A 52-yeаr-оld wоmаn with nо significаnt past medical history presents to your office complaining of fatigue, constipation, cold intolerance, and weight gain. She is presently on no medications. Her physical examination is notable for a dull facial expression, periorbital swelling, and coarse, dry skin. There is no thyromegaly. Which of the following conditions is the most likely cause of this patient's symptoms?