Thyroid problems and the cycle

What it is

The thyroid is a butterfly-shaped gland in the lower front of the neck. Its hormones reach every tissue in the body. They help control how the body uses energy and its temperature, and they support the brain, heart and muscles.¹

Doctors have long known that problems and period problems often go together.² Autoimmune thyroid disease, in which the immune system attacks the gland, happens more often in women.³

Underactive thyroid

Hypothyroidism means the makes too little hormone. Signs include:³

feeling tired
feeling cold
dry skin
constipation
feeling sad or down
forgetting things easily

When the thyroid is underactive, ovulation can become irregular, and the lining of the uterus can build up too much, which can cause heavy periods.⁴ In one 2024 study of 485 women with newly found thyroid problems, unusually heavy periods were reported by 33% of those with an underactive thyroid, compared with 6% of women without thyroid disease.²

Overactive thyroid

Hyperthyroidism means the makes too much hormone. Its most common cause is Graves' disease, and it happens more often in younger women.¹ Signs include:¹

sweating and feeling hot
a racing or irregular heartbeat
shaky hands
anxiety, nervousness and irritability
weight loss even while eating normally or more
frequent bowel movements or diarrhea
trouble sleeping
hair loss
periods that are lighter or less frequent

Easy to mistake for something else

Many of these signs overlap with other things: tiredness, low mood and anxiety can look like PMS, depression or stress. ACOG lists disease among the conditions that can mimic PMS.⁵ That's one reason doctors often check the thyroid when periods or moods change.

How it's checked

Blood tests check hormones. The main ones are TSH and free T4.³ Thyroid conditions are treatable; see the American Thyroid Association for more.

When to see a doctor

See a doctor if your periods change, especially if you also notice several of the signs above. The NHS lists an overactive or underactive among the causes of missed periods, and advises seeing a GP if you've missed a period and also have tiredness, weight changes, or dry or oily skin.⁶

Recap

The thyroid makes hormones that control energy, body temperature, and how the brain, heart and muscles work.
An underactive thyroid can cause tiredness, feeling cold, constipation and low mood, and is linked to heavier periods.
An overactive thyroid can cause a racing heart, sweating, weight loss and anxiety, and lighter or less frequent periods.
Thyroid problems are more common in women. Blood tests including TSH check for them.

Words to know 5

Graves' disease	An autoimmune condition and the most common cause of an overactive thyroid.
hyperthyroidism	An overactive thyroid, which makes too much thyroid hormone.
hypothyroidism	An underactive thyroid, which makes too little thyroid hormone.
thyroid	A butterfly-shaped gland in the front of the neck that makes hormones that control how the body uses energy.
TSH	A hormone from the pituitary gland that tells the thyroid how much to make. Its blood test is the main thyroid check.