An irregular period is usually not an illness in itself, but a signal that something is shifting in your hormone balance. The cause often lies with your thyroid, with PCOS, with stress or with a transition in your life. Six hormones together usually give the first direction: FSH, LH, TSH, prolactin, oestradiol and progesterone.
This is the overview article in our cycle series. I notice that many women only start looking for the cause once the cycle has been erratic for months. That is understandable, because a single off month says little.
Below you read when a cycle is truly irregular, which causes often play a part, which hormones give insight and when a test can be useful.
When is a period irregular?
A cycle is called irregular when the time between two periods varies greatly, is structurally shorter than 21 days or longer than 35 days, or when you menstruate fewer than eight times a year. A single off month is simply part of life and is usually no cause for concern.
A normal cycle lasts 28 days on average, but anything between roughly 21 and 35 days falls within the range. So it is not about one number, but about your own pattern.
Watch especially for these signals:
- The cycle varies greatly in length each month
- You menstruate fewer than eight times a year
- Your period stays away for months while you are not pregnant
- Spotting between periods that keeps coming back
Which causes often play a part?
The cause of an irregular period ranges from a thyroid that works too slowly or too fast, to PCOS, a raised prolactin, stress, strong weight loss or approaching menopause. Which cause fits you depends on your age, your other complaints and your history.
Common causes at a glance:
- Thyroid problem: a slow or fast thyroid can disrupt your cycle (Chaker et al., 2017)
- PCOS: one of the most common hormonal causes, often with acne or hair growth
- Raised prolactin: can inhibit ovulation and suppress the period
- Stress and weight: strong weight loss, intensive exercise or prolonged stress can halt the cycle
- Perimenopause: around menopause the cycle often becomes more erratic
Some of these causes call for a focused approach, others recover on their own once the underlying situation improves. Do you suspect PCOS? Then read our PCOS pillar on symptoms, causes and diagnosis.
Which hormones give insight?
No single hormone points to the cause on its own, but a fixed set of hormones together often reveals a pattern. The table below links each symptom to a possible cause and to the value that usually says the most about it.
| What you notice (symptom) | Possible cause | Value that gives insight |
|---|---|---|
| Fatigue, cold hands, weight gain | Slow thyroid | TSH |
| Acne, hair growth, irregular cycle | PCOS, too many androgens | LH + FSH |
| Absent period, milky discharge | Raised prolactin | Prolactin |
| Hot flushes, changing cycle after 40 | Perimenopause | Oestradiol + FSH |
| Short cycle, complaints in the second half | Possibly low progesterone | Progesterone |
The best time to test depends on what you want to know. FSH, LH and oestradiol are usually measured in the first days of your cycle (day 2 to 5), progesterone around day 21. If you have no period or a very irregular one, blood can be drawn at any time.
The Hormones Women panel measures FSH, LH, oestradiol and prolactin among others in one draw, so you get the whole pattern rather than one isolated number. This table is an aid, not a diagnosis.
What if the cause lies with PCOS?
PCOS is one of the most common hormonal causes of an irregular cycle. An estimated 1 in 10 women have it, and a large share do not know (Bozdag et al., 2016). The picture often combines an erratic cycle with acne, hair growth or trouble getting pregnant.
In PCOS you look at testosterone and SHBG alongside FSH and LH. A raised LH/FSH ratio with signs of too many androgens often fits the picture, though no single value proves PCOS alone.
Want to understand more deeply how your cycle works day by day? Then read understanding your menstrual cycle and hormones.
When is testing useful?
A test is mainly useful when a pattern has lasted a while or when several complaints come together. A single late month rarely calls for action. A cycle that has been erratic for six months, or complaints that affect your daily life, do.
Consider a hormone check if you recognise this:
- Your cycle has been irregular for more than six months
- Your period stays away for months without pregnancy
- You also have fatigue, acne or hair growth alongside cycle complaints
- You are trying to conceive and your cycle is unpredictable
An abnormal result is not a diagnosis on its own. So always discuss your results with your GP, who can place the whole picture.
Testing your cycle through Lunara: how it works
You do not need a GP referral for a hormone test. You order online, book a slot at a draw location near you and have blood taken in the morning. You receive your result digitally, usually within a few working days.
Every result gets context from a BIG-registered doctor, per value. So you know not only what your FSH or TSH is, but what it can mean in your situation. Do you mainly have PMS complaints in the second half of your cycle? Then read about PMS symptoms and what helps. Is your period very heavy instead? Then read about heavy periods and when to test.
Frequently asked questions
How long can my cycle deviate before I do something?
A single off month is normal. If your cycle is structurally shorter than 21 or longer than 35 days, or if the pattern has been erratic for six months, a check can be useful. If you are unsure, discuss it with your GP.
Which hormones to test for an irregular cycle?
You often start with FSH, LH, TSH, prolactin, oestradiol and progesterone. Together they give insight into the thyroid, ovulation and possible causes such as PCOS. No single value makes a diagnosis on its own.
Can stress disrupt my period?
Yes, it can. Prolonged stress, strong weight loss or intensive exercise can inhibit ovulation and temporarily halt the cycle. The cycle often recovers once the underlying situation improves, but have ongoing complaints assessed.
References
- NHG. NHG-Standaard Vaginaal bloedverlies. Dutch College of General Practitioners. Available via richtlijnen.nhg.org.
- Thuisarts.nl. I have an irregular period. Dutch College of General Practitioners. Available via thuisarts.nl.
- Chaker L, Bianco AC, Jonklaas J, Peeters RP. Hypothyroidism. Lancet. 2017;390(10101):1550-1562. PMID: 28336049.
- Bozdag G, Mumusoglu S, Zengin D, Karabulut E, Yildiz BO. The prevalence and phenotypic features of polycystic ovary syndrome: a systematic review and meta-analysis. Hum Reprod. 2016;31(12):2841-2855. PMID: 27664216.
Every blood test result through Lunara includes a professional assessment by a BIG-registered doctor. For treatment decisions, discuss your results with your GP.
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