Hypothyroidism can make periods irregular. You might experience infrequent or sporadic menstruation, heavy menstrual bleeding, or stop having periods altogether.

Also known as an underactive thyroid, hypothyroidism is when your thyroid doesn’t produce enough thyroid hormone.

Its most common symptoms include fatigue, unintentional weight gain, and mood changes. However, it can also affect your menstrual cycle, typically making periods heavier and more irregular or stopping them completely.

That’s because a lack of thyroid hormone makes your body suppress gonadotropin-releasing hormone production, which is necessary for your ovaries to work as they should.

Several studies showed that menstrual symptoms are quite common in people with hypothyroidism.

One 1993 study found that 68% of people with hypothyroidism had irregular menstrual cycles. A 1997 study found the percentage to be lower at 40%.

However, some found menstrual changes to be less common. One study from 2001 noted that only 23% of participants with hypothyroidism had irregular cycles.

What’s known is that the more severe the hypothyroidism, the more likely it can affect your menstrual cycle.

Menstrual symptoms may also occur before receiving a diagnosis of hypothyroidism.

Infrequent periods are the most common menstrual-related symptom of hypothyroidism.

You may skip a period (or more than one) or feel unsure of when you’ll get one because periods are difficult to track.

This skipping is often because high levels of thyroid-releasing hormone increase another hormone called prolactin.

Prolactin affects estrogen production in your ovaries, which can then affect your menstrual cycle’s regularity.

Thyroid conditions like hypothyroidism can make periods stop altogether due to uterine lining thinning.

If this happens for more than 3 months in a row, it’s called amenorrhea (absent menstruation).

A lack of thyroid hormone can change how blood clots, potentially leading to heavier bleeding. Your uterine lining may also excessively thicken and then shed more unpredictably.

According to the American College of Obstetricians and Gynecologists, periods are heavy if you:

  • bleed for more than 7 consecutive days
  • soak through one or more tampons or pads every hour for several hours
  • wear more than one pad at a time
  • change period products at night
  • pass blood clots that are the size of a quarter or bigger

If your body doesn’t produce enough thyroid hormones, processes inside your body can slow down.

The effects can make periods irregular, heavier, or longer lasting. Some may even notice that periods occur more often than once a month.

Hyperthyroidism —when your body produces too much thyroid hormone — can have the opposite effect, like making periods lighter or shorter.

Do medications for hypothyroidism affect menstruation?

A medication called levothyroxine is the standard treatment for hypothyroidism. It can help improve menstrual cycle irregularities by boosting thyroid hormone levels.

If your menstrual irregularities persist, another underlying cause may exist that requires treatment.

Does hypothyroidism affect fertility?

Hypothyroidism can make it harder to become pregnant. Increased prolactin levels can stop ovulation from happening, meaning your ovaries don’t release an egg.

Left untreated, a thyroid disorder like hypothyroidism may cause infertility.

Does hypothyroidism affect pregnancy?

Hypothyroidism can increase your risk of preeclampsia and anemia during pregnancy. It can also increase the risk of pregnancy loss and premature birth.

A newborn may weigh less than 5 pounds or have a faster heart rate, potentially leading to breathing difficulties or heart failure. A baby born with an underactive thyroid may experience more difficulty with growth and brain development than expected.

These risks can decrease significantly with hypothyroidism treatment.

If you’re concerned about irregularities in your period, make an appointment with a healthcare professional as soon as possible.

Early diagnosis and treatment of underlying conditions, including hypothyroidism, can help reduce the risk of complications.

Lauren Sharkey is a U.K.-based journalist and author specializing in women’s issues. When she isn’t trying to discover a way to banish migraines, she can be found uncovering the answers to your lurking health questions. She has also written a book profiling young female activists across the globe and is currently building a community of such resisters. Catch her on Twitter.