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Background: Levothyroxine (L-T4) absorption is a concern in patients who report appropriate self-administration but remain clinically and biochemically hypothyroid. However, there are no established guidelines for L-T4 absorption testing. Clinical Cases: Two women (A and B) with a history of Graves’ disease and radioactive iodine ablation (RAI) on weight-based L-T4 replacement presented to the hospital with severe fatigue and bilateral leg swelling. A third woman (C) with a history of papillary thyroid carcinoma, total thyroidectomy and RAI, on suppressive doses of L-T4 presented to the clinic with fatigue, constipation and weight gain. All patients reported compliance with taking L-T4 with appropriate technique. They had no known medical problems contributing to a malabsorptive state, and were not taking any medications known to interfere with L-T4 absorption. Initial testing of thyroid function revealed TSH (0.57-3.5 μIU/mL)/Free T4 (0.61-1.18 ng/dL) levels of 10.12/0.249, 45.95/&amp;lt;0.2 and 142/&amp;lt;0.2 in patients A, B and C respectively. Patients A and B tested negative for Helicobacter pylori infection, celiac disease and lactose intolerance. Patient C had been successfully treated for H. pylori infection in the past. Celiac testing was negative, but the patient tested positive for anti-parietal cell antibodies and vitamin B12 deficiency. She was referred to Hematology &amp; GI specialists. None of the patients had clinical evidence of heart failure and had normal echocardiograms. In order to evaluate for poor absorption, all patients underwent L-T4 absorption testing. Based on published data, baseline TSH and Free T4 levels were obtained followed by administration of 1000 mcg of L-T4. Next, free T4 levels were obtained at 2 hours for all patients and at 4, 8 and 24 hours for patient A &amp; B. Patients A and B underwent testing in the inpatient setting, while Patient C was tested in the outpatient clinic. In all cases, a &amp;gt; 50% rise in free T4 levels was observed at 2 hours, suggesting non-adherence or “pseudo-malabsorption”. Patients were counseled extensively regarding proper technique and compliance with L-T4. Patients A and B demonstrated rapid clinical improvement in the hospital, and Patient C reported to her next clinic visit with clinical and biochemical improvement. Conclusions: Though guidelines have not been established for L-T4 absorption testing, case reports exist exhibiting the safety and efficacy of L-T4 absorption challenge with a weekly weight-based dose or 1000 mcg of LT4. Our case series show the success of the latter with meaningful results in as little as 2 hours. Case C demonstrates that this test can be done safely and cost-effectively as an outpatient. Standardization of L-T4 absorption test will be of great utility in the management of this common problem encountered by endocrinologists.

SAT-503 Levothyroxine Absorption Testing in Three Patients with Severe Hypothyroidism on Adequate Replacement