What blood work is recommended to investigate irregular periods?

Blood Work for Investigating Irregular Periods

The recommended blood work for investigating irregular periods should include follicle-stimulating hormone (FSH), luteinizing hormone (LH), estradiol, prolactin, and thyroid function tests, measured during the early follicular phase (days 3-6) of the menstrual cycle. 1, 2

Core Laboratory Tests

• FSH and LH should be measured between days 3-6 of the menstrual cycle, with three estimations taken 20 minutes apart for accurate assessment 2
• Estradiol levels should be measured during the early follicular phase to establish baseline ovarian function 1
• Prolactin measurement is essential to rule out hyperprolactinemia, which can cause menstrual irregularity 2, 1
• Thyroid function tests (TSH, T4) are necessary to exclude thyroid dysfunction that may present with similar symptoms 1, 2
• Progesterone levels should be measured during the mid-luteal phase (approximately day 21 of a 28-day cycle) to confirm ovulation; levels <6 nmol/l indicate anovulation 2

Additional Tests Based on Clinical Presentation

• Testosterone levels should be checked if signs of hyperandrogenism (hirsutism, acne) are present 2
• Androstenedione measurement is indicated when testosterone is elevated to rule out adrenal/ovarian tumors 2
• DHEAS (dehydroepiandrosterone sulfate) should be measured if non-classical congenital adrenal hyperplasia is suspected 2
• Fasting glucose and insulin levels (with glucose/insulin ratio calculation) are recommended if metabolic disorders or PCOS are suspected 2
• Anti-Müllerian hormone (AMH) may provide additional information on ovarian reserve, particularly in younger women 1

Timing and Interpretation Considerations

• Laboratory tests should be performed without hormonal contraception for accurate assessment 1
• A single FSH measurement is not diagnostic as levels fluctuate significantly during perimenopause 1
• Normal ranges for hormones vary by laboratory; interpretation should be based on the specific reference ranges provided 2
• For women with irregular cycles, testing should still aim for days 3-6 of the cycle when possible 1

Clinical Correlation with Menstrual Patterns

• Menstrual irregularity is defined as cycles <23 days (polymenorrhea) or >35 days (oligomenorrhea), or absence of bleeding for >6 months (amenorrhea) 2
• Polycystic ovary syndrome (PCOS) is a common cause of irregular periods, characterized by hyperandrogenic chronic anovulation 2
• Low mid-luteal phase progesterone levels indicate anovulation, which is common in PCOS 2
• An LH/FSH ratio >2 is suggestive of PCOS 2

Imaging Studies When Indicated

• Pelvic ultrasound (transvaginal preferred) should be performed between days 3-9 of the cycle if ovarian pathology is suspected 2
• Ultrasound findings of >10 peripheral cysts (2-8 mm diameter) with thickened ovarian stroma suggest polycystic ovaries 2
• Pituitary MRI may be indicated if clinical features or laboratory results suggest hypothalamic-pituitary abnormalities 2

When to Refer to a Specialist

• Referral to an endocrinologist or gynecologist is indicated for persistent abnormal hormone levels or severe menstrual irregularities 1
• Reproductive endocrinology consultation is recommended for women with infertility concerns 1
• Specialist referral should be considered for women with signs of premature ovarian insufficiency (amenorrhea with elevated FSH in women under 40) 2

Common Pitfalls to Avoid

• Relying on a single FSH measurement for diagnosis, as levels fluctuate widely during perimenopause 1
• Measuring hormone levels at incorrect times during the menstrual cycle 2, 1
• Failing to consider non-reproductive causes of menstrual irregularity such as thyroid disorders 2, 1
• Not accounting for the effects of medications, stress, or weight changes on hormone levels 2