SAT-483 Levothyroxine Absorption Test: A Potential Therapeutic Tool for Levothyroxine Malabsorption

INTRODUCTION Persistent hypothyroidism (PH) even on high doses of levothyroxine is a common clinical problem and it is difficult to treat. Levothyroxine absorption test has been used to distinguish between pseudo-malabsorption and malabsorption as one of its causes. This test uses 1000 mcg of levothyroxine to calculate the percentage of levothyroxine absorbed. We present a case of malabsorption in which we used the levothyroxine absorption test to diagnose as well as treat malabsorption. Case: 55-year-old male with history of papillary thyroid cancer status post total thyroidectomy, postsurgical hypothyroidism, recurrent small bowel obstruction, status post jejunal resection, complicated by high output entero-cutaneous fistula, distal high-grade obstruction status post placement of jejunal tube, chronic abdominal pain on narcotics, who initially presented with sepsis and was also found to be hypothyroid. The thyroid stimulating hormone (TSH) level was 45.25 with free thyroxine (FT4) level of 0.54. He was adherent to his levothyroxine (LT4) 175 mcg once daily which is given one hour after tube feeds have been stopped or one hour prior to any administration of other medications. Levothyroxine absorption test was done to distinguish between malabsorption versus pseudo-malabsorption. Three different doses of LT4 were used with results all consistent with malabsorption. The percentage of absorption with 175 mcg, 500 mcg, and 1000 mcg LT4 were 3.4%, 7.2%, and 18% respectively. The formula used to determine the percentage of absorption was (total T4 at 2 hour after LT4 administration - baseline total T4 prior to administration in mcg/L) x plasma volume in liter/dose of administered LT4 in mcg. The final prescription dose of 700 mcg once daily was then derived from the available data, which eventually corrected the patient’s hypothyroidism. Repeat testing after 2 weeks showed TSH of 0.17, FT4 of 1.29 and total T4 of 6.5. The suppressed TSH at this point was attributed to chronic narcotic use but FT4 and total T4 improved appropriately. Conclusion: There is no gold standard protocol for levothyroxine absorption test so we used a different protocol for LT4 dosing compared to the conventional regimen (1000 mcg of LT4). Our patient did show appropriate levothyroxine absorption on the calculated dose. Hence, the Levothyroxine absorption test may be used both as a diagnostic as well as a therapeutic tool for the patients with LT4 malabsorption causing PH. References: G.E. Sun, K.M. Pantalone, C. Faiman, M. Gupta, L. Olansky, B. Hatipoglu, The clinical utility of free thyroxine in oral levothyroxine absorption testing. Endocr. Pract. 20(9), 925–929 (2014)