Concomitant use of levothyroxine and interacting medications in US ambulatory care visits

Background: Levothyroxine (LT4) is the third most commonly prescribed medication in the United States. It is a narrow therapeutic index medication, and thus can be impacted by drugdrug interactions, which are primarily available over-the-counter. The prevalence and associated factors with concomitant interacting drugs with LT4 is limited since over-the-counter products are not routinely captured in many drug databases.Objective: This study aimed to characterize the concomitant use of LT4 with interacting drugs at ambulatory care visits in the United States. Design: A cross-sectional analysis of the National Ambulatory Medical Care Survey (NAMCS) from 2006 to 2018 was completed.Setting and participants: Ambulatory care visits in the United States involving adult patients with a LT4 prescription were included in the analysis.Outcome measures: The primary outcome was initiation or continuation of a selected concomitant interacting drug which impacts LT4 absorption (e.g., proton pump inhibitor) in a patient visit in conjunction with LT4.Results: The authors analyzed 372,942,000 visits (weighted from a sample of 14,880) with a reported LT4 prescription. Concomitant use of interacting drugs with LT4 occurred in 24.4% of visits in which 80% of interacting drugs were proton pump inhibitors. Ages 35-49 years (adjusted odds ratio [aOR], 1.59), 50-64 years (aOR, 2.27), and >65 years (aOR, 2.87) compared to 18-34 years, female (aOR 1.37) versus males, and visits in 2014 or later (aOR, 1.27) versus 2006-2009 were associated with increased odds of concomitant interacting drug use in multivariable analysis.Conclusion: At ambulatory care visits between 2006 and 2018, concomitant use of LT4 and interacting drugs impacted one-quarter of patient visits. Increased age, females, and visits later in the study period were associated with increased odds for concomitant interacting drugs. Additional work is needed to identify downstream consequences of concomitant use.