SHBG: Drugs That Distort This Test and How to Interpret Results

What SHBG Is and Why It Matters for Hormone Testing

Sex hormone-binding globulin is a glycoprotein synthesized primarily by hepatocytes. It circulates in the bloodstream and binds testosterone with high affinity and estradiol with moderate affinity, acting as a buffer that determines how much of each hormone reaches target tissues in its unbound ("free") form [1]. Only about 1 to 3% of circulating testosterone is free. The rest is bound to SHBG (roughly 40 to 60%) or loosely attached to albumin [2].

Why Free Hormone Levels Depend on SHBG

When SHBG rises, more testosterone and estradiol get sequestered. Total hormone on a lab report may look adequate, but the biologically active fraction drops. The reverse is equally misleading: low SHBG frees up more hormone, so a patient with a "normal" total testosterone of 450 ng/dL may actually have supraphysiologic free testosterone if SHBG sits at 12 nmol/L.

What the Endocrine Society Says

The 2018 Endocrine Society Clinical Practice Guideline for testosterone therapy in men with hypogonadism states: "We recommend measuring SHBG when total testosterone concentrations are near the lower boundary of the normal range and in conditions known to influence SHBG" [3]. This recommendation exists because total testosterone alone misclassifies hypogonadal status in roughly 25 to 30% of borderline cases, according to the guideline's supporting evidence review.

The Liver Connection

Because SHBG is a hepatic export protein, anything that changes liver protein synthesis changes SHBG. Estrogen receptor activation in hepatocytes upregulates SHBG gene transcription. Insulin and IGF-1 suppress it. Thyroid hormone stimulates it. This hepatic sensitivity is exactly why so many medications distort the test [4].

Drugs That Raise SHBG

A medication that increases SHBG will bind more circulating testosterone and estradiol, lowering the free fractions. The clinical consequence: patients may develop symptoms of hormone deficiency (fatigue, low libido, mood changes) despite total hormone levels that appear normal on paper.

Oral Estrogens

Oral estradiol and conjugated equine estrogens produce the strongest SHBG elevation of any drug class. Because oral estrogens undergo first-pass hepatic metabolism, they expose the liver to supraphysiologic estrogen concentrations that directly activate SHBG gene transcription. A 2004 study published in the Journal of Clinical Endocrinology and Metabolism found that oral conjugated equine estrogens (0.625 mg/day) increased SHBG by 76% over 12 months, while transdermal estradiol (50 mcg/day) raised SHBG by only 6 to 12% [5]. This difference is the primary reason the Endocrine Society and the North American Menopause Society now prefer transdermal estrogen delivery in many clinical scenarios.

Combined oral contraceptives (COCs) containing ethinyl estradiol raise SHBG even more dramatically. A meta-analysis of 36 studies showed that COCs increased SHBG by 200 to 400% depending on the progestin component [6]. Pills with anti-androgenic progestins like drospirenone or cyproterone acetate produced the largest increases. This is clinically useful for treating hyperandrogenism in PCOS, but it also means that total testosterone measured during COC use is essentially uninterpretable without an SHBG value.

Anticonvulsants

Phenytoin, carbamazepine, and valproate all raise SHBG, though by different mechanisms. Phenytoin and carbamazepine are potent CYP3A4 inducers that accelerate sex steroid metabolism, triggering a compensatory rise in SHBG. One study in Epilepsia (N=89) reported that men on carbamazepine had SHBG levels 40 to 50% higher than untreated controls [7]. The clinical trap: total testosterone may remain normal, but free testosterone drops into the hypogonadal range.

Thyroid Hormones

Levothyroxine replacement in hypothyroid patients raises SHBG as thyroid function normalizes. Overt hyperthyroidism produces SHBG elevations of 2 to 3 times normal [8]. Clinicians starting or adjusting thyroid medication should recheck SHBG 6 to 8 weeks after a dose change, timed with the follow-up TSH draw, to avoid misinterpreting concurrent hormone panels.

Selective Estrogen Receptor Modulators

Tamoxifen and raloxifene act as estrogen agonists in the liver and raise SHBG by 30 to 80%. A 2009 study in the Journal of Clinical Oncology found that premenopausal women on tamoxifen 20 mg/day had a mean SHBG increase of 53% at 6 months [9]. For men prescribed tamoxifen off-label during testosterone therapy to manage estradiol, this SHBG rise partially offsets the benefit of increased total testosterone.

Drugs That Lower SHBG

Medications that suppress SHBG increase the free fraction of testosterone and estradiol. This can mask true hormone status in the opposite direction: total testosterone may read low or low-normal while free testosterone is actually adequate.

