Review Question - QID 214800

A 33-year-old G2P1102 woman presents with 4 months of amenorrhea. Menarche was at age 13. Previously, her menstrual cycles occurred every 28 days with moderate heaviness. Three years ago, her menses started to become irregular. She is sexually active with male and female partners and uses condoms 75% of the time with male partners. After each missed period she has taken a home pregnancy test, all of which have been negative. She does not endorse hot flashes, vaginal dryness, mood changes, acne, hirsutism, constipation, fatigue, muscle weakness, or difficulty concentrating. She endorses significant weight gain in the last 5 years which she attributes to a diet of "junk food" and lack of exercise. She has no other medical conditions and takes no daily medications. Temperature is 98.6°F (37.0°C), blood pressure is 140/90 mmHg, pulse is 75/min, and respirations are 17/min. BMI is 35 kg/m^2. Physical examination, including a pelvic examination, does not disclose any abnormalities. FSH, LH, and prolactin are within normal limits. Other laboratory results are as follows:

Thyroid-stimulating hormone (TSH): 2.50 mIU/L
Free thyroxine (T4): 13.0 pmol/L
Progesterone: Low
Urine pregnancy test: Negative

Which of the following is the most likely cause of this patient's symptoms?