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).

How does hyperthyroidism affect menstruation?

In someone with hyperthyroidism, the thyroid gland produces too many thyroid hormones. This is different from hypothyroidism, where the thyroid gland doesn't produce enough hormones. This causes an acceleration of many of the body's functions, and can lead to changes in the menstrual cycle (2).

Why does hyperthyroidism cause menorrhagia?

Menorrhagia usually resolves with correction of the thyroid disorder. Prolactin-producing pituitary tumors cause menorrhagia by disrupting (GnRH) secretion. This leads to decreased LH and FSH levels, which ultimately cause hypogonadism. Interim stages of menorrhagia result until hypogonadism manifests.