The editorial TLDR.
Sex hormone binding globulin (SHBG) is a protein made by the liver that binds testosterone, estradiol and DHT in the bloodstream. Only the unbound fraction of those hormones — the "free" portion — can leave the bloodstream and act on cells. SHBG, by deciding how much is bound and how much is free, quietly determines how much hormonal activity actually reaches your tissues. Two women with the same total testosterone can have very different free testosterone numbers depending on where their SHBG sits.
Most U.S. labs report SHBG in nanomoles per liter (nmol/L). A typical reference band for non-pregnant adult women runs roughly 18–144 nmol/L, varying widely by laboratory, age and cycle phase. Inside that band, the comfortable middle is roughly 30–80 nmol/L for most reproductive-age women.
SHBG is also one of the most informative metabolic markers in women's health. It is suppressed by insulin — meaning a low SHBG is one of the earliest and most reliable signals of insulin resistance, often years before fasting glucose or HbA1c moves. It is raised by oral estrogen (combined OCPs, oral HRT) and by hyperthyroidism. The number tells you something about hormones, and something about metabolism, in a single read. As always, what you do with it is a conversation worth having with a qualified women's-health clinician or endocrinologist.
What SHBG actually is.
SHBG is a glycoprotein produced primarily by the liver. Its job is to bind sex steroid hormones — particularly testosterone, dihydrotestosterone (DHT) and, to a lesser extent, estradiol — in the bloodstream. While bound to SHBG, those hormones are biologically inactive: they can't leave the circulation, can't bind to receptors, can't do anything until they are released.
The proportion of each hormone bound to SHBG depends on the hormone and on the amount of SHBG. For testosterone in adult women, roughly 60% is typically bound to SHBG, roughly 38% loosely bound to albumin, and roughly 1–2% free. The "free" plus "albumin-bound" fractions are sometimes referred to together as "bioavailable" testosterone — the portion the body can actually access. Estradiol is less tightly bound than testosterone; SHBG affects its biological availability less powerfully but still meaningfully.
"SHBG is the volume knob on your sex hormones. The hormones can be present in the bloodstream — but SHBG decides how loudly they speak."
Crucially, SHBG is not just a passive sponge. Its production by the liver is regulated by a small number of powerful inputs. Insulin lowers SHBG. Chronically elevated insulin — the hallmark of insulin resistance — suppresses SHBG production over time, raising free testosterone and free estradiol while total levels can look unchanged. Estrogen, taken orally, raises SHBG. Oral combined contraception and oral hormone replacement both produce a "first pass" through the liver that meaningfully elevates SHBG. Transdermal estrogen has a much smaller effect. Thyroid hormone raises SHBG. Hyperthyroidism is associated with high SHBG; hypothyroidism with low.
The standard U.S. lab measures SHBG with an immunoassay. Results are reported in nmol/L (occasionally µg/dL in some labs). Reference ranges vary widely; the broad shape below is common for non-pregnant adult women.
Why SHBG matters for women.
SHBG is one of those biomarkers that almost never makes it onto a standard annual physical, and yet meaningfully informs three of the most common clinical conversations in adult women's health.
PCOS and androgen excess. One of the most consistent lab findings in PCOS is a low SHBG. The high insulin that drives many PCOS presentations suppresses SHBG production by the liver, which raises free testosterone even when total testosterone looks unremarkable. A woman presenting with PCOS symptoms — irregular cycles, acne, hair changes, weight-loss resistance — and a low SHBG often has the metabolic-androgen pattern clinicians look for. Total testosterone alone can miss it; SHBG read alongside catches it.
Insulin resistance and metabolic health. SHBG is one of the most under-recognised early markers of insulin resistance in women. Multiple longitudinal studies have shown that declining SHBG predicts type 2 diabetes risk independently of other markers. The mechanism is direct: high circulating insulin tells the liver to make less SHBG. So a low SHBG in a woman without overt PCOS can still be a signal that the metabolic conversation is worth opening — often well before fasting glucose, HbA1c or fasting insulin would flag it alone.
Combined oral contraception. Combined OCPs raise SHBG, sometimes substantially — by 100–300%, particularly with ethinyl estradiol formulations. The clinical consequence is that free testosterone falls, sometimes meaningfully. Many women report libido, mood and energy shifts on the pill that resolve in the months after stopping; a measured SHBG and free testosterone is one way to make sense of the pattern. The effect is smaller with progestin-only contraception and minimal with non-oral routes.
Perimenopause and menopause hormone therapy. Oral estrogen-based hormone therapy raises SHBG meaningfully; transdermal estrogen does not. This matters for women on hormone therapy who continue to experience low-libido or low-androgen-feel patterns — the oral estrogen may be binding up testosterone even while estradiol levels look fine.
Thyroid disease. Hyperthyroidism raises SHBG; hypothyroidism lowers it. An unexpectedly high or low SHBG in a woman with thyroid-suggestive symptoms is worth pairing with a full thyroid panel.
Liver function. SHBG is made by the liver, so meaningful liver disease can affect its production. Worth context with abnormal liver enzymes.
What the ranges generally mean.
