A 38-year-old woman with newly diagnosed Hashimoto's thyroiditis (TSH 12.3 mIU/L, free T4 0.6 ng/dL) is started on thyroid replacement. She reports taking metformin, omeprazole, and a calcium supplement. After 6 weeks on levothyroxine 50 mcg daily, her TSH remains elevated at 9.8 mIU/L. Which single intervention is most likely to improve her response?