An occasional off-schedule period is normal — cycles can shift with travel, illness or a stressful few weeks. But when periods become persistently irregular, unusually far apart, very frequent or stop altogether, it is worth understanding why. Cycles are driven by a chain of hormones, and a blood panel can help pinpoint where that chain is being disrupted.
Common causes of irregular cycles
Several conditions can throw a cycle off, and they often overlap. The most common include:
- Polycystic ovary syndrome (PCOS) — the most frequent hormonal cause, linked to raised androgens and irregular ovulation.
- Thyroid problems — both an under- and overactive thyroid can disturb periods.
- Raised prolactin — a hormone that, when high, can suppress ovulation.
- Perimenopause — the years before menopause, when cycles naturally become erratic.
- Stress, significant weight change or intensive exercise — all of which can pause or delay ovulation.
What a hormone panel checks
A cycle-focused blood panel typically looks at several hormones together: FSH and LH (which govern ovulation), oestradiol, testosterone and other androgens (relevant to PCOS), prolactin, and thyroid function. Together these markers build a picture of where the disruption lies and point towards the likely cause.
Why cycle timing matters
Some hormones vary across the month, so when the blood is taken affects how the result is read. FSH, LH and oestradiol are usually most informative when measured early in the cycle, on day 2 to 5. If your clinician wants to confirm whether you are ovulating, a progesterone test around day 21 of a typical 28-day cycle is used — timed to roughly seven days before your next expected period. When cycles are very irregular, timing can be difficult, so results are always interpreted alongside your history.
Investigate irregular periods
A hormone panel helps identify the cause of irregular cycles — checking the reproductive hormones, androgens, prolactin and thyroid together.
Investigate irregular periodsWhat happens after testing
Results guide the next step. If PCOS is suspected, your clinician may combine the bloods with an ultrasound and a discussion of symptoms. A thyroid or prolactin abnormality has its own specific follow-up. If everything is normal and you are in the right age range, perimenopause or lifestyle factors may be the explanation. The value of testing is turning uncertainty into a clear direction.
When to get checked
Consider testing if your periods have been irregular for several months, have stopped for three months or more without pregnancy, are accompanied by symptoms such as excess hair growth, acne or unexplained weight change, or if you are trying to conceive. Seek prompt medical advice for very heavy bleeding, bleeding between periods or after sex, as these need direct assessment rather than a routine panel.
Investigate irregular periods
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Specialist interest: Cardiovascular Medicine · OneMedicine, CQC-registered clinic