Irregular Periods Explained: What Blood Tests Reveal

But at some point, the vague explanations start to wear thin. How long can you keep blaming irregular periods on stress, hormones, or bad timing without really knowing what’s going on?

Well, there IS a more precise way to figure out what’s going on: your bloodwork. Behind every irregular cycle is a pattern, and those patterns often show up in your labs before they show up in your calendar.

Think of this as your behind-the-scenes look at what doctors are really looking for when they order hormone blood tests for irregular periods. And why understanding your numbers could be the smartest move you make for your cycle, your health, and your peace of mind.

What Does ‘Irregular’ Even Mean?

‘Irregular periods’ is one of the common menstrual cycle-related concerns, but what does it actually mean? For most women, a typical menstrual cycle lasts between 21 and 35 days.
However, studies show that around 14% to 25% of women experience irregular cycles, where periods may come too early or too late, or the bleeding may be unusually heavy or light.

Doctors typically consider your period irregular if:

• You skip periods for more than one month
• Your cycle length keeps changing
• Your periods come too often, less than 21 days apart
• Your bleeding is very light or unusually heavy
• Your period lasts longer than 8 days
• You experience spotting or mid-cycle bleeding

What Causes Irregular Periods?

One of the first things you may ask is, ‘Why is my period late if I’m not pregnant?’ Your period is regulated by a system that responds to stress, blood sugar, and hormone levels. Here are some common reasons that can disrupt this delicate balance:

Thyroid Issues

Your thyroid gland is key in regulating your metabolism and hormone balance. When it’s underactive (hypothyroidism), it can slow down your body’s processes, including the hormones that trigger ovulation. This can cause your periods to become less frequent or even stop altogether. On the other hand, an overactive thyroid (hyperthyroidism) may lead to shorter or lighter cycles. Many people don’t realise that thyroid imbalances are a common yet often overlooked cause of irregular periods.

Chronic Stress

Long-term stress can reduce the hormones needed to keep your cycle regular. Stress affects the hypothalamus, the part of your brain that controls the hormones behind your cycle. When you're under prolonged stress — whether emotional, physical, or mental — your body may suppress ovulation to conserve energy. This can result in missed or delayed periods. Even short-term changes like a sudden illness, rapid weight loss or gain, intense workouts, or travel across time zones can confuse your internal clock and disrupt your cycle temporarily.

Polycystic Ovary Syndrome (PCOS)

This condition affects up to 10% of Indian women. Your body makes more “male-type” hormones than usual. This can prevent your ovaries from releasing an egg regularly, which means your period may not come on time, or it may not come at all. PCOS often shows up alongside acne, facial hair, and weight gain, and it’s closely linked to how your body handles insulin and sugar.

The best blood tests for period problems can help identify whether irregular periods and hormones are connected, and point your doctor toward the right diagnosis.

Lab Tests That Can Tell You What’s Going On With Your Periods

Here's a deeper look at what to test for when you miss a period, when to take it, and how to interpret the results.

1. FSH, LH, Estrogen, And Progesterone 

These are the hormones that run the show when it comes to your cycle. They tell your ovaries when to release an egg, and they help build and later shed your uterine lining.

• FSH (Follicle-Stimulating Hormone) and LH (Luteinizing Hormone) kick things off at the start of your cycle. They signal your ovaries to mature and release an egg (a process called ovulation).
• Estrogen helps build the uterine lining and supports egg development.
• Progesterone takes over after ovulation to keep the lining stable and ready for pregnancy.

When to test:

• FSH, LH, and Estrogen should be tested on Cycle Days 2 - 3.
• Progesterone should be tested around Day 21 (a week before your expected period).

How to interpret the results:

• Low progesterone or no apparent rise in levels suggests you didn’t ovulate during that cycle.
• High progesterone (in mid-luteal phase) confirms that ovulation occurred.
• Low estrogen may cause very light or missed periods due to a poor uterine lining.
• High estrogen can cause heavy periods, breast tenderness, and mood swings, which are often seen in PCOS.
• Low LH often means your hypothalamus (the part of your brain that controls hormone release) isn’t sending the usual signals to your ovaries. This can occur when your body is under stress from factors such as intense exercise, inadequate nutrition, or sudden weight loss.
• High LH (especially if higher than FSH) is common in PCOS and may indicate disrupted ovulation, where there is an imbalance of hormones needed to release the egg.
• Low FSH may mean your brain (hypothalamus or pituitary) isn’t sending strong enough signals to your ovaries, often due to stress, weight loss, or over-exercise.
• High FSH levels suggest that your egg supply may be low. The body produces more FSH to try harder to get the ovaries to respond, which is often seen with age.