Exogenous Androgens and Anabolic Steroids

Testosterone replacement therapy itself lowers SHBG by 15 to 40%, depending on the route and dose. Injectable testosterone cypionate or enanthate produces larger SHBG drops than topical gels because of the higher peak serum levels [10]. Supraphysiologic androgen use (bodybuilding doses) can suppress SHBG below 10 nmol/L, making total testosterone values nearly meaningless for clinical assessment.

Danazol, a synthetic androgen used for endometriosis, is one of the most potent SHBG suppressors in clinical use. Doses of 200 to 800 mg/day reduce SHBG by 50 to 80% [11].

Insulin and Insulin-Sensitizing Agents

Hyperinsulinemia suppresses hepatic SHBG production directly. This is one reason women with PCOS (who typically have insulin resistance) present with low SHBG and clinical hyperandrogenism. The relationship is dose-dependent: a 2012 analysis in Diabetes Care (N=2,820 men from the EMAS cohort) found that each doubling of fasting insulin was associated with a 14% decrease in SHBG [12].

Metformin lowers insulin levels and may raise SHBG modestly as an indirect consequence, but studies show mixed results. A 2019 Cochrane review of metformin in PCOS found inconsistent SHBG changes across trials, with effect sizes ranging from no change to a 20% increase [13]. The effect depends heavily on baseline insulin resistance.

Glucocorticoids

Prednisone, dexamethasone, and other systemic corticosteroids lower SHBG by 20 to 30% within days of initiation. A small pharmacokinetic study (N=12) showed that dexamethasone 0.5 mg/day reduced SHBG by 26% at 5 days [14]. Patients on chronic glucocorticoid therapy may appear to have adequate free testosterone when their hypothalamic-pituitary-adrenal axis is actually suppressed and total testosterone is declining.

Progestins With Androgenic Activity

Norethindrone and levonorgestrel, both 19-nortestosterone derivatives, lower SHBG by 15 to 50% depending on dose. Medroxyprogesterone acetate (injectable or oral) shows a similar effect. The SHBG reduction is clinically relevant in women on progestin-only contraceptives: their total testosterone may appear normal, but signs of androgen excess (acne, hirsutism) emerge because free testosterone is elevated [15].

How to Interpret SHBG in the Context of Medication Use

The practical question for clinicians and patients is not just whether SHBG is high or low, but what it means for treatment decisions.

When to Order SHBG

The Endocrine Society 2018 guideline recommends SHBG measurement in specific scenarios: borderline total testosterone, obesity (BMI >30), suspected medication interference, and conditions like hyperthyroidism, liver disease, or HIV [3]. The American Association of Clinical Endocrinologists (AACE) 2020 consensus statement adds that "SHBG should be measured in all patients in whom clinical suspicion and total testosterone levels are discordant" [16].

Calculating Free Testosterone

Calculated free testosterone using the Vermeulen equation requires three inputs: total testosterone, SHBG, and albumin. The calculation correlates well (r = 0.94 to 0.98) with equilibrium dialysis, the gold standard method [17]. Direct analog immunoassays for free testosterone, by contrast, are unreliable and not recommended by any major endocrine guideline.

Drug-Specific Adjustment Strategies

For patients on oral estrogen with elevated SHBG: switching to transdermal delivery typically normalizes SHBG within 8 to 12 weeks. For patients on anticonvulsants: consider checking free testosterone directly by equilibrium dialysis rather than relying on SHBG-based calculations, as enzyme-inducing anticonvulsants may alter binding dynamics beyond what the Vermeulen equation captures.

Dr. Shalender Bhasin, a professor of medicine at Harvard Medical School and lead author of the Endocrine Society testosterone guideline, has noted: "Measuring SHBG transforms a single, often ambiguous total testosterone number into a clinically actionable assessment of androgen status" [3]. This principle applies equally to estradiol assessment in women on HRT.

Normal SHBG Ranges and What Shifts Them Beyond Drugs

Reference ranges for SHBG vary by age, sex, and BMI. Understanding these physiological influences is necessary to separate drug effects from normal variation.

Male Reference Ranges

For adult men aged 20 to 49, most reference laboratories report a range of 10 to 57 nmol/L [18]. SHBG increases approximately 1 to 2% per year after age 40, driven by declining testosterone and increasing adiposity. By age 70, mean SHBG is roughly 30 to 40% higher than at age 25. This age-related rise contributes to the discrepancy between total testosterone (which may remain borderline-normal) and free testosterone (which falls more steeply).

