A 35-year-old woman c/o 4-month history of irregular menstrual cycles. Menses occur at irregular 15 to 45-day intervals and last 1–2 days with minimal flow. She also reports a milk-like discharge from her nipples for 3 months, accompanied by fatigue and muscle and joint pain. She has recently gained 2.5 kg of weight. She was diagnosed with schizophrenia and hypothyroidism. Pelvic examination shows vaginal atrophy. Which of the following is the most likely explanation for the nipple discharge in this patient?