Estradiol (E2)

What Estradiol Measures

Estradiol is one of three main estrogens the body makes, alongside estrone (E1) and estriol (E3), and it's the dominant one from puberty through the reproductive years.^[1] It's produced when follicle-stimulating hormone (FSH) activates an enzyme, aromatase, inside the cells of a growing ovarian follicle, converting other hormones into estradiol.^[1] As a follicle matures, it makes more estradiol, so the level in blood tracks how far along that follicle's development is.

How It's Measured

Estradiol is measured directly from a blood draw as part of a serum hormone panel, reported in pg/mL, and blood testing is the standard way to get a precise reading.^[2] It can also be tracked indirectly through urine, since the body breaks estradiol down into a metabolite called estrone-3-glucuronide (E3G), which some at-home fertility monitors read from a urine sample rather than a blood draw.^[4]

Why People Track Estradiol

Estradiol's rise and fall is one of the three hormone signals, alongside LH and progesterone, that shape the whole menstrual cycle.^[1] Rising estradiol also changes cervical mucus in a predictable pattern, which is why mucus is a fertile-window sign people can log by hand, no lab test needed.^[1]

For a phase-by-phase walkthrough of how estradiol, LH, and progesterone move together across one cycle, see our hormone curve explainer.

How Go Go Gaia Tracks Estradiol

Go Go Gaia doesn't run lab tests itself, but you can upload results from a serum estradiol panel or a urine E3G reading, and the app charts the values over time next to your logged cycle day and phase. That makes it possible to see where a given estradiol reading fell relative to your period, without needing to line up dates by hand.

What the Research Shows

Estradiol climbs through the follicular phase as a dominant follicle matures and peaks right before ovulation. That sustained peak flips estradiol's usual suppressing effect on LH into a trigger, and the resulting LH surge leads to ovulation about a day and a half later.^[1][3] Our hormone curve explainer covers that mechanism, and the smaller second estradiol rise that follows in the luteal phase, in more depth.^[2]

Because serum estradiol and urinary E3G both come from the same hormone, researchers have compared them directly: one 2024 study found day-specific serum estradiol and urinary E3G from an at-home fertility monitor followed a similar pattern across ovulatory cycles, though serum measurement was more precise at pinpointing the fertile window.^[4]

Related Terms

Frequently Asked Questions

Educational information based on published sources. Not medical advice. For personal concerns, please consult your doctor.

Is estradiol the same as estrogen?

Estrogen is a category, not a single hormone. Estradiol (E2) is one of three main estrogens, along with estrone (E1) and estriol (E3), and it's the dominant one during the reproductive years. Estrone becomes the more prominent estrogen after menopause.

Why does estradiol rise twice in one cycle?

The first, larger rise happens in the follicular phase as a maturing follicle produces more estradiol, peaking right before ovulation. The second, smaller rise happens in the luteal phase, when the corpus luteum, the structure left behind after the egg releases, produces estradiol alongside progesterone.

What's the difference between a serum estradiol test and E3G?

A serum estradiol test measures the hormone directly from a blood draw. E3G (estrone-3-glucuronide) is a urinary metabolite of estradiol, meaning it's what's left after the body processes and excretes it, and it can be measured from a urine sample using an at-home fertility monitor.