Irregular Periods and Hormones

Periods that arrive unpredictably, stop for a while, or change in flow are a common reason people wonder about their hormones. The menstrual cycle is coordinated by several hormone systems, so changes can have many causes. This page explains the possible links and why a clinician's assessment matters more than self-diagnosis.

How irregular periods can relate to hormones

The menstrual cycle depends on a carefully timed conversation between the brain and the ovaries. Signalling hormones from the pituitary gland prompt the ovaries to mature an egg and produce oestrogen and progesterone, and the rise and fall of these hormones drives the lining of the uterus through its monthly changes. When any part of this loop is disrupted, the timing, length, or flow of periods can shift.

Some variation is entirely normal. Cycles often settle into a pattern only gradually after the first period, can be irregular for years approaching menopause, and can be affected by everyday factors. Because the cycle is so sensitive, an irregular period is best understood as a clue rather than a diagnosis.

Which hormones and conditions may be involved

Several conditions can affect cycle regularity. None can be diagnosed from the symptom alone, but a clinician may consider:

• Ovarian sex hormones. Imbalances in oestrogen and progesterone, including cycles where ovulation does not occur, can change timing and flow.
• Androgens and PCOS. Polycystic ovary syndrome is a common cause of irregular or infrequent periods and may involve higher androgen levels.
• Thyroid hormones. Both underactive and overactive thyroid function can disturb the cycle.
• Prolactin. Raised prolactin, sometimes from a pituitary cause, can interrupt periods.
• The menopause transition. In the years before menopause, cycles commonly become irregular as ovarian hormone production changes.

Blood tests a clinician might consider

Testing is guided by the whole clinical picture, not ordered indiscriminately. Depending on the assessment, a clinician might consider:

• Thyroid tests such as TSH to screen thyroid function.
• Prolactin if a pituitary cause is suspected.
• Oestrogen and other sex-hormone tests, interpreted in the context of cycle timing.
• Tests related to androgens when PCOS is being considered.
• A pregnancy test, since pregnancy is a common reason periods stop.

You can read more about individual tests in our blood tests section and about the messengers themselves in the hormones section. The conditions named above are covered in the conditions section.

Non-hormonal causes to keep in mind

Not every change in the cycle reflects a hormone disorder. Pregnancy, significant stress, large changes in weight, intense exercise, certain medicines and contraceptives, and a passing illness can all alter periods. Structural causes within the uterus, and other gynaecological conditions, can change bleeding patterns too. Because these are common, a careful history usually comes before, or alongside, any hormone testing. For a related symptom, see acne and hormones.

When to see a clinician

It is reasonable to seek medical advice when periods stop unexpectedly, become very heavy or painful, occur very frequently or far apart, or come with other symptoms such as unexplained weight change. Bleeding after menopause should always be assessed. A clinician can take a history, examine you, and decide whether any tests are warranted. This is far more reliable than ordering tests for yourself, because results only make sense in context. This page is educational and is not a substitute for personalised medical advice.