How Thyroid Function Affects Menstruation

The thyroid gland, a small butterfly-shaped gland located at the base of the front of your neck, plays an important role in your reproductive health, directly affecting your ovaries and indirectly interacting with sex-hormone binding globulin (a protein that binds sex hormones). Given this, thyroid dysfunction may result in menstrual irregularities—though menstrual problems tend to be more common in those with severe thyroid disease versus those with mild or moderate thyroid disease.

If you are experiencing unusual symptoms during your menstrual cycle, it's worth raising the possibility of a thyroid issue with your doctor.

Hypothyroidism, or an underactive thyroid gland, is a disorder of insufficient thyroid hormone production. There are a number of menstrual irregularities associated with hypothyroidism, ranging from heavy, frequent menstrual bleeding to infrequent, or even absent, menstrual cycles.

According to the American College of Obstetrics and Gynecologists, heavy menstrual bleeding is defined by one or more of the following features:

• Bleeding that lasts more than seven days
• Bleeding that soaks through one or more tampons or pads every hour for several hours in a row
• Needing to wear more than one pad at a time to control menstrual flow
• Needing to change pads or tampons during the night
• Menstrual flow with blood clots that are as big as a quarter or larger

Be sure to see your gynecologist if you are experiencing excessive menstrual bleeding. Tests like a pelvic examination, transvaginal ultrasound, and a thyroid stimulating hormone (TSH) blood test can help your doctor make a diagnosis.

On the opposite end of the spectrum, absent periods (amenorrhea) or infrequent periods (oligomenorrhea) may occur with hypothyroidism. Infrequent menses are more common.

Amenorrhea or oligomenorrhea can occur from the increase in thyroid releasing hormone (TRH) in women with hypothyroidism. The high TRH levels trigger the release of prolactin by the pituitary gland (a pea-sized organ located at the base of the brain).

Prolactin interferes with the production of estrogen from the ovaries, causing infrequent or absent periods, as well as other potential symptoms like infertility, an abnormal milky discharge from the breasts (galactorrhea), and symptoms of menopause, such as hot flashes and vaginal dryness.

Because a lack of thyroid hormone can inhibit ovulation, having hypothyroidism can make it difficult to get pregnant. Moreover, women with hypothyroidism who are pregnant have an increased risk of miscarrying during the first trimester.

The upside here is that treatment of hypothyroidism with thyroid hormone replacement medication (levothyroxine) may correct infertility and decrease the risk of pregnancy loss. That said, some women with hypothyroidism continue to experience abnormal menstrual cycles, despite thyroid medication.

A disorder of an overactive thyroid gland, meaning there is excessive thyroid hormone being produced, is called hyperthyroidism.

Absent or infrequent menses are the most common abnormalities seen with severe hyperthyroidism. This is because an increase in thyroid hormone indirectly causes an increase in sex hormone-binding globulin (SHBG), which can prevent ovulation.

As with hypothyroidism, an overactive thyroid gland that leads to scant or missed periods decreases fertility. In addition, excessive production of thyroid hormone during pregnancy increases the risk of miscarriage.

Treatment with an antithyroid drug (such as methimazole or propylthiouracil) can help prevent this. Another option is radioactive iodine (RAI) therapy, which shrinks the thyroid; this treatment is not safe during pregnancy, however, and can only be used before or after.

While a change in your menstrual cycle may be the first clue to a thyroid disorder, be sure to contact your doctor for a comprehensive workup, as there are many other potential causes. Pregnancy, for example, should be ruled out first and foremost in the event of a missed period.

Keep in mind that the more severe your thyroid disease is, the more likely you are to experience menstrual irregularities. In other words, having normal cycles certainly does not rule out a thyroid problem and vice versa. Abnormal menstruation is a potential clue to an underlying thyroid condition, but not a definite indicator of a problem.