What is the mechanism by which thyroid dysfunction affects the menstrual cycle?
—JULIA ARNETT, ANP, Newton, Mass.

Thyroid dysfunction is associated with a range of menstrual abnormalities, including oligomenorrhea, amenorrhea, and menorrhagia. Women with hypothyroidism may also be at increased risk of pregnancy loss. The connection between thyroid hormone levels and the menstrual cycle is mainly mediated by thyrotropin-releasing hormone (TRH), which has a direct effect on the ovary. Additionally, abnormal thyroid function can alter levels of sex hormone-binding globulin, prolactin, and gonadotropin-releasing hormone, contributing to menstrual dysfunction. For example, increased levels of TRH may raise prolactin levels, contributing to the amenorrhea associated with hypothyroidism. For more information, see Clin Endocrinol (Oxf). 2007;66:309-321 and Speroff L, Fritz MA. Clinical Gynecologic Endocrinology and Infertility. 7th ed. Philadelphia, Pa.: Lippincott Williams & Wilkins; 2005:805-816.
—Lisa Stern, APRN