2. Prolactin 

Prolactin is usually linked to breastfeeding, but even if you’re not nursing, this hormone can disrupt your cycle when its levels are too high, as it blocks the hormones that trigger ovulation. As a result, you may skip your period, it may show up late or be super light during those months.

When to test:

Morning, ideally early in your cycle (Days 2 - 5), after resting (no exercise or stress before the test).

How to interpret the results:

• High prolactin can delay or stop ovulation, leading to missed, late, or very light periods. Some women also notice milky breast discharge (called galactorrhoea) even when they’re not pregnant.
• Low prolactin levels are usually not a concern unless they are extremely low, which is rare. It may point to pituitary gland issues or very high dopamine activity.

3. Testosterone And DHEA-S 

These are often called “male hormones,” but everyone has them, just in different amounts. When they go up in women, it can affect their skin, hair, and period. Testosterone comes from the ovaries, while DHEA-S is made by your adrenal glands.

When to test:

Days 2 - 5 of your cycle.

How to interpret the results:

• High levels may point to androgen excess or PCOS. Symptoms include acne, facial/body hair, scalp hair thinning, and irregular periods.
• Low levels rarely cause significant issues in menstruation, but could affect energy or libido.

4. AMH (Anti-Müllerian Hormone)

AMH is like a snapshot of your remaining egg supply. It doesn’t predict exact fertility, but it gives your doctor an idea of how your ovaries are functioning, as it reflects how many eggs are “in reserve.”

When to test:

Anytime as it doesn’t change much across the cycle.

How to interpret the results:

• High AMH levels can point toward PCOS, where there are lots of tiny follicles that don’t mature properly.
• Low AMH may suggest that your egg count is lower, which can be a normal occurrence with age. AMH and fertility checks are helpful when planning for a possible pregnancy or trying to understand your body’s biological clock better.

5. Thyroid Hormones 

Your thyroid is a butterfly-shaped gland in your neck, which helps control your metabolism, energy levels, and yes, your menstrual cycle. When thyroid hormone levels are off, your periods often are too. A TSH test for periods is one of the first things many doctors will check, along with Free T3 and Free T4 hormone levels.

When to test:

Any day of your cycle, preferably in the morning on an empty stomach.

How to interpret the results:

• High TSH (and low T3/T4) indicates an underactive thyroid (hypothyroidism), which can cause heavy, long, or skipped periods, fatigue, weight gain, and mood changes.
• Low TSH (and high T3/T4) indicates an overactive thyroid (hyperthyroidism), which can lead to light, short, or more frequent periods, sometimes with anxiety, weight loss, or heart palpitations.

6. Blood Sugar And Insulin 

We often think of blood sugar as a diabetes issue, but it can affect your periods, too. When your body doesn’t respond well to insulin (the hormone that helps process sugar), it can throw off your hormone balance. That is why irregular periods and fluctuation in sugar levels often go hand in hand, making the fasting blood sugar and fasting insulin levels the most common tests to check for PCOS.

When to test:

Morning, fasting (8 - 12 hours without food), preferably on Cycle Days 2 - 5 if testing alongside other hormones.

How to interpret the results:

• High insulin usually indicates insulin resistance, which may cause irregular periods, weight gain around the belly, acne, and difficulty managing energy levels.
• High blood sugar can mean poor glucose processing, sometimes a precursor to diabetes.
• Low insulin/blood sugar is less common, but can point to other metabolic or dietary issues.

The Signals Are There. Are You Tuning In?

You don’t have to wait for things to get “serious” before asking for answers. Hormones impact your skin, mood, sleep, metabolism, and your cycle is often the first clue that something’s off.

Check with your doctor to find out which tests are right for your symptoms. Understanding what’s going on in your body is the first and the smartest step toward better health.

While You Figure It Out: Meet Your Everyday Backup

Waiting for your bloodwork won’t help you deal with surprise spotting or cycles that show up early, but Mahina can.

Our Stay Dry Panty is made for unpredictable days:

• Holds leaks without feeling bulky
• No rashes caused by synthetic fibres — just soft, breathable fabric
• Ideal for light flow, spotting, or days when you think your period’s over, but it’s not.

Explore our range and discover the best period panty for unpredictable, irregular bleeding!