Blood Tests for Irregular Periods: Lab Tests That Reveal the Cause

Your period used to be predictable, but now it's all over the place. Or maybe your cycles have never been regular, and you've been told "that's just how your body is." That's not an answer - that's a missed diagnosis.

Irregular periods are a signal that something in your hormonal system is off balance. The good news? Blood tests can identify the cause - whether it's PCOS, thyroid dysfunction, high prolactin, or another hormonal issue.

What Counts as Irregular?

A "normal" menstrual cycle ranges from 21-35 days. Your period is considered irregular if:

• Cycles are shorter than 21 days or longer than 35 days
• Cycles vary by more than 7-9 days month to month
• You skip periods entirely (amenorrhea)
• Bleeding is extremely heavy or very light
• You have breakthrough bleeding between periods

The Hormonal System Behind Your Cycle

Your menstrual cycle is regulated by the hypothalamic-pituitary-ovarian (HPO) axis - a complex feedback loop involving your brain and ovaries. A comprehensive review in Frontiers in Endocrinology explains that dysfunction of this axis, or of other endocrine systems like the thyroid and adrenal glands, can cause irregular cycles and fertility problems.

According to this research, oligomenorrhea (cycles longer than 35 days) is the most common menstrual disturbance across endocrine disorders including thyroid disease, PCOS, Cushing's syndrome, and diabetes.

Common Causes of Irregular Periods

1. PCOS (Polycystic Ovary Syndrome)

PCOS affects up to 10% of women and is the most common cause of irregular periods in reproductive-age women. A review in Journal of Clinical Medicine explains that women with PCOS have elevated LH and decreased FSH, leading to chronic anovulation (lack of ovulation) and cycle irregularity.

PCOS is characterized by:

• Irregular or absent periods
• Elevated androgens (testosterone, DHEA-S)
• Polycystic ovaries on ultrasound
• Often: insulin resistance, weight gain, acne, excess hair growth

2. Thyroid Dysfunction

Both hyper- and hypothyroidism can disrupt your cycle. A study on menstrual disturbances in hypothyroidism found that 23.4% of hypothyroid women had irregular periods, with oligomenorrhea (infrequent periods) and menorrhagia (heavy bleeding) being most common.

Research also shows that subclinical hypothyroidism in PCOS patients is associated with altered thyroid-prolactin axis activity, potentially worsening cycle irregularity.

3. Hyperprolactinemia

Prolactin is the hormone that stimulates breast milk production. When elevated outside of pregnancy/breastfeeding, it can suppress ovulation and cause irregular or absent periods.

According to research on hormone-dependent gynaecological disorders, hyperprolactinaemia can cause galactorrhoea (breast discharge), irregular cycles, or even complete amenorrhoea.

4. Primary Ovarian Insufficiency

If your ovaries aren't functioning properly or are running low on eggs prematurely, cycles become irregular. Testing AMH (Anti-Müllerian Hormone) and FSH can help identify this.

5. Perimenopause

For women in their 40s (sometimes late 30s), cycle changes often signal the transition to menopause.

The Irregular Period Lab Panel: Tests You Need

Core Hormone Panel (Day 2-5 of Cycle)

Timing matters! These should be drawn early in your cycle:

• FSH - Follicle stimulating hormone (high = ovarian insufficiency)
• LH - Luteinizing hormone (elevated LH:FSH ratio suggests PCOS)
• Estradiol - Primary estrogen
• AMH - Ovarian reserve marker

Androgen Panel

• Total Testosterone
• Free Testosterone
• DHEA-S - Adrenal androgen
• SHBG - Low SHBG means more free androgens

Progesterone (Day 19-21)

Confirms whether you ovulated. Low progesterone = likely didn't ovulate that cycle.

Thyroid Panel

• TSH - Optimal: 1.0-2.0 mIU/L
• Free T4
• Free T3
• TPO Antibodies

Prolactin

Elevated prolactin suppresses ovulation. Draw fasting, in the morning.

Metabolic Markers

• Fasting Insulin - Insulin resistance is common in PCOS
• Fasting Glucose
• HbA1c

Red Flags: When to Seek Care Promptly

See a healthcare provider soon if you experience:

• No period for 3+ months (and not pregnant)
• Bleeding between periods
• Periods that last more than 7 days
• Soaking through a pad/tampon every hour
• Severe pain that interferes with daily life
• New cycle changes after age 45

The Bottom Line

Your menstrual cycle is a vital sign - it tells you about your overall hormonal health. Irregular periods aren't something to just "deal with" - they're a signal that deserves investigation.

The right blood tests can identify whether PCOS, thyroid dysfunction, prolactin issues, or another condition is behind your irregular cycles. With a diagnosis comes the ability to treat the root cause - not just manage symptoms.

Your cycle is trying to tell you something. It's time to listen.