Irregular Periods: When It's PCOS and When It's Something Else

Irregular periods are one of the most common reasons Indian women seek a gynaecological appointment. And while PCOS is the first diagnosis that comes to mind — for patients and doctors alike — it is far from the only cause. Irregular cycles can result from several different hormonal conditions, and treating them effectively requires identifying the correct underlying cause.

This article explains what counts as irregular, the main conditions that cause it, how they are distinguished from each other, and what a proper investigation involves.

What Is a Normal Menstrual Cycle?

A normal menstrual cycle runs between 21 and 35 days from the first day of one period to the first day of the next. Duration of bleeding is typically 3 to 7 days. Cycles consistently outside this range, or with more than 7 to 9 days of variation from month to month, are classified as irregular. Amenorrhoea — complete absence of periods for 90 days or more in a non-pregnant, pre-menopausal woman — is a specific presentation that always warrants investigation.

PCOS: The Most Common Cause

PCOS accounts for approximately 70 to 80% of anovulatory irregular cycles in reproductive-age women. In PCOS, the ovaries fail to regularly select and release a mature egg — resulting in long cycles, missed periods, or complete cycle absence. The other features of PCOS (androgen excess, polycystic ovarian morphology on ultrasound) allow confirmation of the diagnosis.

But PCOS is a diagnosis of exclusion — other causes must be assessed and ruled out before the PCOS label is applied.

Thyroid Disease

Both hypothyroidism (underactive thyroid) and hyperthyroidism (overactive thyroid) disrupt the menstrual cycle. Hypothyroidism — particularly autoimmune Hashimoto's thyroiditis, which is extremely common in Indian women — causes heavy, irregular, or absent periods, along with fatigue, weight gain, hair loss, cold intolerance, and cognitive slowing. Even subclinical hypothyroidism (elevated TSH with normal free T4) can disrupt ovulation.

Hyperthyroidism causes light, irregular, or absent periods, along with heat intolerance, palpitations, tremor, and anxiety.

The TSH blood test is the primary screen for thyroid function and should be included in every evaluation of irregular periods. Treatment of thyroid disease typically restores normal cycles within 2 to 3 months.

Hyperprolactinaemia — Elevated Prolactin

Prolactin is the hormone responsible for milk production after childbirth. Abnormally elevated prolactin outside of pregnancy and breastfeeding suppresses the GnRH pulsatility that drives the normal menstrual cycle, causing irregular or absent periods and — in significant excess — galactorrhoea (spontaneous milk discharge from the breasts in a non-breastfeeding woman).

Causes of elevated prolactin include a small benign pituitary tumour called a prolactinoma (the most common cause of significant hyperprolactinaemia), hypothyroidism (elevated TRH stimulates prolactin), and certain medications — antipsychotics, some antidepressants, metoclopramide, and domperidone.

Hyperprolactinaemia is eminently treatable. Cabergoline — a dopamine agonist taken twice weekly — suppresses prolactin effectively, shrinks prolactinomas, and restores ovulation and fertility in the majority of patients.

Premature Ovarian Insufficiency (POI)

POI — previously called premature ovarian failure — refers to loss of normal ovarian function before age 40. It is characterised by irregular or absent periods, low oestrogen, and elevated FSH (reflecting the pituitary working harder to stimulate poorly functioning ovaries). Symptoms include those of oestrogen deficiency: hot flushes, night sweats, vaginal dryness, and reduced libido.

Causes include genetic factors (Fragile X premutation is the most common genetic cause), autoimmune oophoritis, and previous chemotherapy or radiation. POI affects approximately 1% of women under 40.

POI is not a reversible condition. Women with POI need hormone replacement therapy for bone and cardiovascular health. Fertility options are limited — donor egg IVF is the most reliable path to pregnancy, with live birth rates of 55 to 65% per transfer regardless of recipient age.

Hypothalamic Amenorrhoea

In women who are significantly underweight, exercising at extreme intensity, or under severe and chronic psychological stress, the hypothalamus — the brain region that initiates the hormonal cascade driving the menstrual cycle — can suppress GnRH pulsatility as a protective mechanism. The result is functional hypothalamic amenorrhoea: complete cessation of periods without underlying structural disease.

This is common in competitive athletes, dancers, women with restrictive eating disorders, and women under extreme psychological pressure. Distinguishing features: low-to-normal FSH and LH (unlike POI, where FSH is elevated), low oestrogen, and a clear history of extreme exercise, low body weight, or severe stress.

Treatment is addressing the underlying cause — weight restoration, exercise moderation, psychological support. Recovery of the menstrual cycle follows, though it may take several months.

Congenital Adrenal Hyperplasia (CAH) — Non-Classical Form

Non-classical congenital adrenal hyperplasia (NCCAH) is a genetic disorder affecting the adrenal enzyme 21-hydroxylase, causing elevated adrenal androgen production. It presents identically to PCOS — irregular periods, acne, hirsutism, polycystic ovaries on ultrasound — and is diagnosed by elevated serum 17-hydroxyprogesterone. NCCAH is more common in certain ethnic groups and should be excluded before a PCOS diagnosis is confirmed, particularly if there is a strong family history of androgen excess.

The Diagnostic Investigation

A structured investigation of irregular periods includes:

• Day 2 to 3 hormonal panel: FSH, LH, oestradiol, total testosterone, DHEAS — looking at the LH:FSH ratio (elevated in PCOS), total androgen levels, and baseline ovarian function
• 17-hydroxyprogesterone — to exclude non-classical CAH
• Prolactin — to detect hyperprolactinaemia
• TSH and thyroid antibodies — to detect thyroid disease
• AMH — elevated in PCOS, very low in POI, low in hypothalamic amenorrhoea
• Fasting glucose and insulin — HOMA-IR if PCOS is suspected
• Transvaginal ultrasound — ovarian morphology, uterine assessment

This panel allows confident differentiation between PCOS and its mimics, and ensures that a treatable condition is not labelled as PCOS and left managed only symptomatically.

Frequently Asked Questions

Q1. I have always had irregular cycles. Is this normal?

Occasional variation is normal. Consistently long cycles (above 35 days), highly variable cycle lengths, or fewer than 8 periods per year warrant investigation — particularly if you are planning to conceive. Irregular cycles usually indicate irregular or absent ovulation, which directly reduces fertility. Identifying the cause is the first step to addressing it effectively.

Q2. Can stress alone cause irregular periods?

Yes — severe, chronic stress can suppress the hypothalamic-pituitary-ovarian axis through elevated cortisol and reduced GnRH pulsatility, causing delayed or absent ovulation. However, stress is usually one of several contributing factors, and a thorough hormonal evaluation is needed to exclude other causes before attributing irregular cycles to stress alone. "Stress caused it" is frequently used as an explanation without proper testing — which risks missing a treatable condition.

Q3. Do I need to treat irregular periods if I am not trying to conceive?

Yes — particularly if the irregularity is due to chronic anovulation. Long periods without ovulation mean no progesterone production after the missed "ovulation," causing the uterine lining to build up under continuous oestrogen stimulation. Over years, this increases the risk of endometrial hyperplasia and endometrial cancer. Regular withdrawal bleeds — induced by progestogens or combined oral contraceptives — protect the endometrium and are medically recommended for chronically anovulatory women, regardless of fertility intentions.