LH / FSH

What they are

LH (luteinising hormone) and FSH (follicle-stimulating hormone) are two gonadotrophins secreted by the pituitary gland. Together they regulate the ovarian cycle: FSH stimulates the growth and maturation of follicles; LH triggers ovulation (the mid-cycle LH surge) and stimulates the theca cells of the ovary to produce androgens.

Both are measured in the early follicular phase (cycle days 2 to 4), when their basal levels are most representative. In cases of amenorrhoea, the test can be performed at any time.

Why they matter in PCOS

In PCOS, LH secretion is often tonic — persistently elevated rather than properly pulsatile and rhythmic as in a normal cycle. This LH hypersecretion has two major consequences: it over-stimulates theca cells to produce excess androgens, and it disrupts normal follicular maturation, contributing to anovulation.

The LH/FSH ratio is often used as an indicator: in PCOS, this ratio frequently exceeds 2:1, and sometimes 3:1. This imbalance reflects the relative dominance of LH over FSH. It is important to note that this ratio is not an official diagnostic criterion for PCOS (the Rotterdam criteria do not include it), but it can support the diagnosis in context.

FSH, meanwhile, is often normal or slightly low in PCOS. An elevated FSH (especially > 10 IU/L in the follicular phase) points instead towards diminished ovarian reserve or premature ovarian insufficiency — which is distinct from PCOS.

Normal values

• LH (follicular phase): 1–20 IU/L
• FSH (follicular phase): 3–12 IU/L
• LH/FSH ratio in PCOS: often > 2:1, sometimes > 3:1
• Ovulatory LH surge: 25–100 IU/L (not measured in baseline panel)

When is it measured?

LH and FSH are ideally drawn between cycle days 2 and 4 (D2–D4), fasting, in the morning. In cases of amenorrhoea, any time is acceptable. Avoid testing during the ovulatory window to prevent confusing the LH surge with an elevated basal level.

Key takeaways

• LH drives ovarian androgen production; FSH drives follicle growth
• Tonic elevated LH in PCOS → hyperandrogenism + anovulation
• LH/FSH ratio > 2:1 common in PCOS, but not an official diagnostic criterion
• Elevated FSH (> 10 IU/L) suggests diminished ovarian reserve — different from PCOS
• Draw on cycle days 2–4, or any time if amenorrhoeic