A 30-year-old woman, who has 2 healthy children ages 3 and 4, notes that she has had no menstrual periods for the past 6 months. A pregnancy test is negative, and she is not taking any medications. She also complains of headaches for the past 3 months and has had problems with her lateral vision. On physical examination, she is afebrile and normotensive. Laboratory values are within normal ranges for glucose, Na, and K. A magnetic resonance imaging study reveals a pituitary mass. Which of the following is most likely to be present in this woman?