Irregular periods: which hormone tests to have and on which day of your cycle

The menstrual cycle is one of the most sensitive indicators of a woman’s health. It responds to stress, sleep, body weight, thyroid function, prolactin and androgens. That is why cycle disturbances are rarely “just a gynaecological problem”; more often, they signal that something is happening across the hormonal system. This article explains what counts as irregular periods, the most common causes and which blood tests help distinguish between them. Results should always be interpreted by a doctor; the aim of this article is to prepare you for that discussion. The result report is issued in Polish.

What does an “irregular cycle” mean?

A normal cycle lasts from 21 to 35 days (counted from the first day of bleeding to the first day of the next period), and differences between cycles do not usually exceed 7–9 days. Cycles are considered irregular when they are shorter than 21 days or longer than 35 days, their length varies by more than about a week, bleeding occurs between periods, or periods stop for 3 months or longer (secondary amenorrhoea). Very heavy or very light bleeding and severe pain are separate issues.

In the first 2 years after your first period and during perimenopause, irregularity is partly physiological. At other stages of life, a change that persists for several cycles needs investigating.

The most common causes

• Polycystic ovary syndrome (PCOS): the most common cause of infrequent periods in young women; it is associated with excess androgens (acne, excessive hair growth), often insulin resistance and difficulty losing weight.
• Thyroid disorders: both hypothyroidism and hyperthyroidism disrupt the cycle; hypothyroidism more often lengthens cycles and makes bleeding heavier.
• Hyperprolactinaemia: raised prolactin inhibits ovulation; possible causes include stress, certain medicines (including some psychiatric medicines and anti-sickness medicines), hypothyroidism and, less commonly, a pituitary adenoma.
• Low energy availability: intense training, a restrictive diet or rapid weight loss; the body “switches off” ovulation (functional hypothalamic amenorrhoea).
• Premature ovarian insufficiency (before the age of 40) and natural perimenopause.
• Anatomical causes: polyps, fibroids and endometriosis; these require an ultrasound scan and a gynaecological examination, not just blood tests.
• Pregnancy and the period after stopping hormonal contraception.

Blood tests: what to measure and when

Beta-hCG

If your period is late, the first test is a pregnancy test; a quantitative blood beta-hCG test is more sensitive than a urine test.

TSH, FT4, anti-TPO

Thyroid-stimulating hormone (TSH) is a screening test for thyroid function and can be measured on any day of your cycle. Free thyroxine (FT4) helps establish whether a disorder is overt, while anti-TPO indicates an autoimmune cause (Hashimoto’s disease), which is very common in women.

Prolactin

This is measured in the morning, on an empty stomach, after at least 30 minutes of seated rest, avoiding stress, exercise and sexual intercourse on the preceding day. A moderately raised result is worth repeating, because prolactin can respond even to blood collection itself.

FSH, LH, oestradiol: days 2–5 of your cycle

These three hormones are assessed in the early follicular phase. FSH and oestradiol provide information about ovarian reserve and function (high FSH with low oestradiol suggests declining ovarian function), while the LH-to-FSH ratio may be raised in PCOS. If you are not having periods, testing is performed on any day, with this noted on the result report.

Progesterone: about 7 days before your expected period

In a 28-day cycle, this is day 21. A clearly raised result confirms that ovulation has occurred in that cycle; a low result indicates anovulatory cycles, the most common mechanism behind irregularity.

Androgens: testosterone, SHBG, DHEA-S, androstenedione

These are central to investigating PCOS and other conditions involving excess androgens. Total testosterone together with SHBG allows the free androgen index to be calculated. DHEA-S helps distinguish whether the excess comes from the ovaries or the adrenal glands.

AMH

Anti-Müllerian hormone reflects ovarian reserve and is independent of the day of your cycle. It is useful when premature ovarian insufficiency is suspected, when planning a pregnancy and in PCOS (where it may be high).

Glucose, insulin, lipid profile

If you have PCOS or are overweight, it is worth assessing insulin resistance (glucose and insulin on an empty stomach, the HOMA-IR index and, where appropriate, a glucose tolerance test) and having a lipid profile, because metabolic disturbances sustain the hormonal problem.

Full blood count and ferritin

Iron-deficiency anaemia is very common with heavy bleeding. A full blood count and ferritin show whether your body is keeping up with replenishing its stores; correcting these improves well-being faster than any other measure.

How to schedule tests during your cycle

The most convenient option is to have blood collected for most tests on day 3 of your cycle (one visit), with progesterone tested separately in the luteal phase. For all hormone tests, attend in the morning, on an empty stomach, after a good night’s sleep; if you use hormonal contraception, FSH, LH and oestradiol results do not reflect your ovaries’ own activity, so you should tell your doctor.

When not to wait

Very heavy bleeding (changing a sanitary pad every 1–2 hours), bleeding after sexual intercourse, no period for 3 months, a sudden increase in hair growth or acne, milky discharge from the breasts or severe pain: these situations require a visit to a gynaecologist regardless of your blood test results.

Frequently asked questions

Can I have hormone tests without a referral?

Yes. All the hormones listed are available as private tests; it is worth taking your results to your appointment, as this shortens the diagnostic process by one or two consultations.

On which day of my cycle should I have TSH tested?

Any day. The day of your cycle matters only for FSH, LH, oestradiol and progesterone.

Can stress disrupt my cycle?

Yes, by increasing cortisol and prolactin and suppressing the hypothalamic–pituitary–ovarian axis. Your cycle usually returns to normal once the cause resolves, but disturbances lasting more than 3 months need investigating.

Sources

• Teede HJ et al., J Clin Endocrinol Metab, Recommendations From the 2023 International Evidence-based Guideline for the Assessment and Management of PCOS, 2023. academic.oup.com
• Petersenn S. et al., Nat Rev Endocrinol, Diagnosis and management of prolactin-secreting pituitary adenomas: a Pituitary Society international Consensus Statement, 2023. nature.com
• NICE, Thyroid disease: assessment and management (NG145), 2019, updated 2023. nice.org.uk