Sex hormone binding globulin (SHBG) is a protein made mostly in the liver. It attaches to testosterone in the bloodstream and controls how much of it stays free to enter cells. Total testosterone counts everything in the blood. Free testosterone counts only the small unbound portion. SHBG is the main reason those two numbers can tell different stories about the same person.
Roughly 60% of circulating testosterone is bound tightly to SHBG and cannot enter tissue. Another large share is loosely bound to albumin, which can release testosterone to cells fairly easily. Only a small fraction — commonly cited as around 1% to 2% — circulates completely unbound. That free fraction, plus the portion that easily detaches from albumin, is what the body can actually use.
How Shbg Affects Testosterone Free Vs Total
SHBG acts like a sponge in the bloodstream. When SHBG is high, it binds more testosterone, so total testosterone can look normal while free testosterone drops. When SHBG is low, more testosterone stays unbound, so free testosterone can run high even if total testosterone is unremarkable.
This is the core relationship. Total testosterone measures the whole pool. Free testosterone measures what is available to tissues. SHBG determines how that pool gets divided between bound and unbound.
Here is the non-obvious part many people miss: the tightly bound SHBG-testosterone complex is not simply waste. Some tissues can take up the whole complex through specific receptors. The liver and certain other tissues can access testosterone this way. So SHBG is not just a blocker. It also directs where testosterone goes. This is an active area of research and the full picture is still being worked out.
What Does Total Testosterone Actually Measure?
Total testosterone is a blood test that measures all testosterone in the sample — bound to SHBG, bound to albumin, and free. It is the most commonly ordered testosterone test and the most standardized across labs.
Total testosterone reflects the output of the testes and, to a smaller degree, the adrenal glands. It also reflects signals from the brain — luteinizing hormone from the pituitary tells the testes to produce testosterone. When a man’s total testosterone is low on repeat morning testing, that points toward a problem in this signaling chain or in testicular function.
Total testosterone is useful because it is reliable and well studied. The downside is that it does not tell you how much is biologically available. Two men with the same total testosterone can have very different free testosterone if their SHBG levels differ.
What Does Free Testosterone Actually Measure?
Free testosterone measures only the fraction of testosterone not bound to any protein. It is the portion that can slip into cells and activate androgen receptors directly.
Free testosterone is harder to measure accurately than total testosterone. The most reliable method is equilibrium dialysis, a lab technique that separates bound from unbound hormone. Many labs instead calculate free testosterone from total testosterone and SHBG using a formula. Calculated values are convenient and widely used, but they are estimates.
Direct free testosterone immunoassays have a reputation for being less accurate at the lower end of the range. This matters because the free testosterone fraction is small to begin with. Small measurement errors can shift results meaningfully.
Why Do Total And Free Testosterone Sometimes Disagree?
They disagree because SHBG sits between them. Any condition that changes SHBG will pull total and free testosterone in different directions.
When SHBG rises, total testosterone tends to rise with it while free testosterone falls. When SHBG falls, total testosterone tends to fall while free testosterone rises. The two numbers move apart.
This is why a man can have a total testosterone that looks fine on paper yet still have low free testosterone. It is also why a low total testosterone reading does not automatically mean low free testosterone. The SHBG level is the missing piece that explains the gap.
What Causes High Or Low SHBG?
SHBG levels are influenced by hormones, liver function, body weight, age, and some medications. The relationships are well established, though the size of the effect varies from person to person.
Factors linked to higher SHBG:
- Older age — SHBG tends to rise gradually with age in men
- High estrogen levels
- Thyroid hormone excess (hyperthyroidism)
- Liver disease
- Low body weight or very low body fat
- Some medications, including certain anticonvulsants
Factors linked to lower SHBG:
- Higher body fat, particularly around the abdomen
- Insulin resistance and type 2 diabetes
- Obesity
- Treatment with testosterone or anabolic steroids
- Some medications, including certain corticosteroids and progestins
These are associations, not simple cause-and-effect switches. Body fat and insulin resistance, for example, are intertwined, and separating their individual effects on SHBG is difficult. What is clear is that metabolic health and SHBG are closely linked.
When Does The Free Vs Total Distinction Matter Most?
The distinction matters most when symptoms and total testosterone do not match. A man with low libido, fatigue, or reduced muscle mass and a normal total testosterone is a classic case where free testosterone and SHBG add useful information.
It also matters in specific groups. Men with obesity often have low SHBG, which can make total testosterone look low while free testosterone is closer to normal. Older men often have higher SHBG, which can make free testosterone look low while total testosterone is normal. Both patterns are well described in the medical literature.
The distinction is less important when total testosterone is clearly and repeatedly low. In that case, the diagnosis is usually clear without leaning heavily on free testosterone.
Which Test Should You Look At?
Total testosterone is the standard first test. Clinical guidelines generally recommend measuring total testosterone, drawn in the morning, and confirming a low result with a repeat test before drawing conclusions. Free testosterone is usually a second step, not a first one.
Free testosterone is most useful when total testosterone is borderline, when SHBG is suspected to be abnormal, or when symptoms persist despite a normal total testosterone. Calculated free testosterone from total testosterone and SHBG is commonly used in this setting.
There is genuine debate among clinicians about how much weight to give free testosterone. Some guidelines emphasize total testosterone as the primary measure, while others support using free testosterone in specific situations. The evidence does not support using free testosterone alone as a screening test for everyone.
What This Means In Plain Terms
Think of total testosterone as the size of a delivery. Free testosterone is how much actually reaches the door. SHBG is the shipping system that decides how much gets held up along the way.
A normal total testosterone does not guarantee normal free testosterone, and a low total does not always mean low free. The SHBG level explains the difference. When symptoms and lab results seem to conflict, SHBG and free testosterone are often the reason.
None of this is a reason to self-diagnose or self-treat. Testosterone testing and any treatment decisions belong with a clinician who can interpret the full picture, including symptoms, repeat testing, and other health factors.
Frequently Asked Questions
Does high SHBG mean low testosterone?
High SHBG usually means lower free testosterone, not necessarily lower total testosterone. Total testosterone can stay normal or even rise while the free portion falls.
What is a normal free testosterone level?
Normal ranges vary by lab and by the method used, so there is no single universal number. Your result should be interpreted against the reference range printed by the lab that ran the test.
Can you have normal total testosterone and low free testosterone?
Yes. This happens most often when SHBG is high, which binds more testosterone and leaves less free. It is one reason free testosterone is sometimes checked when total testosterone looks normal but symptoms persist.
Does SHBG affect testosterone treatment?
SHBG can affect how testosterone behaves in the body and how results are interpreted during treatment. How much it changes clinical decisions is debated, and monitoring should be guided by a clinician.
