Oestradiol

What it is

Oestradiol (E2) is the primary oestrogen produced by the granulosa cells of growing ovarian follicles. Its level changes considerably across the cycle: low in early cycle (early follicular phase), rising progressively with follicular growth, peaking just before ovulation, then falling and rising slightly again in the luteal phase.

It is oestradiol — not oestrone or oestriol — that is routinely measured in clinical practice to assess ovarian function.

Why it matters in PCOS

In PCOS, early follicular phase oestradiol is often in the low-to-normal range. This is not inherently alarming: it simply reflects the absence of a dominant active follicle, which is consistent with the chronic anovulation characteristic of PCOS.

Oestradiol becomes critical during ovarian stimulation monitoring (ovulation induction or IVF): its level helps estimate the number and size of developing follicles, guides dose adjustments, and helps detect the risk of ovarian hyperstimulation syndrome (OHSS) — a complication more common in PCOS.

Oestradiol is also essential for assessing endometrial preparation before embryo transfer, and for understanding hormone-related symptoms in people with PCOS approaching perimenopause, where oestradiol fluctuations can be erratic and difficult to interpret.

Normal values

• Cycle days 2–4 (basal): 20–150 pmol/L (approx. 5–40 pg/mL)
• Pre-ovulatory peak: 400–1,500 pmol/L
• Luteal phase: 150–900 pmol/L
• Menopause / ovarian insufficiency: < 100 pmol/L (with elevated FSH)

A raised basal oestradiol (> 200–250 pmol/L on days 2–4) may indicate a functional cyst or persistent follicle, and should be taken into account before starting stimulation.

When is it measured?

In the baseline PCOS panel, oestradiol is measured on cycle days 2–4 alongside FSH, LH and AMH. During ovarian stimulation it is drawn every 2–3 days (depending on protocol). In cases of amenorrhoea, an anytime measurement can provide a general indication of hormonal status.

Key takeaways

• Primary ovarian oestrogen, produced by granulosa cells
• Often low-normal on days 2–4 in PCOS — consistent with anovulation
• Critical for stimulation monitoring and OHSS risk detection
• Assesses endometrial readiness before embryo transfer
• Measure ideally on days 2–4 or per stimulation protocol