Biomarker 101: Oestrogen

Oestrogen has a far wider job description than reproduction alone — and like progesterone, it moves so much across the cycle that timing shapes any result.

What oestrogen is

Oestrogen is a group of related steroid hormones rather than a single substance. Three matter.

Oestradiol is the main form in women before menopause, produced by the ovaries, and the one usually measured. Oestrone becomes the predominant form after menopause, produced by fat tissue and the adrenal glands. Oestriol is the weakest and appears in significant amounts only during pregnancy.

What it does

In women, oestrogen regulates the menstrual cycle, prepares the uterus for pregnancy, and drives the development of female sexual characteristics.

Beyond reproduction it does considerably more in both sexes. It contributes to maintaining bone density, influences cholesterol handling and cardiovascular function, affects skin, and plays a role in mood. Men produce oestrogen too, where it supports sperm production and balances testosterone's effects.

What the numbers mean

Reference ranges for oestradiol depend on sex and, in women, on the phase of the menstrual cycle — and the variation across a cycle is dramatic, with levels around ovulation many times those in the early follicular phase.

Ranges in pregnancy are higher again by orders of magnitude, and postmenopausal levels are much lower. Levels also fluctuate considerably through perimenopause, which is one reason a single measurement is a poor way to establish where someone is in that transition.

As with all cycle-dependent hormones, the day of testing is part of the result.

What moves oestrogen

Lower levels occur naturally with menopause. They are also associated with very low body weight, sustained heavy exercise, disordered eating, significant stress, surgical removal of the ovaries, early loss of ovarian function, and pituitary conditions.

Higher levels can relate to hormonal medications, to higher body fat — since fat tissue produces oestrogen — to alcohol intake affecting how the liver processes it, to liver conditions, and less commonly to ovarian growths producing oestrogen directly.

A note on "oestrogen dominance"

The term appears widely online, describing oestrogen being high relative to progesterone. It is worth knowing that it is not a recognised clinical diagnosis and has no agreed definition or threshold.

The underlying idea — that the balance between hormones matters, not just absolute levels — is reasonable. But the label is used loosely to explain a broad range of common symptoms, and a doctor assessing hormonal symptoms will work from measured levels and clinical findings rather than that framing.

What oestrogen testing won't tell you

• It doesn't work without cycle context. A number without a cycle day is difficult to read.
• It doesn't diagnose menopause. Levels fluctuate too much through perimenopause for one result to settle it.
• It isn't read alone. Oestrogen is interpreted with FSH, LH and often progesterone.
• Symptoms don't map neatly onto levels. People with similar readings can feel quite different.

The bottom line

Oestrogen does considerably more than its reproductive reputation suggests, and it varies more across a cycle than almost anything else measured routinely. Read with the cycle day and the rest of the panel, it is informative — read alone, it is easy to misinterpret.