When to Test: How Your Menstrual Cycle Affects Lab Results

Your hormone levels fluctuate dramatically throughout your menstrual cycle. Testing at the wrong time can lead to misleading results and unnecessary worry. Understanding when to test each hormone ensures you get accurate, actionable information about your health.

Why Timing Matters

Your menstrual cycle is divided into distinct phases, each with different hormone profiles. Estrogen, progesterone, LH, and FSH all rise and fall at specific times. Testing at the wrong phase can show "abnormal" results that are actually perfectly normal for that day of your cycle.

For example, progesterone is naturally very low in the first half of your cycle. Testing it on day 5 would show low levels even in a perfectly healthy woman with normal ovulation.

Understanding Your Cycle Phases

Follicular Phase (Days 1-14)

Begins on the first day of your period (Day 1) and lasts until ovulation. During this phase:

• FSH rises to stimulate follicle development
• Estrogen gradually increases as follicles grow
• LH remains relatively low until the surge
• Progesterone stays very low

Ovulation (Around Day 14)

A surge in LH triggers the release of an egg. This is the most fertile window.

Luteal Phase (Days 15-28)

After ovulation, the follicle becomes the corpus luteum and produces progesterone. This phase:

• Progesterone rises significantly
• Estrogen has a secondary peak
• Both drop if pregnancy doesn't occur, triggering your period

When to Test Each Hormone

Days 2-5: Baseline Hormones

Early in your cycle is ideal for testing:

• FSH (Follicle Stimulating Hormone) - Assesses ovarian reserve and function
• LH (Luteinizing Hormone) - Evaluates pituitary function; ratio with FSH is important
• Estradiol (E2) - Baseline estrogen level; high early estradiol can indicate diminished ovarian reserve
• AMH (Anti-Mullerian Hormone) - Can be tested any day, but often done with other baseline hormones
• Testosterone (Total and Free) - Best tested early in cycle
• DHEA-S - Assesses adrenal androgen production
• SHBG - Sex Hormone Binding Globulin

Day 21 (or 7 Days Before Expected Period): Ovulation Confirmation

Mid-luteal phase testing confirms ovulation:

• Progesterone - Should be elevated (typically >3 ng/mL) if ovulation occurred; levels >10 ng/mL indicate strong ovulation
• Estradiol - Has a secondary peak during luteal phase

Any Time in Your Cycle

These tests aren't significantly affected by cycle timing:

• Thyroid hormones (TSH, Free T3, Free T4)
• AMH - Remains relatively stable
• Prolactin - Best tested in the morning
• Vitamin D
• Iron panel and ferritin
• Metabolic markers (glucose, insulin, HbA1c, lipids)
• Inflammatory markers (hs-CRP)

Special Situations

Irregular Periods

If your cycles are very irregular or you're not sure when you'll ovulate:

• Test baseline hormones within the first 5 days of any bleeding
• Your doctor may order tests at any time and interpret accordingly
• Consider serial progesterone testing to catch ovulation
• Use ovulation predictor kits to time progesterone testing 7 days after a positive result

On Birth Control

Hormonal birth control suppresses your natural hormone production. For accurate baseline testing:

• Stop hormonal contraception for at least one full cycle before testing
• Some tests (thyroid, metabolic) can still be done on birth control
• Discuss with your healthcare provider before stopping any medication

Perimenopause

During perimenopause, cycles become unpredictable. Testing may need to be repeated at different times, and results should be interpreted in context of your symptoms.

Fasting Requirements

Some tests require fasting for accurate results:

• Fasting required (8-12 hours): Glucose, insulin, lipid panel, HOMA-IR
• No fasting required: Most hormone tests, thyroid, vitamin D, iron panel
• Morning testing preferred: Cortisol (peaks in early morning), testosterone, prolactin

Creating Your Testing Plan

For a comprehensive hormone assessment, consider this approach:

1. Track your cycle for 1-2 months to understand your pattern
2. Schedule baseline testing (days 2-5) with fasting in the morning
3. Schedule follow-up testing around day 21 for progesterone
4. Add thyroid and metabolic testing at either appointment