SHBG — sex hormone binding globulin — is the protein that binds testosterone in your blood, making it unavailable for cellular use. Your SHBG level determines how much of your total testosterone is actually biologically active. And it should directly influence your TRT protocol design — though many prescribers ignore it entirely.
What SHBG Does
Approximately 40–60% of circulating testosterone is bound to SHBG. Another 35–55% is loosely bound to albumin (and considered bioavailable because albumin binding is weak). Only 2–3% circulates completely free. SHBG acts as a reservoir and regulator — too much, and too little testosterone is available for cells. Too little, and testosterone is cleared too quickly.
High SHBG on TRT
Men with high SHBG (above 50–60 nmol/L) face a specific challenge: their total testosterone may look adequate on labs while their free testosterone is functionally low. The SHBG is absorbing a larger-than-usual share of circulating testosterone, leaving less for biological activity.
Protocol implications for high SHBG:
- Total testosterone targets should be higher — aim for the upper half of the range (700–900 ng/dL at trough) to ensure adequate free T
- Free testosterone is the more relevant metric — track it instead of relying on total T alone
- More frequent injections may help — some data suggests that frequent small doses produce sustained higher free T than infrequent larger doses in high-SHBG men
- Investigate causes: hyperthyroidism, liver disease, certain medications (anticonvulsants, some statins), aging, and low body fat can elevate SHBG
Low SHBG on TRT
Men with low SHBG (below 20 nmol/L) have the opposite issue: more free testosterone is available, but it's also cleared faster. These men often show adequate or high free T early in the injection cycle but rapid decline — they "use up" their testosterone faster.
Protocol implications for low SHBG:
- More frequent injections are strongly favored — daily or every-other-day dosing prevents the rapid clearance from producing symptomatic troughs
- Total testosterone targets can be lower — because more of every milligram is bioavailable, these men may achieve adequate symptom resolution at modest total T levels
- Investigate causes: obesity, insulin resistance, type 2 diabetes, hypothyroidism, and anabolic steroid use lower SHBG
- Addressing the cause may be more important than adjusting TRT — weight loss in obese men often normalizes SHBG
SHBG-Based Protocol Adjustments
- High SHBG (>50): Higher total T target, track free T, consider more frequent injection, investigate thyroid/liver
- Normal SHBG (20–50): Standard protocol — twice-weekly injection, target mid-range total T
- Low SHBG (<20): More frequent injection (daily/EOD), lower total T may suffice, investigate metabolic causes
The Bottom Line
SHBG Is the Context for Your Testosterone Number
A total testosterone of 600 ng/dL means something very different in a man with SHBG of 60 than in a man with SHBG of 15. Any TRT protocol that ignores SHBG is treating a number without context. Measure SHBG at baseline and periodically, use it to interpret your total T, and let it guide injection frequency and target decisions.