There is no single test
People often expect a blood test to settle the question. In practice, the hormone that would be measured, follicle-stimulating hormone, swings so much from day to day during the transition that a single reading is unreliable, which is why testing it is not recommended for women aged 45 and over with typical symptoms (NICE guideline NG23, 2024). Perimenopause is recognised clinically, from the story your body is telling, not from a number.
What to actually look for
The two things that matter most are a change in your menstrual cycle and the arrival of new symptoms that fit. Periods becoming shorter, longer, heavier, lighter, or less predictable is often the first sign. Alongside that, women commonly notice some mix of hot flushes, night sweats, broken sleep, low mood or irritability, brain fog, joint aches, and changes in libido (Journal of Clinical Endocrinology & Metabolism, 2021).
For many women the very first clue is not a hot flush at all, but a run of poor sleep or a dip in concentration turning up around the same time as their cycle starting to shift.
The age question
Age changes the approach. From 45 onward, symptoms and cycle changes are usually enough to recognise perimenopause without tests. Between 40 and 45, a GP may consider blood tests in some circumstances. Under 40, symptoms that suggest menopause need proper assessment rather than being assumed, as they warrant a different pathway (NICE guideline NG23, 2024).
When to talk to a GP
See your GP if symptoms start before 45, if periods become very heavy or close together, if there is bleeding between periods or after sex, or if symptoms are affecting your daily life and you want to discuss options. A diagnosis is not the only reason to go, support is. anna does not diagnose, and this article is not medical advice.