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Estriol – Hormone, Function & Medical Use

Estriol is a naturally occurring female sex hormone belonging to the estrogen group. It plays a key role during pregnancy and is also used therapeutically.

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Things worth knowing about "Estriol"

Estriol is a naturally occurring female sex hormone belonging to the estrogen group. It plays a key role during pregnancy and is also used therapeutically.

What is Estriol?

Estriol (also known as oestriol or E3) is a naturally occurring estrogen produced in large amounts by the placenta during pregnancy. It belongs to the group of steroid hormones and is one of the three major female sex hormones, alongside estradiol and estrone. Compared to estradiol, estriol has a weaker overall estrogenic effect but exerts targeted activity on specific tissues such as the vaginal mucosa and urogenital tract.

Biological Function

Estriol performs several important functions in the body:

Promoting the maturation and blood supply of the uterine lining and vaginal mucosa
Supporting immune tolerance during pregnancy to prevent rejection of the fetus
Regulating cervical mucus in preparation for childbirth
Protecting vaginal mucosa from dryness and atrophy

Outside of pregnancy, estriol levels in the blood are very low. During pregnancy, they rise continuously, reaching their peak in the third trimester.

Medical Applications

Local Hormone Therapy in Menopause

Estriol is widely used as a local estrogen in the form of creams, vaginal tablets, or suppositories to treat conditions affecting the vagina and urinary tract. Common indications include:

Vaginal atrophy: Thinning and dryness of the vaginal mucosa after menopause
Dyspareunia: Painful intercourse due to vaginal dryness
Recurrent urinary tract infections: Improving the mucosal barrier of the urinary tract
Genitourinary syndrome of menopause (GSM): A combination of vaginal and urinary symptoms during menopause

Prenatal Screening

Maternal blood levels of unconjugated estriol (uE3) are an important component of the triple test and quadruple test performed in the second trimester of pregnancy. An abnormally low uE3 level may indicate chromosomal abnormalities in the fetus, such as Down syndrome (Trisomy 21) or Edwards syndrome (Trisomy 18), and is used as a screening marker.

Mechanism of Action

Estriol binds to estrogen receptors (ER-alpha and ER-beta) in target cells. It acts as a weak estrogen receptor agonist and shows a higher affinity for the ER-beta receptor. This means it preferentially exerts its effects in tissues with a high ER-beta density, particularly in the urogenital tract, while systemic effects (e.g., on breast tissue or the endometrium) are weaker than those of estradiol.

Dosage and Usage Notes

For local application of estriol, the following general guidelines apply:

Typical concentration in vaginal preparations: 0.01 to 0.1 mg estriol per application
An initial daily application is usually recommended, followed by a maintenance dose of once or twice per week
Treatment duration should be monitored by a healthcare professional
Systemic absorption with local use is low but possible

Side Effects and Risks

Estriol is generally well tolerated when used locally. Possible side effects include:

Local burning or itching in sensitive mucosa
Mild vaginal discharge
At higher doses or with prolonged use: minor systemic estrogenic effects

Use in women with estrogen-dependent tumors (e.g., breast cancer) should only be considered after careful consultation with a treating physician, as the individual risk profile must be assessed thoroughly.

Estriol Compared to Other Estrogens

The three main natural estrogens differ in terms of potency and clinical application:

Estradiol (E2): The most potent natural estrogen, primarily used systemically
Estrone (E1): The predominant estrogen after menopause, weaker than estradiol
Estriol (E3): The weakest of the three estrogens, preferred for local use, dominant during pregnancy

References

Kuhl, H. - Pharmacology and Toxicology of Hormone Therapy. In: Gynecological Endocrinology (2011).
Sturdee, D.W. & Panay, N. - Recommendations for the management of postmenopausal vaginal atrophy. Climacteric, 13(6), 509-522 (2010).
World Health Organization (WHO) - Sexual and Reproductive Health: Menopause. Available at: https://www.who.int (accessed 2024).

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