Pseudo-Malabsorption in High Dose Levothyroxine–Resistant Hypothyroidism

A 38-year-old woman was diagnosed with hypothyroidism during pregnancy at age 35 years and started on levothyroxine (LT4). Despite increasing the dose to 400 μg/day and adding liothyronine, her thyroid function did not improve, leading to hospitalization. Upon admission, her thyroid-stimulating hormone (TSH), free triiodothyronine (T3), and free thyroxine (T4) levels were 255 μIU/mL (255 mIU/L) (reference range [RR]: 0.35---4.94 μIU/mL; 0.35---4.94 mIU/L), 3.42 pg/mL (5.27 pmol/L) (RR: 1.71-3.71 pg/mL; 2.63-5.70 pmol/L), and 0.153 ng/dL (1.97 pmol/L) (RR: 0.70-1.48 ng/dL; 9.01-19.05 pmol/L), respectively. She reported good adherence to medication and not consuming interfering food or medication. Endoscopic examination revealed no malabsorption. A 1000-μg oral LT4 loading test showed an increase in free T4 level from 0.787 (10.1 pmol/L) to 2.40 ng/dL (30.9 pmol/L), indicating pseudo-malabsorption. After presenting the loading test results, she admitted to nonadherence. A multidisciplinary team intervened, conducting individual counseling and simplifying treatment. Post-discharge, with LT4 200 μg/day, her TSH, free T3, and free T4 levels improved to 0.496 μIU/mL (0.496 mIU/L), 5.23 pg/mL (8.05 pmol/L), and 2.19 ng/dL (28.2 pmol/L), respectively. When addressing treatment-resistant hypothyroidism, it is crucial to evaluate patient history and medication schedule and to check for malabsorption. Comprehensive interventions are recommended if nonadherence is suspected.