Irregular, late or missing periods: which blood tests your doctor checks.

If your periods are irregular, late or have stopped, the usual first step is a small set of blood tests: a pregnancy test, FSH, LH, oestradiol, prolactin, TSH and testosterone. Together they check the common causes, from PCOS and thyroid to perimenopause. Here is what each one looks for, how to time them, and when to see a doctor promptly.

What counts as irregular?

A cycle is counted from the first day of one period to the first day of the next. For adults, roughly 24 to 38 days is considered normal on the current international definition (older sources, still widely quoted in Australia, say 21 to 35 days), and a few days of variation from month to month is ordinary. Doctors call cycles irregular when they fall outside that window, or when the length swings by more than about a week from one cycle to the next.

There are some useful terms you may see on a request form or in a report:

• Oligomenorrhoea: infrequent periods, usually cycles longer than 35 days or fewer than about nine periods a year.
• Secondary amenorrhoea: periods that have stopped for three months or more in someone who previously had regular cycles, or six months or more if cycles were already irregular.
• Primary amenorrhoea: no first period by about age 15, or within three years of breast development starting.
• Heavy menstrual bleeding: a separate issue, covered in low iron and heavy periods, though it often travels with irregular cycles.

In the first few years after periods begin, and in the years before menopause, irregular cycles are common and expected. In between, a persistent change is worth understanding.

The common causes doctors consider

Most irregular or missing periods come down to a handful of causes. Your doctor weighs your age, history and other symptoms before deciding which are most likely.

• Pregnancy. Always checked first when a period is late, whatever your contraception.
• Polycystic ovary syndrome (PCOS). The most common hormonal cause of irregular periods in women of reproductive age, often alongside acne, excess hair growth or weight that is hard to shift. Not every irregular cycle is PCOS.
• Thyroid function. Both an under-active and an over-active thyroid can lengthen, shorten or stop cycles.
• Raised prolactin. The hormone behind breast milk production can rise outside pregnancy and switch ovulation off.
• Perimenopause. From the early forties, and sometimes earlier, cycles commonly shorten, lengthen and skip before they stop.
• Weight change, heavy training and stress. Significant weight change, intense exercise, illness and prolonged stress can all pause ovulation.
• Coming off hormonal contraception. Your own cycle can take a few months to settle after stopping the pill or an implant. After the contraceptive injection it commonly takes six to ten months from the last dose, and sometimes a year or more.
• Less common causes, including premature ovarian insufficiency (loss of ovarian function before 40), early menopause (between 40 and 45) and pituitary conditions, which your doctor considers if the first tests do not explain things.

The blood tests that sort them out

Each test in the standard set checks one or two of the causes above. Reading them together is what makes the pattern clear.

Test	What it is looking for	Typical adult range
hCG (pregnancy hormone)	Rules pregnancy in or out before anything else is interpreted	Negative outside pregnancy
FSH and LH	Whether the ovaries are being driven normally, are winding down (high FSH), or are switched off from above (low FSH and LH); an LH that runs higher than FSH is a supporting clue for PCOS	Varies by cycle day and age; the lab range is phase-specific
Oestradiol	Whether the ovaries are producing oestrogen; read alongside FSH	Varies by cycle day; phase-specific range
Prolactin	Raised prolactin, which can stop ovulation	Usually below about 500 mIU/L in women who are not pregnant
TSH	Thyroid function, under- or over-active	About 0.4 to 4.0 mIU/L
Testosterone, SHBG and free androgen index	Raised androgens, one of the features of PCOS	Lab-specific; the free androgen index is calculated from the other two

Ranges vary by laboratory; your report shows the range your lab uses. Typical ranges on this page are drawn from Pathology Tests Explained (RCPA) and the sources listed below.

Depending on your history, your doctor may add DHEA-S (another androgen), AMH (often raised in PCOS, low as the ovaries wind down), or fasting glucose and HbA1c if PCOS is likely. These are explained in our PCOS diagnosis article.

When in your cycle should you test if you have no regular cycle?

