg/day (common in hair-and-nail supplements and some multiple sclerosis protocols) competes with biotin-streptavidin capture chemistry used in many immunoassays. The FDA issued a safety communication in 2019 warning that biotin interference has caused both falsely high and falsely low hormone results depending on assay design. Because SHBG is often measured by the same immunoassay platforms, a high-biotin patient could have both total testosterone and SHBG distorted simultaneously, producing an unpredictable effect on calculated bioavailable testosterone.

Patients should stop biotin supplementation for at least 72 hours before hormone panels.

Spironolactone

Spironolactone is an aldosterone antagonist with anti-androgenic properties. It does not reliably change SHBG in most patients, but it competes with testosterone at the androgen receptor and cross-reacts in some older testosterone immunoassays. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) is less susceptible to this interference. A review in Steroids notes that immunoassay-based testosterone measurements may be unreliable in patients taking spironolactone at doses above 100 mg/day.

Clomiphene Citrate and SERMs

Clomiphene and tamoxifen raise LH and FSH, increasing endogenous testosterone production and raising bioavailable testosterone through a real biological effect rather than a measurement artifact. This is not interference per se, but it does mean bioavailable testosterone values on clomiphene do not reflect baseline gonadal function. A 2013 trial in BJU International (N=36) showed bioavailable testosterone rising by a mean of 89% after 3 months of clomiphene 25 mg every other day. If the clinical question is diagnosing primary or secondary hypogonadism, clomiphene should be stopped and a 5-half-life washout observed before testing.

How to Raise Bioavailable Testosterone

Bioavailable testosterone can be raised through both pharmacological and lifestyle approaches.

Pharmacological Options

Testosterone replacement therapy (TRT) with injectable testosterone enanthate or cypionate, transdermal gels, or pellets raises both total and bioavailable testosterone directly. The Endocrine Society's clinical practice guideline recommends initiating TRT when total testosterone is below 300 ng/dL on two morning measurements with consistent symptoms, and to confirm with free or bioavailable testosterone when SHBG may be altered.

Clomiphene citrate at 25-50 mg every other day stimulates pituitary LH and FSH release, raising endogenous testicular testosterone. This approach preserves fertility, which exogenous testosterone suppresses.

Anastrozole (1 mg three times weekly in some protocols) reduces aromatization of testosterone to estradiol, which secondarily lowers SHBG and raises bioavailable testosterone. Use should be guided by an estradiol measurement.

Lifestyle Modifications

Weight loss lowers insulin, which raises SHBG, which sounds paradoxical at first. Net bioavailable testosterone tends to rise with fat loss in overweight men because total testosterone production increases more than SHBG. A 2016 meta-analysis in Obesity Reviews covering 24 intervention studies found significant increases in total and free testosterone after structured weight-loss programs. Resistance training provides an additional, modest short-term boost in testosterone with some evidence of longer-term SHBG modulation.

How to Lower Bioavailable Testosterone

Reducing bioavailable testosterone is the goal in conditions such as PCOS, androgen-secreting tumors, and gender-affirming hormone therapy in transgender women.

Medical Therapies

Combined oral contraceptives raise SHBG sharply (as described above), lowering bioavailable testosterone by 40-60% in women with PCOS. A Cochrane review on pharmacological interventions for PCOS identifies combined oral contraceptives as first-line agents for hyperandrogenism management. Spironolactone at 50-200 mg/day provides anti-androgenic receptor blockade and modestly lowers androgen production.

Gonadotropin-releasing hormone (GnRH) agonists such as leuprolide suppress LH and FSH, dropping testosterone to castrate levels. This approach is used in prostate cancer and in transgender women seeking full gonadal suppression.

Monitoring After Intervention

Recheck bioavailable testosterone 6-8 weeks after starting any SHBG-altering drug before adjusting doses. Single-point measurements taken within the first two weeks of a new medication can be misleading because SHBG shifts lag behind drug initiation.

A Practical Framework for Ordering and Interpreting This Test

The following decision approach is used by the HealthRX clinical team when bioavailable testosterone results seem inconsistent with the patient's clinical picture.

Step 1. Reconcile the medication list first. Before interpreting any bioavailable testosterone value, confirm the patient is not on oral estrogens, antiepileptics, glucocorticoids, exogenous androgens, insulin, or high-dose biotin. Each of these shifts the result independently of gonadal function.

Step 2. Confirm the assay method. Ask the lab whether total testosterone was measured by LC-MS/MS or immunoassay, and whether SHBG was measured on the same platform. Immunoassay-based total testosterone has a coefficient of variation up to 15% in some platforms, which propagates into the calculated bioavailable result.

Step 3. Time the draw correctly. In men, draw between 7 a.m. And 10 a.m. Testosterone peaks in the morning and falls 20-30% by afternoon. The Endocrine Society guideline states: "Testosterone concentrations are highest in the morning in young men, and this diurnal variation diminishes with age but is still present." Source: JCEM 2010 Clinical Practice Guideline.

Step 4. Repeat before acting. A single low or high value alone does not justify treatment changes. The same 2010 Endocrine Society guideline specifies that a diagnosis of androgen deficiency requires at least two morning measurements.

Step 5. Wash out interfering agents when feasible. For drugs with short half-lives (biotin, prednisone short course), five half-lives typically means stopping 3-7 days before retesting. For oral contraceptives (ethinyl estradiol half-life roughly 24 hours but SHBG normalization takes 3-6 months), plan the washout period accordingly.

The Endocrine Society guidelines state: "We recommend against the routine measurement of free testosterone by direct immunoassays because of their inaccuracy." The bioavailable testosterone calculation remains the preferred alternative when SHBG is known to be abnormal. Endocrine Society 2010 Guideline, JCEM.

Special Populations Requiring Extra Caution

Older Men

SHBG rises approximately 1.2% per year after age 50. A man aged 75 with "normal" total testosterone of 350 ng/dL might have bioavailable testosterone well below the age-matched reference range. The European Male Ageing Study (N=3,369) showed that symptoms of androgen deficiency correlated more tightly with free testosterone than with total testosterone, supporting bioavailable T as the better clinical marker in this group.

Women With PCOS

The Androgen Excess and PCOS Society recommends measuring total testosterone and a calculated free or bioavailable testosterone in the initial PCOS androgen workup. Their 2006 position statement notes that SHBG is frequently low in PCOS due to hyperinsulinemia, making bioavailable testosterone an especially sensitive marker of androgen excess in this population.

Transgender Men on Testosterone Therapy

Exogenous testosterone suppresses SHBG. Bioavailable testosterone may reach or exceed male reference ranges within weeks of starting therapy even before total testosterone reaches target. Monitoring total testosterone alongside SHBG at 3 and 6 months after initiation, then annually, allows accurate calculation of the bioavailable fraction throughout transition.