A 28-year-old woman presents to the gynecology clinic with a 6-month history of amenorrhea. She reports recent weight loss of 8 kg over 3 months, increased stress at work, and denies pregnancy. Physical examination reveals BMI 18.5 kg/m², normal breast development, and normal external genitalia. Serum β-hCG is negative. FSH level is 3.2 mIU/mL (normal follicular phase: 3.5–12.5 mIU/mL), LH is 2.1 mIU/mL (normal: 2.4–12.6 mIU/mL), and prolactin is 12 ng/mL (normal: 4–29 ng/mL). TSH and free T4 are normal. Pelvic ultrasound shows normal uterus and ovaries with no follicles visible. What is the most likely diagnosis?