As a male with sex hormone‑binding globulin (SHBG) of 99 nmol/L and total testosterone of 40 nmol/L, my libido was normal last year but is now absent—did the elevated SHBG cause the loss of libido rather than a drop in testosterone?

What Likely Happened to Your Libido

Your elevated SHBG (99 nmol/L) is almost certainly the primary culprit—it has likely risen over the past year, binding most of your testosterone and leaving insufficient free (bioactive) testosterone to maintain libido, even though your total testosterone remains at 40 nmol/L (≈1,154 ng/dL). 1, 2

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Understanding the SHBG-Testosterone-Libido Connection

Why SHBG Matters More Than Total Testosterone

• SHBG binds testosterone tightly, rendering it biologically inactive—only free (unbound) testosterone can enter tissues and drive sexual function, including brain centers that regulate libido. 2, 3
• With SHBG at 99 nmol/L (reference range 7–50 nmol/L), you have nearly double the upper limit of normal binding capacity, meaning the vast majority of your testosterone is locked up and unavailable. 2, 4
• The free testosterone index (total testosterone ÷ SHBG × 100) is the key metric—a ratio < 30 (or total testosterone/SHBG < 0.3) reliably identifies functional hypogonadism and predicts hypogonadal symptoms better than total testosterone alone. 2

Your Likely Scenario

• Your total testosterone at 40 nmol/L (≈1,154 ng/dL) appears high, but with SHBG at 99 nmol/L, your calculated free testosterone index is approximately 40 ÷ 99 × 100 = 40.4, which is borderline low-normal. 2, 3
• If your SHBG was lower last year (e.g., 30–40 nmol/L), your free testosterone would have been substantially higher even with the same total testosterone, explaining why your libido was normal then. 2, 4
• A subset of men with elevated SHBG and normal-to-high total testosterone will have low free testosterone levels that cause symptomatic hypogonadism, particularly diminished libido and erectile dysfunction. 2, 5

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What Causes SHBG to Rise

Common Reversible Causes (Address These First)

• Hyperthyroidism—excess thyroid hormone stimulates hepatic SHBG synthesis; treating hyperthyroidism normalizes SHBG. 2
• Chronic liver disease or cirrhosis—hepatic dysfunction raises SHBG production. 2
• Medications—anticonvulsants, oral estrogens, and supratherapeutic thyroid hormone increase SHBG; discontinuation (when feasible) is a first-step intervention. 2
• Aging—physiological aging modestly raises SHBG, though this is less likely to explain a sudden change over one year in a younger man. 2, 4

Less Common Causes

• HIV infection—suboptimal antiretroviral therapy or HIV-associated wasting can elevate SHBG. 2
• Low insulin states—conditions that lower insulin (e.g., caloric restriction, weight loss) can raise SHBG. 1

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Diagnostic Algorithm to Confirm the Problem

Step 1: Measure Free Testosterone Accurately

• Calculated free testosterone using the Vermeulen formula is the preferred method when equilibrium dialysis (gold standard) is unavailable. 1, 3
• Direct immunoassays for free testosterone are inaccurate and should be avoided, especially when SHBG is abnormal. 1, 3
• Repeat morning (8–10 AM) total testosterone, SHBG, and calculated free testosterone to confirm the pattern, as single measurements can be misleading. 1, 2

Step 2: Investigate Causes of Elevated SHBG

• Check thyroid function (TSH, free T4) to exclude hyperthyroidism. 2
• Perform liver function tests (ALT, AST, bilirubin, albumin) to exclude chronic liver disease. 2
• Review medications for SHBG-elevating agents (anticonvulsants, oral estrogens, thyroid hormone). 2

Step 3: Measure LH and FSH

• Low or low-normal LH/FSH with low free testosterone indicates secondary (hypothalamic-pituitary) hypogonadism. 1
• Elevated LH/FSH with low free testosterone indicates primary (testicular) hypogonadism. 1
• Normal LH/FSH with low free testosterone and high SHBG indicates functional hypogonadism due to SHBG elevation. 2

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Treatment Algorithm

First-Line: Address Reversible Causes

• Treat hyperthyroidism if present—this will normalize SHBG and restore free testosterone. 2
• Discontinue SHBG-elevating medications when feasible. 2
• Optimize metabolic health—address liver disease, improve body composition, and optimize insulin sensitivity. 1, 2

Second-Line: Testosterone Replacement Therapy (If Free Testosterone Remains Low)

• Testosterone replacement therapy produces small-to-moderate but statistically significant improvements in sexual function and libido (standardized mean difference ≈ 0.35), representing a clinically meaningful benefit. 2
• Transdermal testosterone gel (1.62% at 40.5 mg daily) is preferred due to stable day-to-day levels and lower risk of erythrocytosis compared to injectable preparations. 1
• Monitor free testosterone levels 2–3 months after initiation to ensure adequate replacement, targeting mid-normal free testosterone. 1, 2

Critical Caveat: Testosterone Therapy Has Minimal Impact on Other Symptoms

• Testosterone replacement shows little to no impact on physical functioning, energy, vitality, depressive symptoms, or cognitive performance—the primary benefit is sexual function. 1, 2
• If you have other symptoms (fatigue, low energy, mood changes), do not expect testosterone to fix them—these require separate evaluation and management. 1

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Common Pitfalls to Avoid

• Do not rely on total testosterone alone—it misses approximately half of hypogonadism diagnoses when SHBG is elevated. 1, 3
• Do not use direct immunoassays for free testosterone—they are inaccurate when SHBG is abnormal. 1, 3
• Do not start testosterone replacement without first investigating and addressing reversible causes of elevated SHBG—treating hyperthyroidism or discontinuing offending medications may restore free testosterone without exogenous therapy. 2
• Do not assume your total testosterone has dropped—the more likely scenario is that your SHBG has risen, reducing free testosterone availability. 2, 4

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Expected Outcomes with Treatment

• If free testosterone is confirmed low and you start testosterone replacement, expect small-to-moderate improvements in libido and sexual function within 3–6 months. 1, 2
• If a reversible cause (e.g., hyperthyroidism) is identified and treated, expect SHBG to normalize and free testosterone to rise, restoring libido without exogenous testosterone. 2
• Approximately 5% of men with erectile dysfunction have hypogonadism as the underlying cause, and testosterone treatment may ameliorate the response to PDE5 inhibitors (sildenafil, tadalafil) in men with low free testosterone. 6, 5