Irregular Periods: What Lab Tests Can Tell You

Irregular periods are your body's way of signaling that something is off balance. Lab tests can help identify whether the cause is hormonal, thyroid-related, or something else.

What's Considered Irregular?

• Cycles shorter than 21 days or longer than 35 days
• Significant variation in cycle length (>7 days)
• Skipped periods
• Very heavy or very light bleeding

Common Causes

• PCOS: The most common cause in reproductive-age women
• Thyroid dysfunction: Both hypo and hyperthyroidism affect periods
• Hyperprolactinemia: Elevated prolactin suppresses ovulation
• Perimenopause: Hormonal fluctuations as ovarian function declines
• Hypothalamic amenorrhea: From stress, underweight, or overexercise
• Primary ovarian insufficiency: Early decline in ovarian function

The Irregular Periods Lab Panel

Baseline Hormones (Days 1-5)

• FSH and LH
• Estradiol
• Testosterone (total and free)
• DHEA-S
• Prolactin

Thyroid

• TSH, Free T3, Free T4
• Thyroid antibodies

Other Tests

• AMH (ovarian reserve)
• Progesterone (days 19-21 if cycling)
• Fasting insulin and glucose (PCOS screening)

If Periods Are Absent

• Pregnancy test
• Prolactin
• FSH (elevated suggests menopause/POI)

References

• Practice Committee of the American Society for Reproductive Medicine. (2008). Current evaluation of amenorrhea. Fertility and Sterility, 90(5 Suppl), S219-S225.
• American College of Obstetricians and Gynecologists. (2017). ACOG Practice Bulletin No. 128: Diagnosis of abnormal uterine bleeding in reproductive-aged women. Obstetrics & Gynecology, 120(1), 197-206.