LBODP105 A Case of Significantly Increased Levothyroxine Requirement With Rifampin Therapy

INTRODUCTION: Rifampin has a wide range of drug interactions through increased hepatic microsomal enzyme activity. We are reporting a case of a patient with post-surgical hypothyroidism on stable thyroid hormone replacement therapy who required significantly higher doses of levothyroxine after starting rifampin. CASE REPORT: Patient is a 78-year-old woman with history of Papillary Thyroid Cancer (PTC) in remission status post thyroidectomy in 1994 and radioactive iodine therapy in 1997. Patient had been on a stable dose of Levothyroxine of 75-88mcg daily until a visit in January 2013 when her TSH was found to be 39 despite compliance with levothyroxine. Further history revealed that she was diagnosed with Mycobacterium Avium Intracellulare (MAI) infection of the lungs for which she was given rifampin therapy. Her Levothyroxine dose was progressively increased to 150mcg daily. She received rifampin therapy for 18 months, from August 2012 until April 2013. Her Levothyroxine dose was later changed to average 139 mcg /day based on TSH levels and continued to be the same through the rifampin course. In April 2014, and following discontinuation of rifampin, Levothyroxine was decreased to 100mcg daily and subsequently 75mcg daily. During a relapse of MAI in March 2016, her levothyroxine dosage was similarly again increased to 125mcg daily for TSH of 24.58. In June 2017, rifampin was discontinued after completion of second course of MAI treatment, and Levothyroxine dose decreased to 88 mcg daily, and subsequently 75 mcg in September 2017, and average daily 64mcg most recently given age and long-term remission with target mid normal TSH. DISCUSSION: Rifampin is used to treat active tuberculosis and MAI infection among other uses. It is a hepatic enzyme inducer which significantly interacts with other drugs. It has been reported previously to cause hypothyroidism in patients with underlying thyroid disorder and increased serum TSH while on levothyroxine, but its impact range on thyroid function has not been clearly described. Our patient was on stable levothyroxine dose following thyroidectomy, and required almost a 100% increase of levothyroxine dose when started on rifampin therapy, returning to baseline levothyroxine requirements off rifampin. This case highlights the severe hypothyroidism and significant impact on levothyroxine dose requirements following rifampin therapy, and the importance of close thyroid function monitoring in patients with hypothyroidism and on rifampin, especially in context of post-surgical hypothyroidism. Presentation: No date and time listed