What does SHBG do on TRT?
SHBG on TRT does the same job it does without treatment: it decides how much of your testosterone is free. SHBG (sex hormone-binding globulin) is a protein made by the liver that grips testosterone tightly in the blood. About 40% of testosterone is bound to SHBG, about 2% is free, and the rest is held loosely by albumin and other proteins.
So two men with the same total testosterone can have quite different free testosterone if their SHBG differs. Guidelines use SHBG mainly for that reason. It tells them when the total is misleading and free testosterone is worth measuring.
It is not part of the routine TRT lab panel. So it shows up only when your prescriber has a reason to order it.
What is SHBG?
SHBG is a carrier protein made in the liver. It binds the sex steroids and moves them around the body. These are mainly testosterone, its stronger relative DHT, and estradiol.
The key difference between the carriers is grip. Albumin holds testosterone loosely and lets go easily, while SHBG holds it tightly. The free part is thought to be the part that acts on tissues.
How does SHBG change free testosterone?
When SHBG goes up, more testosterone is locked away and the free share goes down. When SHBG goes down, the free share goes up. The total can stay the same while the free part changes.
That is why the Endocrine Society asks for free testosterone in men who have a condition that alters SHBG. It prefers a lab method called equilibrium dialysis, with an accurate formula as the fallback. The formula most calculators use needs your total testosterone and your SHBG from the same draw.
We cover the three ways to measure free testosterone, and why their ranges differ, on our free vs total testosterone page. This page stays on SHBG itself.
How big is the effect?
Large enough to change how a result reads. Take a typical albumin and a total testosterone of 450 ng/dL. The most widely used formula gives about 120 pg/mL of free testosterone when SHBG is 20 nmol/L.
When SHBG is 60 nmol/L, it gives only about 62 pg/mL. Same total, roughly half the free testosterone.
The reverse matters on TRT. A man with low SHBG may see a total testosterone that looks modest at his trough, the lowest point before his next dose, while his free testosterone is in a comfortable range. Reading the total without the SHBG next to it can lead to the wrong conclusion.
That is why many prescribers look at the two together.
How does estradiol fit in?
Estradiol, the main estrogen, also binds to SHBG. Estrogens also push the liver to make more SHBG. On treatment, estradiol tends to rise because part of the testosterone is turned into it.
In the Testosterone Trials, small changes in estradiol had a bigger effect on the free share of testosterone than changes in testosterone itself. Our estradiol page explains why that hormone rises on TRT and how it is measured.
What raises and lowers SHBG?
The Endocrine Society lists conditions that change SHBG as one reason to measure free testosterone. The table groups them.
| Tends to raise SHBG | Tends to lower SHBG |
|---|---|
| Ageing | Obesity |
| Liver cirrhosis and hepatitis | Diabetes mellitus |
| Overactive thyroid (hyperthyroidism) | Underactive thyroid (hypothyroidism) |
| Some anti-seizure medicines | Glucocorticoids, some progestins and androgens, including testosterone |
| Estrogens | Nephrotic syndrome (a kidney condition) |
| HIV infection | Acromegaly (excess growth hormone) |
The guideline’s table also lists changes (variants) in the SHBG gene, which can push SHBG either way.
The biology behind some of these is well described. Thyroid hormone and estrogens make the liver release more SHBG, while signals from inflammation reduce it. In cell and animal studies, the sugars glucose and fructose reduce it too.
In liver cell and mouse studies, these sugars cut SHBG output by driving fat production in the liver. That is lab work, not a diet plan. It helps explain why SHBG tends to be low in overweight people and in those with metabolic syndrome.
Some of these change quickly. Starting or stopping a medicine on the list, or a large change in weight, can move SHBG within months. A result from before the change may not describe you now.
Tell your prescriber about thyroid or liver problems, new medicines and big changes in weight. Each one can move SHBG and so change how your testosterone results should be read.
Does SHBG change with age?
Yes, it rises. In a large US study of men aged 40 to 70, SHBG rose about 1.6% a year. Total testosterone fell about 0.8% a year and free testosterone about 2% a year.
So free testosterone falls faster than total with age, because SHBG holds more of a shrinking total. An older man’s total can look fine while his free testosterone is low. For total testosterone ranges by age, see our page on normal testosterone levels by age.
What happens to SHBG on TRT?
The studies do not agree. None of the three below used weekly cypionate or enanthate injections, and we did not find one that did. Treat them as a rough guide only.
- No change. In the Testosterone Trials, older men on testosterone gel for 12 months had higher total and free testosterone, but SHBG did not change.
