A 45-year-old woman who had two normal pregnancies 13 and 20 years ago presents with the complaint of amenorrhea for 9 months. She expresses the desire to become pregnant again. After exclusion of pregnancy, which of the following is the next best test indicated in the evaluation of this patient's amenorrhea?

Patient has secondary amenorrhea, which rules out abnormalities associated with primary amenorrhea such as congenital Mullerian abnormalities and chromosomal abnormalities. The most common cause for secondary amenorrhea in a woman of reproductive age is pregnancy, which should be evaluated first. Other possibilities include chronic endometritis or scarring of the endometrium (Asherman syndrome), hypothyroidism, and ovarian failure. The latter is the most likely diagnosis in a woman at this age. Persistently elevated gonadotropin levels (especially when accompanied by low serum estradiol levels) are diagnostic of ovarian failure.