A 42-year-old woman has noticed dry skin, fatigue, and weight gain over the past 3 months. Her blood pressure is 110/70 mm Hg, pulse 60/min, and heart and lungs are normal. Her skin feels rough and dry, but the rest of the examination is normal. Her biochemistry is normal but the thyroid-stimulating hormone (TSH) is 39 mU/L (0.5-5 mU/L). Which of the following is the most likely cause for her elevated TSH?