Missing periods: 9 causes of amenorrhoea

If your period stays away for three months or longer while you are not pregnant, that is called amenorrhoea. An estimated 3 to 5 percent of women of reproductive age deal with it. In nearly every case there is a traceable cause, and it is reversible more often than women expect.

What strikes me about most pages on this topic: they list the causes but not which test belongs to which cause. So you arrive at your GP without knowing what you want to discuss.

These nine are ordered by how often they occur, with a table at the end linking each suspicion to the blood values that fit it.

What counts as amenorrhoea?

Amenorrhoea means your period stays away for three months or longer after a previously regular cycle, or six months after a previously irregular one. If you have never menstruated and you are older than fifteen, that is called primary amenorrhoea. Everything in between usually falls under oligomenorrhoea.

That distinction is not nitpicking. Oligomenorrhoea means a cycle longer than 35 days, so you do still menstruate, just rarely. The two have partly different causes and partly different work-ups.

Always count your cycle from the first day of bleeding. That is day 1.

1. Pregnancy

It sounds obvious, and yet this is where every GP starts. A pregnancy test costs a few euros and rules out the most common cause in five minutes.

Including on contraception, including after sterilisation, and including when you think it cannot be the case. Take the test first, and the rest of the conversation gets more useful.

2. Hormonal contraception and the months after

With the hormonal coil, the injection and continuously taken pills, periods often stop altogether. That is an expected effect of the method, not a disorder.

After stopping the pill, most women see their own cycle return within a few weeks to a few months. If nothing happens after three months, that is a reason to look further. What happens in that window is covered in coming off the pill.

3. Too little available energy

This is the cause missed most often, and the reason is awkward: your BMI can look entirely normal. It is not about your weight but about the gap between what you eat and what you burn.

With a sustained shortfall, your hypothalamus dials down the signal to your ovaries. That is called functional hypothalamic amenorrhoea, and in younger women it is one of the most common causes (PMID 28368518).

Take two women of 29, same height, both with a BMI of 21. One trains three hours a week, the other ten and does not eat up to match. Only the second runs a real risk of this form, while at first glance their blood values look alike: low oestradiol with an LH and FSH that are notably not raised.

A 2024 review set out the lifestyle factors involved and stresses how often recovery tracks with energy balance rather than with a medicine (PMID 39275282).

4. PCOS

In PCOS ovulation regularly fails to happen, and without ovulation there is no normal period. More often that produces oligomenorrhoea than full amenorrhoea, but both occur.

The telling pattern is the combination with signs of androgen excess: acne along your jawline, more hair growth on your face or abdomen, or thinning at the crown. Which values fit is covered in PCOS testing.

5. A raised prolactin

At high levels prolactin suppresses the hormones that drive your ovulation. The result is a cycle that first turns irregular and can then stop.

Sometimes there is milky nipple discharge, often there is not. Medication is a common reason, a benign pituitary growth another (PMID 21296991). The full list is in high prolactin.

6. Thyroid too slow or too fast

Both an underactive and an overactive thyroid can disturb your cycle. With an underactive thyroid your prolactin often rises alongside, so two causes act at once.

TSH is the first value here, with free T4 behind it. How to read that result is in thyroid values explained.

7. Early menopause

In women under 40 who stop menstruating, primary ovarian insufficiency may be involved. The ovaries then run down earlier than expected.

The picture differs from the causes above: FSH is high rather than low, usually with a low oestradiol alongside. The ESHRE and ASRM guideline describes the confirmation that belongs with it (PMID 39652037). Hot flushes and night sweats can be present, but need not be.

8. Stress, illness and rapid weight change

A bereavement, surgery, burnout, or ten kilos lost in a few months: your cycle responds to anything your system reads as an emergency. Usually the period returns on its own once the situation settles.

Chronic conditions such as poorly controlled diabetes or coeliac disease can do the same. Cortisol plays a role in the stress route, see high cortisol.

9. Rarer causes your GP rules out

Adhesions in the uterus after a procedure, a congenital difference in uterus or vagina, and adrenal conditions sit at the bottom of this list because they are uncommon. They are on it, because they need a different investigation than blood testing alone.

If your period stays away while every hormone value is normal, this is the direction the search takes next.

Which blood values fit which suspicion?

No single blood value explains amenorrhoea. What your GP does is request a small basic set and extend it guided by your symptoms. This table links the suspicion to the values that fit, so you know what you are discussing.

I would take it to your appointment. Not to decide what gets measured, but so the conversation runs on your pattern rather than on one isolated result.

The bottom two rows look almost identical on oestradiol, and yet the approach is opposite. The difference sits in FSH: high in early menopause, low or normal with an energy shortfall. That is why FSH and oestradiol are nearly always requested together, see FSH testing. The partner to FSH is covered in LH levels.

Timing matters less here than for a routine cycle check. If you are not menstruating there is no cycle day to wait for, so you can draw blood at any point. If you want that picture taken, hormones for women fits these values.

When do you contact your GP?

At three months without a period an appointment is reasonable, even if you feel fine. Call sooner with milky discharge, with new headaches or a change in your vision, with hot flushes under 40, and with rapidly increasing hair growth or a deeper voice.

Thuisarts and the NHG information for Dutch GPs describe the same route: rule out pregnancy, then a limited hormone set, then extend on indication. What helps is bringing an overview of your last six cycles, your weight trend and your medication.

The wider overview of which hormone test fits when is in hormone testing in women. If you do still menstruate but irregularly, irregular periods is the better fit.

References

1. Gordon CM, Ackerman KE, Berga SL, et al. Functional Hypothalamic Amenorrhea: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2017;102(5):1413-1439. PMID 28368518.
2. Dobranowska K, Plevin D, Smith H, et al. Dietary and Lifestyle Management of Functional Hypothalamic Amenorrhea: A Comprehensive Review. Nutrients. 2024;16(17):2967. PMID 39275282.
3. ESHRE, ASRM, CREWHIRL and IMS Guideline Group on POI. Evidence-based guideline: Premature Ovarian Insufficiency. Fertil Steril. 2025. PMID 39652037.
4. Melmed S, Casanueva FF, Hoffman AR, et al. Diagnosis and treatment of hyperprolactinemia: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2011;96(2):273-288. PMID 21296991.
5. Thuisarts. Ik ben niet ongesteld. Available via thuisarts.nl.
6. RIVM and VZinfo. Fertility and reproductive health: figures and context. Available via vzinfo.nl.

Every blood test result at Lunara includes a professional assessment by a BIG-registered doctor. For treatment decisions, discuss your results with your GP.