Women's health and hormones
Irregular Periods Blood Test UK: Which Hormones to Check
Reviewed by a qualified clinician · analysed at UKAS-accredited UK labs (ISO 15189)
Last reviewed July 20267 min read
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Periods are irregular when the gap between them is under 21 days or over 35 days. The most common hormonal cause is PCOS, though thyroid changes and perimenopause matter too. A blood test checks the hormones behind your cycle, so you can see what is driving it rather than guess.
See which hormones to check →As of July 2026.Irregular periods are one of the most searched women's health questions in the UK. This guide gives the honest read: what counts as irregular, what tends to cause it, and the one step that turns worry into an answer.
Your cycle is a monthly hormone conversation between the brain and the ovaries. When that signal is disrupted, the timing of your period drifts. The pattern is a clue. A blood test is how you read it.
1. What counts as an irregular period?
The average cycle runs about 28 days, counted from the first day of one period to the first day of the next. But normal covers a wide band. The NHS draws the line clearly.
“A woman's periods are irregular if the gap between them is less than 21 days or more than 35 days.”
— NHS, Irregular periods
A single odd month is rarely a worry. Cycles shift with a bad week or a hard training block. What matters is a lasting change: periods that keep coming too close together, too far apart, or that vary a lot in length.
Some drift is expected at either end of your reproductive life. Cycles are often irregular in the first year or two after they start, and again as you approach the menopause. Between those points, a new and settled change is worth looking into.
2. What causes irregular periods?
The NHS lists the usual drivers: puberty, the approach to menopause, pregnancy, hormonal contraception, big weight changes, stress and heavy exercise. Two hormonal causes are worth singling out.
PCOS. Polycystic ovary syndrome is the most common hormonal cause of irregular periods, and it affects around 1 in 10 women in the UK. It raises androgens (male hormones), which can disrupt ovulation. Our PCOS blood test guide explains how it is assessed.
Thyroid changes. An under- or over-active thyroid can speed up, slow down or stall your cycle. The symptoms overlap with PCOS, so a thyroid blood test is part of a proper look.
Perimenopause. In the years before menopause, hormone levels swing and cycles become unpredictable. If you are over 40 with new irregularity, our perimenopause blood test guide covers what changes and why.
3. Symptoms aren't proof: which hormones to check
Irregular periods have several possible causes that look alike from the outside. Blood work is how you tell them apart. A first-line hormone check looks at:
- FSH and LH, the two brain hormones that steer the cycle. Their pattern gives a read on ovulation and the menopause transition.
- Testosterone, SHBG and the free androgen index, the androgen markers used to investigate PCOS. SHBG is the protein that binds testosterone, and the free androgen index shows how much is active.
All five sit in Helvy's Hormone Balance panel (£99), which measures FSH, LH, SHBG, total testosterone and the free androgen index. It is a home finger-prick kit from a UKAS-accredited UK laboratory, with results in about 5 working days.
Thyroid is the other big one to rule out, and a TSH test covers it. If your periods are also heavy, they can leave you low in iron, so a full blood count is worth adding to check for anaemia.
4. When to see your GP
A test gives you numbers. Some signs still call for a clinician. The NHS says to see a GP if your periods are irregular, if they last longer than 7 days, or if the gap keeps changing.
Book sooner if irregularity comes with weight gain, tiredness, hair growth on your face, or skin changes, as these can point to PCOS or a thyroid issue. Also see a GP if periods stop for a while and you could be pregnant, or if you are trying to conceive.
A blood test and a GP work well together. Numbers in hand make the appointment sharper, and a qualified clinician is the right person to interpret them alongside your history.
READY TO TEST?
Check the hormones behind your cycle
Irregular periods can point to PCOS, a thyroid change or perimenopause. Helvy's home finger-prick Hormone Balance panel measures FSH, LH, SHBG, total testosterone and the free androgen index, so you can act on numbers rather than a hunch.
Frequently asked questions
What counts as an irregular period?
The NHS says periods are irregular if the gap between them is less than 21 days or more than 35 days. The average cycle is about 28 days, but normal covers a wide range. A one-off odd month is rarely a concern; a lasting change is what to look into.
Can a blood test tell why my periods are irregular?
It can point to the cause. Checking FSH, LH, SHBG, testosterone and the free androgen index shows whether PCOS or the menopause transition is in play, and a thyroid test rules that cause in or out. Bloods support the picture rather than diagnose on their own, so a clinician reads them with your symptoms.
Is PCOS the cause of my irregular periods?
PCOS is the most common hormonal cause, and it affects around 1 in 10 women in the UK. It is not the only cause, though. Thyroid changes, perimenopause, weight changes, stress and heavy exercise can all disrupt a cycle, which is why testing the hormones is worthwhile.
When in my cycle should I test?
Days 2 to 5 of your cycle, counting from the first day of your period, is ideal for cycle hormones. If your periods are absent or very irregular, a sample at any time is acceptable, and it is worth noting that when results are interpreted.
When should I see a GP about irregular periods?
The NHS says to see a GP if your periods are irregular, last longer than 7 days, or the gap keeps changing. Book sooner if you also have weight gain, tiredness, facial hair or skin changes, if you could be pregnant, or if you are trying to conceive.
Related guides
PCOS Blood Test UK
Which hormones confirm PCOS, how diagnosis works, and what your results actually mean.
LH & FSH Blood Test UK
What the two brain hormones that steer your cycle reveal, and how to read them.
Perimenopause Blood Test UK
What changes in the years before menopause, and which markers are worth checking.
Women's Hormones Hub
The full set of women's hormone guides, from PCOS to perimenopause and beyond.