TSH (thyroid-stimulating hormone) is a hormone produced by the pituitary gland that regulates the production of thyroid hormones by the thyroid gland. A low TSH level in a woman indicates that the thyroid gland is producing too much thyroid hormone, a condition known as hyperthyroidism. The normal range of TSH levels in non-pregnant adult women is 0.5 to 5.0 mIU/L. However, during the menstruation cycle, pregnancy, or after menopause, TSH levels may fall slightly outside the normal range because of fluctuating levels of estrogen. During the first trimester of pregnancy, TSH levels fall, and total T3 and T4 levels go up. Free T4 and T3 are also high-normal in the first trimester and return to normal in the second trimester. Pregnant women will generally have lower TSH levels. It is important to note that TSH levels can vary widely depending on a persons age, sex, and weight, and reference TSH ranges remain controversial.