SHBG and Low Testosterone: The Most Ignored Number in Your Panel

What SHBG Actually Does

Sex hormone-binding globulin (SHBG) is a glycoprotein made by the liver that binds sex steroids in your blood. In men, it binds testosterone and DHT tightly, and estradiol loosely.

Why it matters: only free testosterone (and to a lesser extent loosely albumin-bound T) is biologically active. SHBG-bound testosterone is locked up. So the total amount of testosterone you're producing matters less than the fraction that's actually available.

Roughly:

• 45-65% of your testosterone is SHBG-bound (inactive)
• 30-55% is albumin-bound (loosely — considered bioavailable)
• 1-2% is truly free

Change SHBG and you change the bioavailable fraction — often without changing total testosterone at all.

Why SHBG Is the Most Ignored Number

Most men get a "testosterone level" test and see a single number. That number is almost always *total* testosterone. If SHBG is missing, the total number is nearly uninterpretable in the borderline range.

Consider two men, both with total testosterone of 400 ng/dL:

• Man A: SHBG 52 nmol/L (normal) → free T about 7 ng/dL (low-normal)
• Man B: SHBG 14 nmol/L (low) → free T about 10 ng/dL (mid-normal)

Same total. Different free T. Different clinical implications. Different treatment discussions.

Without the SHBG, you can't distinguish them.

What Drives Low SHBG

Low SHBG (<20 nmol/L in men) is essentially a biomarker for insulin resistance. The liver makes less SHBG when insulin and inflammatory cytokines are chronically elevated.

Common drivers:

• Obesity (especially visceral)
• Type 2 diabetes and prediabetes
• Metabolic syndrome
• Nonalcoholic fatty liver disease
• Hypothyroidism
• Nephrotic syndrome
• Chronic high-carbohydrate intake with poor metabolic health
• Anabolic-androgenic steroid use
• Exogenous testosterone (TRT) — expected, 20-30% suppression
• Growth hormone use
• Some genetic polymorphisms

The useful clinical insight: a man with SHBG of 11 and total T of 370 probably has underlying insulin resistance driving both. Fixing the metabolic problem often resolves the hormonal one.

What Drives High SHBG

High SHBG (>60 nmol/L) is classically the aging pattern, though it has other causes:

• Aging — SHBG rises steadily after 40; expect +1 nmol/L per year on average
• Hyperthyroidism — thyroid hormone upregulates hepatic SHBG synthesis
• Oral (not transdermal) estrogens — e.g., older forms of HRT, oral contraceptives for cycling women
• Chronic liver disease / cirrhosis
• Anorexia nervosa — significant caloric restriction
• Celiac disease (well-documented)
• Hepatitis C (well-documented)
• Certain anticonvulsants (phenytoin, carbamazepine)
• HIV on some antiretrovirals

A 68-year-old with SHBG of 78 and total T of 520 may be functionally hypogonadal despite the "adequate" total, because his free T is sitting at 5.5 ng/dL.

How to Use SHBG Clinically

If your SHBG is low

Step 1: Assess metabolic health.

• Fasting glucose, A1C
• Lipid panel with triglyceride/HDL ratio
• Waist circumference
• Liver enzymes
• TSH

Step 2: Interpret testosterone in context.

• Total T may look lower than symptom burden suggests
• Free T is often preserved relative to total
• Focus on free T for treatment decisions

Step 3: Treat upstream if possible.

• Weight loss (especially visceral) raises SHBG
• Resistance training raises SHBG
• Metformin, if clinically indicated, raises SHBG
• Sleep apnea treatment helps

If your SHBG is high

Step 1: Rule out treatable causes.

• TSH + free T4 (hyperthyroidism)
• Medication review (oral estrogens, anticonvulsants)
• Liver panel
• Celiac screen if clinically suggested

Step 2: Interpret testosterone in context.

• Total T may look better than symptoms warrant
• Free T is what correlates with how you feel
• A "normal" total with low free T is still clinical hypogonadism

Step 3: Treat as indicated.

• Treatment of hypogonadism proceeds based on free T and symptoms, not total alone
• High-SHBG men may need slightly higher TRT doses to bring free T into range

SHBG on TRT: What to Expect

When you start testosterone, SHBG predictably drops 20-30%. Your free T on a given total T will therefore run higher than pre-TRT. This is normal, not a side effect — but it changes how you interpret follow-up labs.

Example, before and after starting weekly testosterone cypionate 100 mg:

| Lab | Baseline | 8 weeks in |

|---|---|---|

| Total T | 285 | 710 |

| SHBG | 38 | 27 |

| Free T (calc) | 6.2 | 18.4 |

| Estradiol | 15 | 32 |

The free T rose proportionally more than total because SHBG compressed. This is the normal pattern; monitor both total and free at each follow-up.

Is There Such a Thing as "Optimal" SHBG?

Most healthy men land between 20 and 50 nmol/L. Values in this range usually signal reasonable metabolic health and no major hormonal noise. Outside this range:

• Under 15 nmol/L is a red flag for significant insulin resistance
• Over 70 nmol/L warrants a thyroid and hepatic review

Neither extreme is "optimal." A reasonable goal for a man with low SHBG is to raise it through lifestyle work, which also tends to raise total and free testosterone. A reasonable goal for a man with high SHBG is to identify and treat the underlying cause (usually thyroid or medication-related) rather than try to suppress SHBG directly.

Bottom Line

SHBG is the context that makes testosterone numbers interpretable. Without it, total testosterone is a number, not a diagnosis. Low SHBG points at insulin resistance; high SHBG points at aging, thyroid disease, or specific medications. Either extreme distorts the relationship between total and free testosterone, and free is what correlates with how you feel.

Always get SHBG in your panel. If it's abnormal, treat the underlying driver — don't try to manipulate SHBG directly.

Sources

1. Goldman AL et al. "A Reappraisal of Testosterone's Binding in Circulation: Physiological and Clinical Implications." Endocr Rev, 2017.
2. Laurent MR et al. "Sex Hormone-Binding Globulin Regulation of Androgen Bioactivity in Vivo." Mol Cell Endocrinol, 2016.
3. Ding EL et al. "Sex Hormone-Binding Globulin and Risk of Type 2 Diabetes in Women and Men." N Engl J Med, 2009.
4. Hammond GL. "Diverse Roles for Sex Hormone-Binding Globulin in Reproduction." Biol Reprod, 2011.
5. Bhasin S et al. "Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline." J Clin Endocrinol Metab, 2018.