Free Testosterone vs Estradiol: Key Differences Explained
In men, estradiol is made largely from testosterone, so the two are read as a pair. Learn how free testosterone and estradiol differ and what each adds in men and women.
- Published
- April 23, 2026
- Last updated
- September 25, 2026
- Read time
- 4 min read
- Medically reviewed by
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Rhonda Collins, FNP-C
Free testosterone and estradiol are linked by one enzyme. Aromatase converts testosterone into estradiol, so in men much of the estradiol in the blood starts out as testosterone. Free testosterone is the share of testosterone not bound to proteins and available to act on tissues. Estradiol is the main estrogen. In men the two are read as a pair, because each one carries out different jobs that the other cannot fully cover. In women they answer different questions.
What Is Free Testosterone?
Free testosterone is testosterone that is not attached to proteins. Most testosterone in the blood is bound to sex hormone-binding globulin (SHBG) or albumin, and bound testosterone is harder for the body to use. A free testosterone test measures only the unattached form, which is more available for building muscle and bone. In both sexes, testosterone helps control sex drive and supports muscle and bone. Women have it in smaller amounts.
What Is Estradiol?
Estradiol (E2) is the main estrogen in women of childbearing age who are not pregnant. In women it is made mainly by the ovaries, and it rises and falls across the menstrual cycle. Men make smaller amounts, largely from testosterone. Results are usually reported in picograms per milliliter (pg/mL).
How Testosterone Becomes Estradiol
Aromatase converts testosterone into estradiol in several tissues. That means a man’s estradiol tends to move with his testosterone. The conversion also matters for testosterone therapy, because some of the added testosterone is turned into estradiol.
The clearest evidence for what each hormone does in men comes from a trial of 400 healthy men aged 20 to 50. Researchers suppressed the men’s own sex hormones, gave different doses of testosterone, and in half the men also blocked the conversion to estradiol with an aromatase inhibitor. Low testosterone accounted for losses in lean mass, muscle size, and strength. Low estradiol mainly accounted for gains in body fat. Both contributed to the decline in sexual function.
Free Testosterone vs Estradiol at a Glance
Feature | Free Testosterone | Estradiol |
|---|---|---|
What it is | The unbound share of testosterone, available to act on tissues. | The main estrogen, reported as a total of bound and unbound hormone. |
Main source in men | The testicles. | Largely converted from testosterone by the enzyme aromatase. |
Main source in women | The ovaries and adrenal glands, in smaller amounts. | Mainly the ovaries, changing across the menstrual cycle. |
Roles in men | Muscle and lean mass, strength, sex drive, sperm and red blood cell production. | Body fat regulation and, together with testosterone, sexual function. |
Most common reason to test | Symptoms of low testosterone in men, or of high testosterone in women. | Cycle and menopause questions in women; estrogen balance in men. |
Main influence on the result | SHBG: more SHBG leaves less testosterone free. | Cycle day and life stage in women; testosterone levels in men. |
Usual units | pg/mL or ng/dL | pg/mL |
Reading Free Testosterone and Estradiol Together
- In men with low free testosterone: estradiol can be low as well, because there is less testosterone to convert. Some symptoms, such as rising body fat, may reflect the low estradiol as much as the low testosterone.
- In men on testosterone therapy: some of the added testosterone is converted to estradiol, so an estradiol result is read with the therapy in mind.
- In men with high estradiol: a high estradiol is read next to testosterone and SHBG, since more testosterone or more conversion can both raise it.
- In women: estradiol is read against the cycle day or life stage. Free testosterone is mostly checked for signs of high testosterone, such as facial or body hair growth, where causes include polyendocrine metabolic ovarian syndrome (PMOS, formerly PCOS) and adrenal disorders.
Labs report each hormone against its own reference range rather than as a ratio, so read each result first on its own terms. For how an estradiol value is read by age and life stage, see how to read estradiol levels by age and life stage.
FAQs
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Is estradiol bad for men? No. Men need estradiol. In the trial above, suppressing it led to more body fat and contributed to lower sexual function, even when testosterone was maintained.
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Why is SHBG often tested with free testosterone? SHBG binds testosterone tightly, so it largely decides how much of total testosterone is free.
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When should a free testosterone test be drawn? Testosterone samples are usually taken in the morning, between 7 and 10. Follow the timing on your order.
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Can women have free testosterone tested? Yes, mostly when there are signs of high testosterone, such as abnormal hair growth or irregular periods.
References
- MedlinePlus: Testosterone levels test
- MedlinePlus: Estrogen levels test
- MedlinePlus: SHBG blood test
- Finkelstein et al. Gonadal steroids and body composition, strength, and sexual function in men. N Engl J Med, 2013
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