Moleculium

CID 5757

C18H24O2

Estradiol

  • C 18
  • H 24
  • O 2
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44 atoms, 47 bonds.
Ball-and-stick model of estradiol: 18 carbon atoms, 24 hydrogen atoms and 2 oxygen atoms, 47 bonds (44 single, 3 double). Carbon is grey, hydrogen white, oxygen red.
IUPAC name
(8R,9S,13S,14S,17S)-13-methyl-6,7,8,9,11,12,14,15,16,17-decahydrocyclopenta[a]phenanthrene-3,17-diol
Molar mass
272.388(43) g/mol
Atoms
44
SMILES
CC12CCC3C(C1CCC2O)CCC4=C3C=CC(=C4)O
InChIKey
VOXZDWNPVJITMN-ZBRFXRBCSA-N
PubChem CID
5757

Made of

C , Carbon, nonmetal H , Hydrogen, nonmetal O , Oxygen, nonmetal
Structural formula of estradiol. Each unlabelled corner is a carbon atom, and the 19 hydrogens on carbon are not drawn. Wedges come toward you, hashes go back. It shows which atoms are joined to which, not the shape. The model above shows the shape. Structural formula of estradiol, drawn flat from PubChem's 2D record: 18 carbon atoms, 24 hydrogen atoms and 2 oxygen atoms, 47 bonds (44 single, 3 double). Each unlabelled corner is a carbon atom, and the 19 hydrogens on carbon are not drawn. Two bonds are drawn as wedges toward the reader. Symbols carry their usual colours: oxygen red. A structural formula shows how the atoms are joined, not the shape of the molecule.

Each menstrual cycle, estradiol helps to release an egg and thickens the lining of the uterus, and by doing both, MedlinePlus says, it makes pregnancy possible. The lining thickens as oestrogen rises, preparing to hold a fertilised egg, according to NICHD, the US child-health institute. The rest of the cycle belongs to the progesterone page.

MedlinePlus calls estradiol, also written oestradiol or E2, the main oestrogen (estrogen in US spelling) of females of childbearing age who are not pregnant. Medical Subject Headings (MeSH) rates it the most potent of the mammalian oestrogenic steroids. Inside cells it binds an oestrogen receptor, which the National Cancer Institute (NCI) Thesaurus locates in the female organs, breasts, hypothalamus and pituitary. The bound pair promotes the activity of genes that maintain fertility and female secondary sexual characteristics. MeSH defines oestrogens as a class by that receptor: compounds that act on it with effects like estradiol’s.

Structure and bonding

Only one of estradiol’s four fused rings is flat. Three rings have six carbon atoms and the fourth has five, fused edge to edge. That takes 17 carbons; the eighteenth is a methyl group, CH3. In the Stick view all three doubled sticks, the C=C double bonds, lie in the flat ring. Turned edge-on in Ball-and-stick, its six carbons sit in one plane.

The model draws that ring with single and double bonds alternating. In a benzene-type ring the electrons of the double bonds are shared around all six carbons, so the six bonds are alike. Such a ring is called aromatic, and MeSH describes estradiol as an aromatised steroid.

In Space-filling the two oxygen atoms sit at opposite ends of the outline, and Labels shows a hydrogen on each: two O–H groups. The hydroxyl at position 3 is on the flat ring, as in phenol; the one at position 17 is on the five-membered ring, beside the methyl-bearing carbon. Of two isomers, molecules made of the same atoms arranged differently, the model is the 17-beta form. The Hazardous Substances Data Bank (HSDB) calls the alpha form relatively inactive.

Where it turns up

Most of the body’s estradiol is made in the ovaries, on MedlinePlus’s account, and males make small amounts in the testicles. According to the NCI Thesaurus the placenta makes it too, with a small amount from the adrenal cortex, which the cortisol page describes. In both sexes oestrogen helps to regulate blood glucose and bone growth, according to MedlinePlus Genetics. HSDB records its isolation from the fluid of ovarian follicles, placental tissue, and the urine of pregnant women and mares.

Since the 1950s the US Food and Drug Administration (FDA) has approved steroid hormone drugs, estradiol among them, to speed growth in beef cattle and sheep. They are typically pellets, called implants, set under the skin on the back of an animal’s ear. FDA holds the meat safe to eat because the added hormone is very small beside the natural hormones of untreated meat and the human body. FDA residue tolerances exist for estradiol in meat.

Why it matters

The body makes its oestrogens from androgens, hormones built on 19 carbons. The enzyme aromatase makes an androgen’s ring A aromatic, giving the flat ring, and removes one methyl group, which leaves an 18-carbon oestrogen, on MeSH’s description.

As a medicine, estradiol is used in menopausal hormone therapy, usually oestrogen alone or oestrogen with a progestin, which the progesterone page explains. The US National Toxicology Program (NTP) lists steroidal oestrogens, as a class, as known to be human carcinogens, on sufficient evidence from studies in humans.