Most U.S. labs report SHBG in nmol/L. Reference ranges vary widely by laboratory and by life stage; the broad shape below is for non-pregnant adult women in their reproductive years.
SHBG reference, non-pregnant adult women
nmol / LIllustrative ranges, not diagnostic. Reference ranges vary widely by laboratory, by age, and by medication context. Always discuss your specific result with a qualified healthcare provider.
What may drive SHBG low or high.
The patterns below come up most often when SHBG reads outside the comfortable middle in U.S. women.
What may drive SHBG low.
- Insulin resistance. The single most reliable driver. Chronically elevated insulin suppresses hepatic SHBG production.
- Polycystic ovary syndrome. A low SHBG is one of the most consistent lab features of PCOS — often paired with elevated free testosterone and elevated fasting insulin.
- Hypothyroidism. An underactive thyroid is associated with reduced SHBG. Worth pairing with TSH.
- Cushing's syndrome and exogenous glucocorticoids. Excess cortisol lowers SHBG.
- Acromegaly and growth hormone excess. Less common, but associated with reduced SHBG.
- Excess androgens (endogenous or exogenous). High testosterone itself can feedback-suppress SHBG.
- Obesity, particularly visceral adiposity. Closely linked to the insulin-resistance pathway above.
- Progestin-only contraception. Some progestin-only methods modestly lower SHBG.
- Very low calorie or very-low-carbohydrate eating, sustained. Can affect SHBG via metabolic and thyroid pathways.
What may drive SHBG high.
- Oral estrogen-containing medications. Combined oral contraception and oral hormone replacement raise SHBG meaningfully via first-pass hepatic effects. The single most common cause of elevated SHBG in U.S. women.
- Hyperthyroidism. Overactive thyroid raises SHBG. Worth pairing with TSH if a high SHBG is unexplained by medication.
- Anorexia nervosa and severe weight loss. Often associated with very high SHBG alongside low estradiol and amenorrhea.
- Pregnancy. SHBG rises substantially during pregnancy. The non-pregnant reference range does not apply.
- Liver disease. Some chronic liver patterns elevate SHBG.
- Aging. SHBG tends to rise gradually with age in women, particularly through and after menopause.
- Genetic variation. Some women run constitutionally higher or lower SHBG. Trend over time within an individual is often more informative than the absolute number.
Why low SHBG is an early metabolic flag.
The standard U.S. metabolic workup focuses on fasting glucose and HbA1c. Both are useful — but both are lagging indicators. By the time fasting glucose drifts upward, insulin has typically been climbing for years.
SHBG moves earlier. Because insulin directly suppresses SHBG production by the liver, a low SHBG can be one of the earliest signs that insulin resistance is developing. In women with PCOS-suggestive symptoms, weight-loss resistance, or perimenopause-era metabolic shifts, SHBG is one of the most informative single numbers to add to a workup — particularly alongside fasting insulin. A low SHBG with a normal glucose is not a diagnosis. It's a useful clue worth bringing to a clinical conversation.
Questions worth asking your healthcare provider.
Conversation starters, not a script. These are designed to make sure SHBG actually appears on the requisition — and is read in the context of testosterone and insulin:
- Can we add SHBG to the next hormone panel, alongside total and free testosterone?
- Given my PCOS / perimenopause / cycle changes, what does my SHBG suggest about insulin sensitivity?
- My SHBG is low / high — could that be related to current medications or thyroid status?
- If my total testosterone looks normal but free testosterone is high, how does SHBG explain that?
- I'm on combined oral contraception — should we read my SHBG and free testosterone in that context, and consider transdermal options?
- If SHBG keeps trending down at successive visits, what would prompt a fuller metabolic workup?
Your provider will guide the conversation based on your full medical context. These prompts are designed to keep SHBG part of the picture — as the lens through which testosterone and insulin patterns become legible together.
When to test, and how it's measured.
The SHBG test is a standard venous blood draw, almost always bundled with sex steroid hormones (total testosterone, free testosterone, DHEA-S, estradiol) and often with insulin and metabolic markers. No fasting is required for SHBG itself, but many of the markers commonly paired with it (insulin, lipids) do require fasting — so a fasting morning draw is the most efficient.
Timing matters in two ways. Cycle day: SHBG shifts modestly across the menstrual cycle, but less than estradiol or progesterone. A cycle-day-3 draw is the convention when the full reproductive panel is being measured. Time of day: a morning draw, ideally before 10 a.m., aligns with the convention for the rest of the hormone panel (testosterone peaks in the morning).
If you are on combined oral contraception or any oral estrogen-containing medication, mention it on the requisition — both meaningfully raise SHBG. Transdermal estrogen has a much smaller effect; the distinction is clinically important. If you've recently started, stopped or changed hormonal therapy, give SHBG at least two to three months to stabilize before drawing a baseline.
If you are taking high-dose biotin (often in hair, skin and nail supplements), pause it for at least 48 hours before the test — biotin can interfere with several hormone immunoassays.
Direct-to-consumer at-home tests — Function Health, LetsGetChecked, Modern Fertility's broader panels — include SHBG in their PCOS-aware and women's-health-focused panels. They're useful for a baseline; they don't replace the clinical conversation. For the trade-offs across services, see our guide to at-home blood tests for women.