Exploring Estriol vs Estradiol & What They Mean to You

We all know that estrogen is a key hormone for reproductive health. But did you know that there are actually multiple different types of estrogen found in the body? 

Read on to learn all about the three primary forms of estrogen including estrone, estradiol, and estriol. We’ll cover how they’re similar, how they’re different, and why it may matter to you along your TTC journey. 

Estrogen 101

Estrogen is a female sex hormone that plays a key role in early sexual development, fertility, and pregnancy. It also supports overall health and organ functioning. 

There are three main types of estrogen, including estradiol (E2), estriol (E3), and estrone (E1). Estradiol is the dominant form of estrogen during reproductive years, estriol is dominant during pregnancy, and estrone is dominant during menopause.

It’s normal for estrogen levels to fluctuate throughout the menstrual cycle and over the course of an individual’s life. Testing estrogen may be necessary if are struggling to get pregnant, have a high-risk pregnancy, or if you are experiencing uncomfortable menopause symptoms.

What is estriol?

Estriol, often spelled “oestriol” and denoted as E3, is the dominant form of estrogen found during pregnancy. It is produced by the placenta, and its primary function is to control the production of other pregnancy hormones while also supporting fetal development and uterine growth. Estriol also helps to prepare the body for childbirth and breastfeeding.

It is common to have estriol levels checked periodically during pregnancy, as levels that are too high or too low may indicate certain risk factors. For example, research shows that low levels of estriol could be a sign of Down syndrome, and heightened estriol levels before week 37 is also a risk factor for premature labor.  

During menopause, laboratory-made estriol can be prescribed as part of hormone replacement therapy (HRT). While estriol alone is not FDA-approved, it is still commonly used in combination with estradiol to help ease symptoms of menopause such as hot flashes, night sweats, insomnia, and vaginal dryness. 

What is estradiol?

Estradiol, sometimes spelled “oestradiol” and denoted as E2, is the dominant form of estrogen found in individuals of reproductive age. It is produced by the ovaries, and it helps to maintain vaginal wall thickness while also supporting menstrual cycle regularity, follicular development, and uterine lining growth.

During puberty, estradiol levels increase and remain at their highest through an individual’s reproductive years. Estradiol then gradually decreases in perimenopause and remains low throughout the duration of menopause. 

When estradiol levels are out of balance, this may indicate an underlying problem and can cause symptoms. For example, low estradiol levels in younger individuals can delay puberty and disrupt the menstrual cycle. High estradiol levels on the other hand may cause acne, low libido, weight gain, and in more severe cases lead to infertility. 

As natural estradiol levels decrease in menopause, it may be necessary to take estradiol medication as part of hormone replacement therapy (HRT). This medication can help to ease common symptoms of menopause such as hot flashes and vaginal dryness. 

Estriol vs estradiol

Similarities

Even though estriol and estradiol technically perform different functions in the body, they still share a number of similarities. This is because they are both forms of estrogen that bind to estrogen receptors. It’s also important to note that the name “estrogen” can be used interchangeably to describe either estriol or estradiol. Another key similarity is that both hormones can be used as part of hormone replacement therapy (HRT) to help manage symptoms of menopause. 

Differences

The key difference between estriol and estradiol is their function. Estriol is dominant during pregnancy and helps to support both the mother and fetus, while estradiol is dominant in nonpregnant individuals and helps to regulate the menstrual cycle. 

Another key difference between estriol and estradiol is their “potency”. Estradiol is considered the more “potent” form of estrogen, and when taken as part of hormone replacement therapy (HRT), it may increase the risk of uterine cancer. Estriol is only 8% as potent as estradiol, and many doctors use estriol in combination with estradiol to offset this risk during HRT.  

What about estrone?

Estrone, sometimes spelled “oestrone” and denoted as E1, is considered the “weakest” form of estrogen and is predominant during menopause. It is produced in the ovaries, as well as the adrenal glands and adipose tissue (i.e. body fat). 

High estrone levels are associated with both breast cancer and endometrial cancer. It is also common to see elevated levels of estrone in cases of obesity due to increased production of estrone by adipose tissue. 

Low estrone levels during menopause may be associated with an increased risk of osteoporosis, however, this has yet to be confirmed by research. 

Estriol vs estradiol: why it might matter to you

If you’re TTC

Before conception, you will primarily be concerned with your estradiol (E2) levels. This is because estradiol helps to regulate the menstrual cycle and drive follicular development. If your estradiol levels are out of balance, you may struggle to ovulate regularly. In turn, this will have a negative effect on your ability to conceive. If you are undergoing in-vitro fertilization (IVF), your doctor may also prescribe estradiol to support uterine lining growth. 

Once you do become pregnant, estradiol levels will decrease and estriol levels (which are produced by the corpus luteum and placenta) will increase. It’s common to measure estriol periodically as a way to monitor the health of the pregnancy. This is especially true if your pregnancy is high risk (for example, if you are over the age of 35 or have miscarried in the past).

If you’re trying to balance your hormones

If you are not trying to conceive but are simply trying to balance your hormones, you will primarily be concerned with estradiol. Without hormone testing, it may not be clear whether your levels are too high or too low. However, low estradiol levels are known to cause irregular periods, while higher than normal estradiol levels may cause acne, weight gain, low libido, mood swings, insomnia, fatigue, and irregular cycles. 

If you suspect that your estradiol levels are out of balance and you want to do something about it, the best place to start is by examining your lifestyle. To balance estradiol (and your hormone levels in general), you should aim to:

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• Maintain a healthy weight. 
• Keep stress levels to a minimum. 
• Minimize alcohol, tobacco, and caffeine consumption. 

If you are still unable to balance your hormones after at least 6-12 months, it may be worth seeking further support from your doctor. 
It’s important to note here that the urine metabolite of estradiol (E3G) can be measured and tracked with the Mira App and Analyzer. This may be helpful if you are looking to plan a pregnancy or balance your hormones. To learn more, check out our article Estrogen Testing: How to Test Estrogen Levels at Home.