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

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/