By Dr. Allison Smith, ZRT Laboratory
Sex Hormone Binding Globulin (SHBG) is one of the most overlooked variables in hormone health. It directly controls how much testosterone is biologically active in your body — yet it rarely appears on standard blood panels. Whether you're dealing with symptoms of low testosterone, androgen excess, or unexplained hormonal imbalance, understanding SHBG is essential to getting the full picture.
What is SHBG?
SHBG is a protein produced primarily in the liver that binds tightly to testosterone — and to a lesser degree, oestradiol (E2) and dihydrotestosterone (DHT) — and carries them through the bloodstream. Because the bond is so strong, testosterone bound to SHBG is considered inactive: it cannot enter cells or exert its effects. Only free testosterone — the fraction not bound to SHBG — is biologically available.
Think of SHBG as a sponge: when SHBG goes up, free testosterone goes down. When SHBG goes down, more testosterone is available to tissues. This makes SHBG a critical modulator of androgen excess and androgen deficiency alike.
Key facts about SHBG:
- Role: Binds and transports testosterone (and E2, DHT) through the bloodstream; controls clearance and bioavailability
- Production sites: Liver (primarily), testes, brain
- Raised by: Oestradiol, triiodothyronine (T3)
- Suppressed by: Insulin
- Also known as: ABP (androgen binding protein), SSBG, TEBG, GBG
Prescription Drugs & SHBG Levels
Most oral medications have some effect on the liver and therefore on SHBG. Some drugs alter binding at androgen receptor sites; others raise prolactin, which suppresses testosterone and elevates SHBG. Key examples:
Drugs that raise SHBG via increasing prolactin:
- Antipsychotics (typical and atypical)
- Antidepressants (SSRIs, Tricyclics, MAO-Is)
- Xanax and Buspar
- H2 Antagonists (Cimetidine, Ranitidine)
- Morphine
- Some antihypertensives
Drugs that raise SHBG:
- Raloxifene (Evista)
- Tamoxifen
- Spironolactone
- Anticonvulsants (Phenytoin)
- Oral (but not transdermal) oestradiol
- Ethinyl oestradiol (oral contraceptives)
- Metformin
- Exogenous insulin in Type 2 diabetes
- Coffee
Drugs that suppress SHBG:
- Progestins
- Glucocorticoids
- Exogenous insulin in Type 1 diabetes
Genetics: Some individuals carry a genetic variation in SHBG structure that slows or speeds its degradation, leading to persistently high or low levels. Current research is examining specific SHBG SNPs in relation to Type 2 diabetes risk.
Areas of Clinical Application
| Condition | SHBG Pattern | Effect |
|---|---|---|
| Insulin resistance / Type 2 diabetes | Low SHBG | More free testosterone → androgen excess in women; paradoxically low T in men |
| Hypothyroidism | Low SHBG | Reduced T3 → less SHBG production |
| Hyperthyroidism | High SHBG | Excess T3 → elevated SHBG → low free testosterone |
| Oral oestrogen therapy | High SHBG | Reduces free testosterone significantly |
| Obesity | Low SHBG | Driven by high insulin and low T3 |
| Andropause / ageing men | High SHBG | Progressive rise with age → declining free testosterone |
| PCOS | Low SHBG | Insulin-driven suppression → androgen excess |
How to Influence SHBG
Rather than simply trying to raise or lower SHBG in isolation, the most effective approach is to identify and address the underlying cause. Always consider the three main endocrine inputs that drive SHBG:
- Insulin — the most powerful suppressor of SHBG. High fasting insulin = low SHBG.
- Oestradiol — raises SHBG; relevant in women on oral HRT or contraceptives.
- Thyroid hormones (T3) — T3 stimulates SHBG production; low thyroid = low SHBG.
Recommended Accompanying Tests
When SHBG is abnormal, the following markers help identify the root cause:
- Fasting insulin and HbA1c
- Sex steroids: oestradiol (E2), testosterone, DHEA-S
- Thyroid panel: TSH, free T3, free T4
- LH and FSH
- IGF-1
- Prolactin
- Liver function tests
Treatment Approaches by Scenario
| Scenario | Goal | Approach |
|---|---|---|
| High SHBG, low free T (men) | Lower SHBG | Address thyroid if low; reduce oral oestrogen exposure; optimise insulin sensitivity |
| Low SHBG, androgen excess (women with PCOS) | Raise SHBG | Improve insulin sensitivity (diet, exercise, Metformin); consider oral oestradiol if appropriate |
| Low SHBG, high free T (men on TRT) | Monitor | Adjust TRT dose; monitor haematocrit and cardiovascular markers |
| High SHBG on oral contraceptives | Assess symptoms | Consider transdermal oestrogen alternatives; test free testosterone |
Should You Test Your SHBG?
If you're experiencing symptoms of low testosterone (fatigue, low libido, poor recovery, mood changes) despite "normal" total testosterone on a standard blood test, SHBG may be the missing piece. High SHBG can make total testosterone look normal while free testosterone — the active fraction — is actually deficient.
Our SHBG Hormone Test measures your SHBG level from a simple at-home dried blood spot sample, with results in 3–5 working days.
For a broader picture that includes testosterone, oestradiol, DHEA, and cortisol alongside SHBG, the Male Saliva Hormone Profile provides a comprehensive baseline in a single at-home test. If thyroid function is also a concern — given T3's direct role in SHBG production — the Complete Hormone & Thyroid Test Kit combines both panels in one kit.