Female Reference Ranges

Premenopausal women: 18 to 144 nmol/L. Postmenopausal women: 15 to 120 nmol/L [18]. The wide range reflects the powerful influence of estrogen status and body composition. SHBG drops after menopause as estrogen declines, which is one reason some postmenopausal women develop new-onset acne or facial hair even without a pathological androgen source.

Body Composition Effects

Obesity is the strongest non-pharmacological suppressor of SHBG. A 2010 study in the Journal of Clinical Endocrinology and Metabolism (N=1,849) found that each 1 kg/m² increase in BMI was associated with a 2.1% decrease in SHBG in men and a 2.7% decrease in women [19]. Weight loss produces the reverse: bariatric surgery patients show SHBG increases of 50 to 200% within 12 months, which can temporarily lower free testosterone even as total testosterone improves.

Liver and Metabolic Disease

Non-alcoholic fatty liver disease (NAFLD/MASLD) is independently associated with low SHBG, even after adjusting for BMI and insulin resistance. A 2019 meta-analysis in Liver International (15 studies, N=44,044) found that individuals with NAFLD had SHBG levels 30% lower than matched controls [20]. Conversely, hepatitis and cirrhosis causing liver inflammation can paradoxically raise SHBG as part of the acute-phase response.

How to Raise SHBG (When Clinically Indicated)

Some clinical scenarios call for intentionally increasing SHBG, most commonly in women with hyperandrogenism or PCOS who need androgen suppression.

Lifestyle Interventions

Weight loss raises SHBG. A 2015 randomized trial (N=117 overweight men) showed that a 10% body weight reduction over 12 months increased SHBG by 26% [21]. Dietary composition also matters: high-fiber, low-glycemic-index diets are associated with higher SHBG, likely through reduced postprandial insulin secretion.

Pharmacological Options

Oral estrogen (in women) produces the most reliable SHBG elevation. COCs containing ethinyl estradiol with drospirenone raise SHBG by 200 to 400% and are first-line therapy for PCOS-related hyperandrogenism per the 2023 International Evidence-Based Guideline for PCOS [22]. In men, SHBG elevation is rarely a therapeutic goal, but clinicians should recognize that starting thyroid replacement or switching from injectable to oral medications may produce unintended SHBG increases.

How to Lower SHBG (When Clinically Indicated)

Men with high SHBG and symptomatic hypogonadism despite normal total testosterone may benefit from strategies to reduce SHBG and increase free testosterone.

Route-of-Delivery Changes

Switching from oral to transdermal estrogen in women on HRT typically lowers SHBG by 30 to 60% within 3 months [5]. For men on clomiphene citrate (which raises both testosterone and SHBG), adding low-dose testosterone or switching to direct testosterone replacement may be more effective if the goal is to increase free testosterone.

Treating Underlying Insulin Resistance

Because insulin suppresses SHBG, improving insulin sensitivity with lifestyle changes, metformin, or GLP-1 receptor agonists can modestly shift SHBG. This indirect approach is most relevant in lean women with PCOS who have SHBG above 100 nmol/L and symptoms of relative estrogen deficiency.

Boron Supplementation

A small pilot study (N=13) found that boron supplementation at 10 mg/day for 7 days reduced SHBG by 9% in healthy men [23]. The evidence base is limited, and no professional society recommends boron for SHBG modification. Larger, longer trials are needed before any clinical recommendation can be made.

Timing, Fasting, and Specimen Collection for Accurate SHBG Results

Getting a reliable SHBG value requires attention to specimen handling and clinical timing.

Morning Fasting Draw

SHBG has modest diurnal variation, but testosterone (the hormone it binds) varies by 20 to 30% across the day. The Endocrine Society recommends a fasting morning blood draw before 10 AM for testosterone assessment, and SHBG should be drawn from the same sample [3]. Fasting matters because postprandial insulin acutely suppresses SHBG, potentially lowering results by 10 to 15%.

Medication Timing

For patients on oral estrogen, the SHBG draw should occur at trough (before the daily dose) for consistency. For patients on injectable testosterone, draw at mid-cycle (for weekly injections) or trough (for biweekly injections) to capture the steady-state SHBG level, not the acute suppression from a peak testosterone spike.

Repeat Testing

SHBG should be repeated 6 to 8 weeks after any medication change that could affect it. A single abnormal SHBG value should be confirmed with a repeat draw before clinical decisions are made, just as the Endocrine Society recommends confirming low testosterone with two separate morning measurements [3].