Which Blood Tests Matter for Irregular Periods and Why

Topic: Lab tests explained

You have tracked your cycle. You have realized it is irregular. You are ready to see a doctor. Now comes the question: what are we looking for?

Blood work is the detective work of hormonal health. It gives us a snapshot of what is happening chemically inside your body. But a “routine blood test” (like a CBC) won’t tell you anything about your hormones. You need a targeted endocrine panel.

Here is a breakdown of the essential blood tests for investigating irregular periods, what they measure, and why they matter.

1. The Pituitary Drivers: FSH and LH

These two hormones come from your brain (pituitary gland) to talk to your ovaries.

• FSH (Follicle Stimulating Hormone): Tells the ovaries to grow an egg. Very high levels can indicate ovarian insufficiency (early menopause).
• LH (Luteinizing Hormone): Triggers the release of the egg (ovulation).
• The Ratio: In specific cases of PCOS, LH can be significantly higher than FSH (often a 2:1 or 3:1 ratio). This “loud shouting” from the LH confuses the ovaries and stalls ovulation.

When to test: Day 2 or 3 of your cycle (if you have one).

2. The Androgens: Testosterone and DHEAS

These are the “male” hormones.

• Total Testosterone: The overall amount in your blood.
• Free Testosterone: The bioavailable amount that actually affects your cells. This is often more important than total testosterone for diagnosing complications like acne or hirsutism.
• DHEAS: An androgen that comes primarily from the adrenal glands (stress glands), not the ovaries. High DHEAS indicates “Adrenal PCOS,” which might need a different management approach (stress reduction) compared to ovarian PCOS.

3. The Imitators: Prolactin and TSH

These tests rule out other conditions that look like PCOS but aren’t.

• TSH (Thyroid Stimulating Hormone): Thyroid issues cause irregular periods. Treating the thyroid often fixes the cycle completely.
• Prolactin: High levels (Hyperprolactinemia) shut down ovulation. This can be caused by benign pituitary tumors or even stress.

4. The Tracker: Progesterone

This is the only way to confirm ovulation occurred after the fact.

• What it does: Progesterone rises only after an egg is released.
• When to test: 7 days after suspected ovulation (usually Day 21 of a classic 28-day cycle). If it is low, you didn’t ovulate (anovulatory cycle).

5. The Metabolic Panel: Insulin and HbA1c

Since PCOS is a metabolic condition, knowing your blood sugar status is non-negotiable.

• HbA1c: Your average blood sugar over the last 3 months.
• Fasting Insulin: This is critical. You can have normal blood sugar/HbA1c but high insulin (because your pancreas is overworking to keep sugar stable). High insulin drives PCOS symptoms. Most standard panels skip this—ask for it specifically.
• Lipid Profile: Cholesterol and triglycerides, to assess long-term heart health.

6. Diagnosis: AMH (Anti-Müllerian Hormone)

• What it is: Produced by the small follicles in the ovaries.
• Why it matters: Very high AMH often correlates with PCOS (many follicles producing hormone). Very low AMH indicates low ovarian reserve. It replaces the need for an ultrasound in many guidelines.

How to Prepare

• Timing is Key: Most of these (FSH, LH, Estradiol) must be done on Day 2-4 of your period to be accurate baselines. Progesterone must be done mid-luteal.
• Fasting: Insulin and Lipids usually require 8-12 hours of fasting.
• ** Supplements:** Stop taking Biotin (Vitamin B7) 48 hours before thyroid tests, as it can skew results.

Empower yourself by knowing what these tests mean. When you understand your numbers, you understand your body.