OR19-3 Utility of the Levothyroxine Absorption Test: The Mayo Clinic Experience

Introduction: While hormone replacement with levothyroxine (LT4) is the most effective management for hypothyroidism, there is a subset of patients who remain hypothyroid despite high doses of LT4. These patients warrant evaluation for conditions that may contribute to poor LT4 absorption. Several case reports have documented a condition known as LT4 pseudomalabsorption, a factitious disorder in which overt hypothyroidism persists despite endorsed medication adherence and absent evidence for gastrointestinal malabsorption. LT4 absorption testing (LT4AT) is a non-invasive method for effectively distinguishing malabsorption from pseudomalabsorption. Given the lack of standardization of this test and the morbidity associated with uncontrolled hypothyroidism, we reviewed our institution’s experience with LT4AT and investigated the impact of this test on long-term management in patients with primary refractory hypothyroidism. Methods: We conducted chart reviews between 2015 to 2018 for patients who had undergone LT4AT at our institution for primary refractory hypothyroidism. Pertinent information collected included medications known to interfere with LT4 absorption or metabolism, adherence and timing of ingestion in relation to food intake, and comorbidities including celiac disease, heart failure, nephrotic syndrome, cirrhosis, gastritis, or short bowel syndrome. Our LT4AT protocol involves observed administration of LT4 dosed according to age and BMI (600mcg, 1000mcg, or 1500mcg). Total thyroxine is measured at baseline, 1 hour, 2 hours, 3 hours, 4 hours, and 6 hours. TSH is measured at baseline and at 6 hours. Percent LT4 absorption is calculated using the following formula: [[Increment TT4 (mcg/dL) x 10] / total administered LT4 (mcg)]] x Vd (L) x100. Absorption of >60% is considered normal. Results: Fifteen patients completed LT4AT during the search period, and 14 demonstrated normal absorption. One patient had virtually no LT4 absorption and was diagnosed with atrophic gastritis. Ten patients had follow up ranging from 2 months to 2 years. Three achieved normal TSH values within one year, 5 demonstrated improvement, but had yet to achieve TSH normalization, and 2 remained poorly controlled. Direct observation of LT4 administration was performed in 2 patients, both of whom achieved a normal TSH at 3 and 7 months, respectively. Conclusions: Our review confirms the utility of LT4AT for differentiating pseudomalabsorption from gastrointestinal malabsorption. Most of our patients had improvement in thyroid function during follow up, contending that performance of this test may enhance compliance by establishing an objective framework from which both clinicians and patients can work to develop effective, long-term management strategies.