SHBG (sex hormone binding globulin)

SHBG is a protein produced by the liver that binds sex hormones in the bloodstream, primarily testosterone and estradiol. Bound hormone is biologically inactive; only the small unbound (free) fraction can enter cells and bind hormone receptors. SHBG is one of the most important and most ignored variables on a standard hormone panel.

What SHBG does

SHBG acts as a hormone reservoir and transport system:

  • Binds testosterone and estradiol with high affinity. Roughly 65 to 75% of total testosterone in women is SHBG-bound.
  • Releases hormone slowly. Bound hormone unbinds as free hormone is used by tissues, maintaining a steady free hormone supply.
  • Modulates net hormonal activity. Two women with identical total testosterone can have very different symptoms depending on SHBG.

The total-hormone reading on a standard lab panel includes both bound and free fractions. The free fraction is what biology actually responds to.

Why SHBG matters for cycle syncing

In practice, SHBG explains a lot of the variation in why women with apparently "normal" hormones experience strong symptoms (or apparently abnormal hormones experience few symptoms):

  • A woman with PCOS and insulin resistance often has normal total testosterone but very low SHBG, producing high free testosterone and the full picture of androgen excess.
  • A woman on combined hormonal contraceptives has high SHBG (driven by synthetic estrogen), low free testosterone, and sometimes presents with low libido and mood changes.
  • A perimenopausal woman with rising SHBG can have functional androgen deficiency even though total levels look normal.

What raises SHBG

Several factors push SHBG up, decreasing free hormone activity:

  • Estrogen exposure. Endogenous estrogen, oral HRT, and especially oral combined hormonal contraceptives. The first-pass liver effect of oral estrogen is the main driver.
  • Hyperthyroidism. Thyroid hormone increases SHBG production.
  • Pregnancy. SHBG rises significantly.
  • Anorexia and very low body weight.
  • Cirrhosis or significant liver disease.
  • Certain anti-seizure medications (phenytoin, carbamazepine).

What lowers SHBG

The mirror pattern, pushing SHBG down and increasing free hormone activity:

  • Insulin resistance and hyperinsulinemia. The most common driver in reproductive-age women. Insulin directly suppresses liver SHBG production. This is a major mechanism in PCOS.
  • Obesity (overlapping with insulin resistance).
  • Hypothyroidism.
  • Androgen excess (negative feedback to SHBG production).
  • Glucocorticoids and Cushing's syndrome.
  • Growth hormone and IGF-1.
  • Non-alcoholic fatty liver disease.

SHBG, insulin, and PCOS

The insulin-SHBG-androgen loop is central to PCOS pathophysiology:

  1. Insulin resistance raises insulin levels.
  2. High insulin suppresses liver SHBG production.
  3. Low SHBG means more free testosterone is available to tissues.
  4. High insulin also directly stimulates ovarian theca cells to produce more testosterone.
  5. Higher testosterone worsens insulin resistance.

This is why insulin-sensitizing interventions (metformin, inositol, strength training, dietary changes) often produce broad PCOS symptom improvement: they raise SHBG, lower free testosterone, and restore cycles indirectly rather than acting on the ovary directly.

SHBG and hormonal contraceptives

Oral combined hormonal contraceptives raise SHBG by 2 to 4x in most users. The clinical implications:

  • Free testosterone drops sharply. This explains the acne-clearing effect for many users and the low libido side effect for others.
  • The SHBG elevation can persist for months after stopping the pill, longer than the explicit hormones themselves. This contributes to post-pill amenorrhea and post-pill mood and libido issues.
  • Hormonal IUDs and progestin-only methods raise SHBG less because they do not deliver synthetic estrogen at the same dose.

This is part of why cycle syncing on birth control does not work the same way: the entire SHBG-mediated hormonal landscape is different.

Testing

SHBG is a simple blood test, can be drawn any day of the cycle. Standard reference ranges in adult women run roughly 18 to 144 nmol/L, with substantial inter-lab variation.

Useful interpretations:

  • SHBG below 30 nmol/L in a woman with cycle irregularity strongly suggests insulin resistance and warrants testing fasting insulin and glucose.
  • SHBG above 100 nmol/L in a woman with low libido or fatigue suggests low free hormone activity. Check what is raising SHBG (oral estrogen, thyroid status).
  • Trending SHBG over months is more useful than a single point, especially for tracking response to interventions like inositol or metformin.

Free testosterone calculation

The most informative testosterone reading is calculated free testosterone, which uses total testosterone, SHBG, and albumin in a formula. Direct free testosterone immunoassays are less reliable. Most major labs (Quest, LabCorp) offer the calculated value.

If your provider orders total testosterone without SHBG, request SHBG be added. The two readings together are far more informative than total alone.

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