A 55-year-old woman with hypothyroidism on levothyroxine 75 mcg daily presents with persistent fatigue and weight gain. Vital signs: BP 128/82, HR 58, RR 14, Temp 36.8°C. TSH is elevated at 8.2 mIU/L despite medication adherence. Free T4 is low-normal at 0.8 ng/dL. She denies dyspnea or palpitations. Recently, she started taking calcium supplements and a multivitamin for bone health. Which over-the-counter agent is most likely impairing her levothyroxine absorption?

1. A)Propylthiouracil
2. B)Tolvaptan
3. C)Desmopressin
4. D)Octreotide
5. E)Ferrous sulfateGABARITO