LH/FSH ratio: the classic, imperfect PCOS pattern
LH (luteinising hormone) and FSH (follicle-stimulating hormone) are two hormones from the pituitary gland that help govern the menstrual cycle. The ratio between them was once treated as a hallmark of PCOS. It's still measured, but it's far more nuanced than that reputation suggests — and when in your cycle the blood is taken matters a great deal.
What this test measures
This test measures LH and FSH and looks at the balance between them. In PCOS, LH is sometimes raised relative to FSH, producing a higher ratio — the 'classic' pattern. But both hormones swing across the menstrual cycle, so a result taken on one day can differ markedly from another. Reference ranges and the way the ratio is interpreted vary by lab and by cycle phase, so there's no fixed figure to quote, and we don't. The number only means something with its timing attached.
Why it comes up in PCOS care
The raised LH/FSH ratio is part of PCOS history, and it still gets checked — but it's recognised as imperfect. Plenty of women with PCOS don't show it, and it isn't required to make the diagnosis. Because the hormones fluctuate, clinicians pay attention to where in the cycle the blood was drawn; an early-cycle sample is read differently from a mid-cycle one. The NHS describes hormone tests as part of the diagnostic process, and this ratio is one input among several, not a deciding test.
Keeping results across years
Because timing is everything with LH and FSH, a result is only interpretable if you also know the cycle day it was taken on — precisely the detail that gets stripped away as numbers move between systems. A dated vault that records the result and lets you note where you were in your cycle keeps that context attached. Over time, that turns a scatter of context-free figures into a series a clinician can actually compare.
PCOS Diary keeps each LH/FSH result dated and next to your cycle in the lab vault, so the timing a result depends on travels with it in your PDF.
Common questions
- Do I need a raised LH/FSH ratio to have PCOS?
- No. The raised ratio is a classic association but an imperfect one — many women with PCOS don't show it, and it isn't required for diagnosis. It's read as one input alongside your symptoms and other tests.
- Why does the timing of the blood test matter?
- LH and FSH change across the menstrual cycle, so the same person can get quite different readings depending on the day. Knowing the cycle day the sample was taken is essential to interpreting the result, which is why it's worth recording.