A 24-year-old woman presents 6 months after the delivery of her first child, a healthy girl, for evaluation of fatigue. She suspects that the fatigue is related to getting up at night to breastfeed her baby, but she has also noticed cold intolerance and mild constipation. She recalls having a tremor and mild palpitations for a few weeks, beginning 3 months after delivery. On examination, her BP is 126/84 and her pulse rate is 56. The thyroid gland is two times normal in size and nontender. The rest of the physical examination is normal. Laboratory studies reveal a free T4 level of 0.7 ng/mL (normal 0.9-2.4) and an elevated thyroid-stimulating hormone (TSH) at 22 microU/mL (normal 0.4-4). What is the likely course of her illness?