How do thyroid hormones affect fertility?

The importance of the thyroid hormone for the adequate functioning of multiple organ systems within the body cannot be overstated. The production of adequate quantities of thyroid hormones (thyroxine [T4] and triiodothyronine [T3]) is necessary for normal fetal and neonatal growth and development, as well as proper functioning of adult organ systems, including the skeletal system, the cardiovascular system, lipid and carbohydrate metabolism, and the neuromuscular system.

The major effects of abnormal thyroid levels relates largely to changes in ovulation and menstruation. Ovulation may be impaired by changes in the production of: sex hormone binding globulin (SHBG), follicle stimulating hormone (FSH), estrogen, and androgens to name a few. The body compensates by altering the production of thyroid releasing hormone (TRH) from the hypothalamus. The changes in TRH will affect the feedback loop between the hypothalamus, pituitary, and the ovary, leading to changes in ovulation and menstruation. These changes can be subtle, especially when symptoms of thyroid dysfunction are not obvious and do not lead to changes in menses or ovulation. Early stages of thyroid dysfunction (before symptoms are manifest) can lead to subtle changes in ovulation and endometrial receptivity, which then may have profound effects on fertility.