Those studies, NTP reports, tied oestrogen therapy after menopause to a consistent rise in the risk of endometrial cancer, cancer of the uterus lining. The rise in breast cancer risk was less consistent. NCI finds that adding a progestin can raise breast cancer risk, and lists a lower risk with systemic oestrogen alone, the kind given to reach the whole body. On 10 November 2025 FDA announced it was working with companies to remove references to breast cancer, cardiovascular disease and probable dementia from the labelling of these therapy products.

Properties

Properties of estradiol Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 5757.
Property Value Unit Source
Molecular formula (Hill order) C18H24O2
Molar mass 272.388(43)   g/mol g/mol IUPAC/CIAAW 2024
Carbon atoms 18
Hydrogen atoms 24
Oxygen atoms 2
Bonds 47
Single bonds 44
Double bonds 3
PubChem molecular weight 272.4   g/mol g/mol PubChem

Safety

Pure estradiol, a white crystalline powder, may damage fertility or the unborn child: 82.4% of the company reports behind its Globally Harmonized System (GHS) classification say so. Those reports, filed with the European Chemicals Agency, also include suspected of causing cancer (57.9%) and may cause cancer (39.6%). Smaller shares add harm to breast-fed children, organ damage from prolonged or repeated exposure, and very toxic to aquatic life. The PubChem Laboratory Chemical Safety Summary for estradiol is the full sheet for anyone handling the powder.

Questions

What is estradiol used for?

Estradiol is a medicine used in menopausal hormone therapy, which treats symptoms of menopause. The Hazardous Substances Data Bank lists tablets taken by mouth, vaginal tablets, a skin gel and skin patches, and the ChEMBL database marks it prescription only. Oestrogens as a class are also used in birth control, on the National Cancer Institute's account. Estradiol is also among the steroid hormones approved in the United States for implants in beef cattle and sheep.

Is estradiol the only oestrogen?

No. Oestrogens are a group, and three are commonly measured in tests, MedlinePlus says: estrone (E1), estradiol (E2) and estriol (E3). Estrone is the only one females go on making after menopause, on its account. The body converts estradiol reversibly to estrone, and either of them to estriol, the major metabolite in urine, according to the Hazardous Substances Data Bank. Commercial estradiol, too, is usually prepared from estrone.

What does aromatase do?

Aromatase is the enzyme that turns androgens into oestrogens, the class estradiol belongs to. It sits in the oestrogen-producing cells of the ovaries, placenta, testes, body fat and brain, according to Medical Subject Headings. In the placenta, MedlinePlus Genetics says, the conversion stops androgens from directing the sexual development of female foetuses. The enzyme works on one ring of the androgen, making it aromatic, and takes away one methyl group.

Why is one ring of estradiol flat?

Each of the flat ring's six carbons is bonded to only three atoms, and Chem1 describes a carbon bonded this way as holding its three neighbours in one plane, in a trigonal arrangement. The model draws the ring with three C=C double bonds. Estradiol's other twelve carbons have four neighbours each, and a carbon with four bonds directs them towards the corners of a tetrahedron.

Does estradiol in wastewater harm fish?

That is the concern the US Geological Survey (USGS) describes where treated wastewater concentrates estradiol and other oestrogenic chemicals: raised levels could feminise fish and wildlife. USGS scientists also showed that hormones such as oestrogen can biodegrade in streams and groundwater. In groundwater from a sandy aquifer on Cape Cod, they found naturally occurring bacteria degrading the estradiol.

Made of the same elements

All 51 compounds made of carbon, hydrogen and oxygen

Sources

  1. PubChem CID 5757, estradiol
  2. IUPAC/CIAAW, Abridged Standard Atomic Weights 2024
  3. PubChem Laboratory Chemical Safety Summary for estradiol (CID 5757)
  4. MedlinePlus, Estrogen Levels Test
  5. NICHD (National Institutes of Health), About Menstruation
  6. NLM Medical Subject Headings (MeSH), Aromatase
  7. MedlinePlus Genetics (National Library of Medicine), Aromatase deficiency
  8. National Cancer Institute, Menopausal Hormone Therapy and Cancer
  9. National Cancer Institute, Risk Factors: Hormones
  10. National Cancer Institute, NCI Dictionary of Cancer Terms: estrogen
  11. National Toxicology Program, Report on Carcinogens, Fifteenth Edition: Estrogens, Steroidal
  12. US Food and Drug Administration, HHS Advances Women's Health, Removes Misleading FDA Warnings on Hormone Replacement Therapy (2025)
  13. US Food and Drug Administration, Steroid Hormone Implants Used for Growth in Food-Producing Animals
  14. US Geological Survey, Hormones Degrade in the Environment! (2010)
  15. Stephen Lower, Chem1 General Chemistry, §9.6 The Hybrid Orbital Model (LibreTexts, CC BY 3.0)
  16. Stephen Lower, Chem1 General Chemistry, §9.7 The Hybrid Orbital Model II (LibreTexts, CC BY 3.0)