A 42-year-old woman presents to her primary care provider with complaints of irregular menses over the last 8 months and her last menstrual period was two months ago. She is particularly distressed due to irregular, unpredictable episodes of increased skin temperature and sweating lasting about 3 to 4 minutes at a time. These sudden feelings of sweating disrupt her sleep and impact her day-time productivity at home and work. Review of systems is remarkable for irritability, mood swings, vaginal dryness, discomfort during intercourse, decreased sex drive, and increased urinary frequency. She is sexually active and still has the uterus. Her vital signs and physical examination are normal. Which of the following laboratory tests is the most important test to be considered in this patient?
