Irregular periods: what they could mean
Irregular or missing periods can mean several things. This page helps you narrow the likely cause and points you to the next step.
Evidence-based · Sources cited · Updated July 2026
The most common hormonal cause of irregular periods is PCOS (now officially PMOS), usually alongside acne, unwanted hair, or weight that will not shift.[2] But thyroid disease, undereating or overtraining, high prolactin, and normal variation all cause it too. The move is not to guess. It is to get the labs that tell these apart.
The most common cause: PCOS
PCOS is the most common hormonal reason periods go irregular. It usually travels with other signs: acne, unwanted hair growth, or weight that will not shift.[2] If that pattern sounds familiar, it is worth checking properly rather than waiting it out.
What else it could be
Other causes include thyroid disease, and hypothalamic amenorrhea, where stress, undereating, or heavy training switch the cycle off. High prolactin can do it. So can perimenopause in older women. And some of it is simply normal variation, common in the first years after periods start or when coming off the pill.
When it is worth checking
Get it looked at if your cycles are regularly shorter than 21 days or longer than 35 days, if you miss three or more in a row, or if irregular periods come with other symptoms.[1] Those are the signs that this is more than normal variation.
The next step
Do not guess between these causes. The labs separate them, starting with thyroid function and androgens. Bring the pattern of your cycle to the appointment and ask for the blood work. That is what turns a vague symptom into an answer.
Not sure where you stand?
Take the 2-minute PCOS check. Get your likely pattern and the exact labs to ask your doctor for.
Take the PCOS checkKeep reading
Sources
- [1] 2023 International Evidence-Based Guideline for the Assessment and Management of PCOS. PMC10505534
- [2] Polycystic ovary syndrome, WHO fact sheet. who.int
This article is educational and is not medical advice. Talk to a clinician before starting, stopping, or combining treatments.