Amenorrhea is the medical word for absent periods. It is a symptom, in the same way that a headache is a symptom — useful information, but not an explanation on its own. The whole job of the next few weeks is working out which explanation applies to you.
There are two versions of it. Primary amenorrhea is when periods have not started by around age 15. Secondary amenorrhea is when periods that were happening stop for three months or more, or for six months if they were irregular to begin with.
Pregnancy comes first
It is the most common reason a period does not arrive, and it is the quickest thing to rule out. Take a test even if you are confident it is not that — your GP will ask, and having already done it saves a step.
Low energy availability
This is the cause most often missed, and the one this site spends most time on.
Running a menstrual cycle costs energy. When the body is not getting enough fuel for everything it is being asked to do, the brain reduces the hormonal signals that drive the cycle. The clinical name is functional hypothalamic amenorrhea.
It is not only about eating too little, and it is not only about athletes. It is the gap between what you take in and what you spend — so a heavy training block, a stressful year, an illness, or a gradual drift towards eating less can each open it up. Importantly, it happens at every body size, including weights that look entirely unremarkable. That is a large part of why it gets overlooked.
Polycystic ovary syndrome
PCOS is common and often presents as irregular or absent periods. It may come with acne, unwanted hair growth, or difficulty with weight — though plenty of people with PCOS have none of those. It is diagnosed on a combination of symptoms, blood tests and sometimes a scan, which is a job for a clinician rather than a questionnaire.
Thyroid conditions
Both an underactive and an overactive thyroid can disturb the cycle. Thyroid problems are easy to test for and very treatable, which is why thyroid function is usually on the first round of blood tests.
Raised prolactin
Prolactin is the hormone involved in breastfeeding. If levels are raised when you are not breastfeeding — sometimes because of a small, benign pituitary growth, sometimes because of medication — periods can stop. A classic additional sign is milky discharge from the nipples, or headaches and changes to your vision. Mention any of those to your GP promptly.
Premature ovarian insufficiency
Sometimes the ovaries stop working as expected before the age of 40. Alongside absent periods there may be hot flushes, night sweats or vaginal dryness. It needs proper diagnosis and ongoing care, including attention to bone health.
Contraception and other medication
Several forms of hormonal contraception reduce or stop bleeding, and that is often expected rather than a problem. Some other medications affect the cycle too. Do not stop or change any prescribed medication to see what happens — discuss it instead.
What happens at the appointment
Expect a conversation about your cycle history, your weight, your training, your stress and your medication, and then usually blood tests: a pregnancy test, thyroid function, prolactin, and hormones including FSH, LH, oestradiol and testosterone. A scan is sometimes arranged. From there, treatment depends entirely on what is found.
Why bone health keeps coming up
If oestrogen stays low for months or years, bone density suffers, and that matters at any age. It is one of the main reasons clinicians treat absent periods as worth investigating rather than waiting out.
The honest limit of this article
This explains the landscape. It cannot tell you which part of it you are standing in — nobody can do that without seeing you and testing you. If your periods have stopped, book the appointment.
General education, not a diagnosis. If your periods have stopped, your GP is the next step.
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