Functional Medicine · Hormonal Health

The Journal

by Dr. Natasha Ryan, ND

Cycles · Root Causes

Irregular Periods in Your 20s and 30s: The Causes That Aren't Perimenopause

5 min read

Your period used to come every 28 days. Now it's 40, then 52, then it skips a month entirely. Or it never really settled down in the first place. You're nowhere near menopause, so what's going on?

A cycle isn't just about periods. It's a monthly report on your hormones, your thyroid, your stress, and how well you're fueled. When it changes, it's worth finding out why.

What counts as irregular

Using the international FIGO definitions, a normal cycle in adults:

  • Comes every 24 to 38 days, counted from the first day of one period to the first day of the next
  • Varies by no more than 7 to 9 days between your shortest and longest cycles
  • Lasts up to 8 days of bleeding

Cycles that are regularly longer than 35 days, or that vary a lot month to month, are worth looking into. Going three months or more without a period, when you used to have them, is called secondary amenorrhea and always deserves an evaluation.

The first test is always a pregnancy test. After that, these are the most common causes.

1. PCOS (now called PMOS)

PCOS is the most common hormonal cause of irregular cycles in women of reproductive age, affecting about 1 in 8. Long cycles often come with acne, excess hair growth, scalp hair thinning, cravings, and weight around the middle. Insulin resistance is a core driver.

In 2026, PCOS was renamed polyendocrine metabolic ovarian syndrome (PMOS). See PCOS Symptoms and Root Causes and How PCOS Is Diagnosed.

2. Under-fueling, overtraining, and stress

This one is easy to miss, especially in women who look healthy and work hard at it.

The brain's hypothalamus controls the cycle. When it senses that energy is short, from eating too little for how much you train, rapid weight loss, or ongoing stress, it turns down the signal to the ovaries. Periods spread out or stop. This is called functional hypothalamic amenorrhea.

It's common in runners, dancers, and women who combine intense exercise with careful eating. But it can also happen with high stress and normal exercise. The 2017 Endocrine Society guideline recommends considering it when cycles persistently run longer than 45 days or periods stop for three months or more, after other causes are ruled out.

It matters because low estrogen over time leads to bone loss. The guideline advises against using birth control pills just to bring back a bleed, because the pill creates a withdrawal bleed without fixing the underlying problem. The real treatment is enough food, enough recovery, and less stress, often with support from a dietitian or therapist.

If you're unsure whether your eating or exercise has crossed a line, that's a good conversation to have with a clinician you trust.

3. Thyroid problems

Both an underactive and an overactive thyroid can throw off your cycle. An underactive thyroid tends to cause heavier, longer, or less frequent periods, along with fatigue, cold intolerance, constipation, and hair loss.

Hashimoto's, the autoimmune form, is very common in women and can be present for years before TSH goes out of range. See Hashimoto's With a Normal TSH and Weight Loss Resistance and a "Normal" Thyroid.

4. High prolactin

Prolactin is the hormone that drives breast milk. When it's high outside of breastfeeding, it shuts down ovulation. Signs can include irregular or missing periods, milky nipple discharge, headaches, or vision changes.

Causes include certain medications (some antipsychotics, antidepressants, and anti-nausea drugs), an underactive thyroid, and small benign pituitary tumors called prolactinomas. A simple blood test checks it.

5. Coming off the pill

The pill causes a withdrawal bleed, not a true period. It doesn't regulate your cycle. It replaces it. So any irregularity you had before often returns when you stop.

Most women's cycles return quickly. In one Columbia University study of women taking a continuous pill, 99% had their period back or were pregnant within 90 days of stopping. If your cycle hasn't returned within about three months, don't put it down to "post-pill." It's worth testing, because the pill may have been masking PCOS, thyroid problems, or another cause all along.

6. Early drop in ovarian function

Primary ovarian insufficiency (POI) is when the ovaries slow down before age 40. It's more common than once thought. The 2024 international guideline puts it at about 3.5% of women.

Signs look like early perimenopause: irregular or missing periods, hot flashes, night sweats, vaginal dryness, and low mood. The guideline says a single FSH level above 25, together with at least four months of irregular or missing periods, is enough for diagnosis. Hormone therapy is recommended for most women with POI, to protect bone, heart, and brain until the usual age of menopause.

7. Other causes

  • Significant weight change, up or down
  • Celiac disease and other conditions that affect absorption
  • Medications, including some for mood, seizures, and blood pressure
  • Postpartum and breastfeeding, which suppress ovulation for months

The labs to ask for

The American Academy of Family Physicians recommends this starting point for missing or irregular periods:

  • Pregnancy test
  • FSH and LH, to see how the brain and ovaries are talking
  • Prolactin
  • TSH, and ideally free T4 and thyroid antibodies

Depending on your symptoms, your clinician may add:

  • Total and free testosterone, SHBG, and DHEA-S if there are signs of high androgens
  • 17-hydroxyprogesterone to rule out non-classic congenital adrenal hyperplasia
  • Estradiol and AMH to look at ovarian function
  • Glucose testing, ideally an oral glucose tolerance test, if PCOS is suspected
  • A pelvic ultrasound

Timing matters for some of these. FSH and estradiol are best drawn on days 2 to 5 of your cycle if you're still bleeding.

What you can do now

  1. Track your cycle for three months: start dates, length, and symptoms.
  2. Look honestly at fuel and recovery. Are you eating enough for how much you're training and how much stress you're under?
  3. Ask for the labs above, not just a prescription for the pill.
  4. Don't wait it out if you've gone three months without a period.

An irregular cycle is information. It's your body telling you something specific, and the right tests usually find it.

References

  1. Munro MG, Critchley HOD, Fraser IS; FIGO Menstrual Disorders Committee. The two FIGO systems for normal and abnormal uterine bleeding symptoms and classification of causes of abnormal uterine bleeding in the reproductive years: 2018 revisions. Int J Gynaecol Obstet. 2018;143(3):393-408. Table in the Merck Manual Professional Edition. View source
  2. Klein DA, Paradise SL, Reeder RM. Amenorrhea: a systematic approach to diagnosis and management. Am Fam Physician. 2019;100(1):39-48. View source
  3. Gordon CM, et al. Functional hypothalamic amenorrhea: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2017;102(5):1413-1439. Summary in HCPLive. View source
  4. Columbia University Irving Medical Center. Rapid return to menses once oral contraception stopped. 2006. View source
  5. ESHRE, ASRM, CRE-WHiRL, IMS. Evidence-based guideline: premature ovarian insufficiency. Climacteric. 2024. Summary in HealthDay. View source
  6. New guidelines aim to revolutionize POI diagnosis and management. Contemporary OB/GYN. December 2024. View source
  7. Endocrine Society. Polyendocrine Metabolic Ovarian Syndrome: new name to improve diagnosis and care. May 12, 2026. View source