FSH, LH and oestradiol are usually collected on day 2 to 5, counting the first day of full bleeding as day one, because that is when the reference ranges are set. If your cycles are irregular, you have three practical options:

• Wait for a period and test on day 2 to 5 if you still get them, even far apart. This gives the most interpretable FSH and oestradiol.
• Test on any day and record the date of your last period. If it has been months, the doctor reads FSH, LH and oestradiol as a "no cycle" picture. Prolactin, TSH, testosterone, SHBG and AMH do not depend on cycle day, so they are informative whenever you collect.
• Test in the morning, rested. Prolactin rises with stress, poor sleep and a recent meal, so a calm morning collection avoids a falsely high reading. Testosterone is also highest in the morning.

On combined hormonal contraception, and on the injection or implant, FSH, LH and oestradiol mostly reflect the contraception rather than your own ovaries, and combined methods raise SHBG; a hormonal IUD affects them much less. Thyroid and prolactin are still informative. See contraception and hormone tests and the "day 3" hormone test for how to count day one.

What the results pattern can point to

Blood results do not name a cause on their own. They form a pattern that a doctor reads against your age and symptoms. These are the patterns most often seen, explained rather than diagnosed:

• Positive hCG: pregnancy, which changes everything else on the form.
• High FSH with low oestradiol: the ovaries are winding down. In your forties this fits perimenopause; before 40 it needs prompt follow-up and usually a repeat test.
• Normal or low FSH, LH running higher than FSH, raised testosterone or free androgen index: fits a PCOS picture, which a doctor confirms using the full criteria, not the bloods alone.
• Low FSH, low LH and low oestradiol: the signal from the brain has quietened, as happens with significant weight loss, heavy training, illness or stress.
• Raised prolactin: often mild and explained by stress, a recent meal or some medicines; a repeat fasting sample is usual, and persistently high levels are investigated further.
• TSH outside the range: thyroid function is likely contributing and is checked further with free T4 and antibodies.

Any one of these results is a starting point for a conversation, not a verdict. Mild deviations are common, which is why doctors often repeat a test before acting on it.

When to see a doctor promptly

Irregular cycles are rarely an emergency, but some situations need prompt attention:

• A positive pregnancy test with pain or bleeding.
• Periods that have stopped for three months or more and you are not pregnant, breastfeeding or on contraception that stops bleeding.
• No first period by age 15.
• Very heavy bleeding, soaking through a pad or tampon every hour for several hours, or bleeding that lasts longer than seven days.
• Bleeding between periods or after sex, or any bleeding after menopause.
• Irregular periods with new headaches, vision changes or milky nipple discharge.
• Irregular periods with rapidly increasing hair growth, a deepening voice or severe acne.
• Symptoms of perimenopause before 40, such as hot flushes with skipped periods.

The KnowLuna panels that cover this

If you would like the standard set of bloods done in one visit, three KnowLuna options cover the ground above, each collected at an accredited Australian centre, or with a home kit or a nurse visit, and each with a plain-English report.

• The female hormone panel covers FSH, LH, oestradiol, progesterone, testosterone, SHBG, free androgen index, prolactin, DHEA-S and cortisol: the core cycle picture.
• The PCOS panel adds the metabolic markers (fasting glucose, HbA1c and lipids) and TSH, for irregular cycles with acne, hair changes or weight that will not shift.
• The thyroid panel checks TSH, free T4, free T3 and thyroid antibodies as a single test, if thyroid is the main question.

A pregnancy blood test (hCG) is available as a single test when a period is late.

Common questions

Sources and further reading

• Pathology Tests Explained (RCPA): follicle stimulating hormone (FSH)
• Pathology Tests Explained (RCPA): prolactin
• Pathology Tests Explained (RCPA): thyroid function tests
• Monash University: International Evidence-based Guideline for the Assessment and Management of PCOS 2023
• Jean Hailes for Women's Health: irregular periods
• Jean Hailes for Women's Health: absent periods
• Healthdirect: irregular periods

General information only, not medical advice. If your symptoms are severe or sudden, see a doctor promptly. This page does not replace a consultation with a doctor who knows your history; read our medical disclaimer.