SHBG and Testosterone: Why Your Total T May Be Lying to You

Total testosterone is the most commonly ordered testosterone test—and the most commonly misinterpreted. If your doctor checks your testosterone and tells you it's "fine" at 450 ng/dL without measuring SHBG and free testosterone, they may be missing the actual story. For a substantial number of men, high SHBG is silently stealing the benefits of their seemingly adequate testosterone levels.

What Is SHBG?

Sex hormone-binding globulin (SHBG) is a glycoprotein produced primarily in the liver. Its job is to transport sex hormones—testosterone, estradiol, and dihydrotestosterone (DHT)—through the bloodstream. When testosterone is bound to SHBG, it cannot enter cells, bind to androgen receptors, or exert biological effects. It is, for practical purposes, inactive.

Of the total testosterone circulating in the blood:

• ~60–70% is bound to SHBG (inactive)
• ~25–35% is bound to albumin (loosely bound; bioavailable)
• ~1–3% is unbound, or "free" testosterone (the most biologically active fraction)

"Bioavailable testosterone" refers to the sum of free and albumin-bound testosterone. "Free testosterone" is the most potent fraction. Both are more clinically meaningful than total testosterone alone when SHBG is abnormal.

Why SHBG Matters: Two Clinical Scenarios

Scenario 1: High SHBG with "Normal" Total Testosterone

A 42-year-old man reports fatigue, low libido, reduced muscle mass, and depression. His total testosterone is 450 ng/dL—just above the typical lab lower limit of 300. His doctor tells him he's "fine." But his SHBG is 75 nmol/L (well above normal range of 10–57 nmol/L), which means the vast majority of his testosterone is bound and inactive. His calculated free testosterone is 6 pg/mL—significantly below the normal range of 9–30 pg/mL. Clinically, he is functionally hypogonadal despite an "adequate" total T.

Scenario 2: Low SHBG with Low-Normal Total Testosterone

A 55-year-old man with obesity and insulin resistance has total testosterone of 380 ng/dL. His SHBG is 18 nmol/L (low end of normal), meaning a larger proportion is free. His free testosterone is 14 pg/mL—adequate. He may not be functionally hypogonadal despite lower total T. His low SHBG, however, signals metabolic problems that require their own attention.

What Raises SHBG? (Reducing Bioavailable Testosterone)

Factor	Effect on SHBG	Notes
Aging	⬆ Increases ~1%/year	Major driver of age-related testosterone decline
Elevated estrogen	⬆ Increases	Estrogen directly stimulates SHBG synthesis in the liver
Hyperthyroidism	⬆ Increases significantly	Thyroid hormones upregulate SHBG production
Cirrhosis / liver disease	⬆ Increases	Paradoxically; impaired steroid catabolism raises E2
Anorexia / caloric restriction	⬆ Increases	Severe restriction acutely raises SHBG
Certain medications	⬆ Increases	Anticonvulsants (phenytoin, carbamazepine), HIV medications
High dietary fiber diet	⬆ Modest increase	Through reduction of free fatty acids and insulin

What Lowers SHBG? (Increasing Bioavailable Testosterone)

Factor	Effect on SHBG	Notes
Obesity / insulin resistance	⬇ Decreases	Insulin directly suppresses hepatic SHBG production
Type 2 diabetes	⬇ Decreases	Hyperinsulinemia downregulates SHBG
Hypothyroidism	⬇ Decreases	Low thyroid hormones reduce SHBG synthesis
High-dose anabolic steroids	⬇ Decreases significantly	Exogenous androgens suppress SHBG production
Glucocorticoids (prednisone, etc.)	⬇ Decreases	Steroid hormones reduce SHBG
Progestins	⬇ Decreases	Particularly androgenic progestins
Growth hormone / IGF-1	⬇ Decreases	GH signaling suppresses SHBG

Low SHBG is not always desirable. It often signals metabolic dysfunction (insulin resistance, hypothyroidism) and is associated with cardiovascular risk. Artificially lowering SHBG to increase free testosterone is not a sound clinical strategy without addressing the underlying cause.

How to Calculate Free Testosterone

The gold standard for free testosterone measurement is equilibrium dialysis—an accurate but expensive and not widely available lab method. Most labs offer a "free testosterone" calculated from total testosterone, SHBG, and albumin using the Vermeulen equation. While not perfect, it is clinically adequate for most purposes.

Providers can use online calculators (such as the Vermeulen calculator available at ISSAM.ch) to determine calculated free testosterone when serum total testosterone and SHBG are known. The Free Androgen Index (FAI = total T / SHBG × 100) is a simpler but less accurate alternative.

SHBG and TRT Management

SHBG levels affect how men respond to and how they should be managed on testosterone replacement therapy:

• High SHBG men — May need higher total testosterone targets to achieve adequate free T; may benefit from more frequent dosing (daily vs. weekly injections) to maintain more stable free T levels without large peaks that could convert to estrogen
• Low SHBG men — Often respond well to lower TRT doses; are more prone to elevated free T, which can increase hematocrit and estradiol; may benefit from more frequent smaller doses
• Monitoring — SHBG should be measured at TRT baseline and periodically during treatment to guide dosing interpretation

For more on how testosterone is measured and interpreted, see our detailed guide on free vs. total testosterone.

Strategies for Optimizing SHBG

If SHBG Is Too High

• Optimize thyroid function (hyperthyroidism raises SHBG significantly)
• Review medications that may raise SHBG (anticonvulsants are major culprits)
• Ensure adequate caloric intake (severe restriction raises SHBG)
• Resistance training modestly reduces SHBG over time
• Zinc (important cofactor in testosterone binding; deficiency associated with elevated SHBG)
• In TRT patients, dosing adjustment to achieve adequate free testosterone is the primary clinical tool

If SHBG Is Too Low

• Address insulin resistance: weight loss, low-glycemic diet, exercise
• Treat hypothyroidism if present
• Reduce alcohol consumption (moderate alcohol directly suppresses SHBG)
• Increase dietary fiber

The Takeaway

SHBG is the hidden variable in testosterone interpretation. A single total testosterone number, without SHBG and calculated or measured free testosterone, is an incomplete picture. If your symptoms suggest low testosterone but your total T looks "normal," SHBG may explain everything. Conversely, a low total testosterone with equally low SHBG may not indicate clinical hypogonadism at all.

The right question is not "what is my total testosterone?" but "how much testosterone is actually available to my cells?"—and that answer requires SHBG. For a broader look at men's hormone health, explore our article on low testosterone vs. depression.