A 38-year-old woman with a 3-year history of treated hypothyroidism (on levothyroxine 75 μg daily) presents with persistent fatigue, weight gain, and muscle aches despite reported compliance with medication. Her TSH is 6.2 mIU/L and free T4 is 1.1 ng/dL (both within or near-normal range). Serum ferritin is 8 ng/mL (normal 12–150), hemoglobin 10.2 g/dL, and celiac serology (tissue transglutaminase IgA) is positive. What is the most likely explanation for her persistent symptoms despite apparent adequate levothyroxine dosing?