- A small fall. In a 24-week trial of 39 men aged 50 to 70 with type 2 diabetes, testosterone gel lowered SHBG by about 2 nmol/L compared with placebo.
- A larger fall over years. In a registry of 321 men on long-acting testosterone undecanoate shots every 12 weeks, SHBG fell by about 10.5 nmol/L over up to 12 years.
Testosterone and other androgens are on the list of things that lower SHBG. So a modest fall on treatment would not be surprising. How much it falls likely depends on the product, the levels reached and the person.
What does a fall mean in practice? If SHBG drops while your free testosterone holds steady, your total can read a little lower without anything being wrong. If SHBG rises, the total can look better than the free part really is.
That is why a change in your total between draws is worth reading alongside SHBG. Ask your prescriber how they want to read the pair.
Does injection frequency change SHBG?
A common question is whether more frequent injections change SHBG. We did not find a controlled study comparing weekly with twice-weekly (or more frequent) injections that measured SHBG. The guidelines do not set injection schedules by SHBG.
We also did not find trial evidence that a schedule change will lower SHBG. Your schedule is your prescriber’s decision. The testosterone level estimator shows how frequency changes the shape of your testosterone curve, using your current prescription.
Is low SHBG a health problem?
In men not on treatment, low SHBG travels with metabolic problems. A review reports that studies link low SHBG with a higher chance of type 2 diabetes, apart from sex hormone levels. In one study, repeated in a group of men, those in the top quarter of SHBG had a much lower risk of type 2 diabetes than those in the bottom quarter.
A pooled analysis of 20 studies found that each step down in SHBG went with a higher chance of metabolic syndrome. That is the cluster of high blood pressure, high blood sugar, high triglycerides and a large waist. The link was strongest in men who were not overweight, so SHBG may be more than a marker of body weight.
These studies looked at men’s own SHBG, not SHBG lowered by testosterone treatment. Nobody has shown that a fall caused by TRT carries the same risk. A low SHBG is a reason to look at weight, blood sugar and liver health with your prescriber, not a number to chase.
Is high SHBG a problem?
High SHBG mostly matters for what it does to free testosterone. With more testosterone locked up, a man can have symptoms of low testosterone while his total looks normal. That is exactly when the Endocrine Society measures free testosterone.
High SHBG can also be a clue to something else, such as an overactive thyroid or liver disease. That is why your prescriber may look further rather than treat the number.
The AUA mentions SHBG in another case. A man on treatment may have a high hematocrit (the share of blood made up of red cells) and a low-normal testosterone. For him, the AUA suggests measuring SHBG and free testosterone with a reliable test.
What is a normal SHBG range?
SHBG is usually reported in nmol/L, and every lab prints its own range. Many labs use separate ranges for younger and older men because SHBG rises with age. One large US lab, for example, lists 16.5 to 55.9 nmol/L for men aged 20 to 49 and 19.3 to 76.4 nmol/L above 49.
The guidelines do not define a “low” or “high” SHBG. They leave that to the lab range and the wider clinical picture.
Most labs measure SHBG with an immunoassay, a test that uses antibodies to detect the protein. If you change labs, expect the range to change too. Do not read a small shift between two labs as a real change in your body.
So compare your result with the range on your own report, and compare results from the same lab over time. Some SHBG gene variants are missed by immunoassays or hold testosterone less tightly. That can make a result hard to read.
How should you time an SHBG test?
SHBG varies much less across the day than testosterone does. In a study of 66 men sampled over clinic hours, testosterone was 20 to 25% lower at 4 pm than at 8 am in men aged 30 to 40. SHBG changed much less across the day.
We did not find a study of SHBG across an injection cycle, so ask your prescriber when to draw it. What matters is that it comes from the same draw as the total testosterone it is paired with. Free testosterone is calculated from both.
Draw it with your usual TRT labs. The draw-date planner helps with timing for the full panel. Estradiol, which also binds to SHBG, is often checked in the same draw.
What should you ask your prescriber about SHBG?
- Is my SHBG high or low for my age, and on which lab’s range?
- Could thyroid, liver, weight or a medicine explain it?
- Does it change how you read my total testosterone?
- Would a calculated or measured free testosterone help?
- Is anything else worth checking, such as blood sugar or liver tests?
How can you track SHBG over time?
Because SHBG moves slowly, its trend over months tells you more than any single value. Log it with the total and free testosterone from the same draw. Then you can see whether a change in free testosterone came from your testosterone or from SHBG.
TRTracker keeps SHBG next to your other hormone results, each against its own lab range. Our guide to what each TRT blood test is for runs from testosterone and hematocrit to PSA.