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Background: Levothyroxine is a medication with a narrow therapeutic window. Precise dosing and patient adherence to administration guidance are critical for optimizing drug performance. This is particularly important as under- or overexposure to levothyroxine can have negative therapeutic consequences. This study is a continuation of several previous studies examining the real-world use of levothyroxine. The primary objective was to quantify the percentage of patients taking levothyroxine outside of the administration guidance in the Food and Drug Administration (FDA)-approved labeling. This includes the timing of administration (≤30 minutes before breakfast) and coadministration with foods, beverages, or medications known to interfere with levothyroxine absorption. Secondary objectives included assessment of information provided by prescribers regarding levothyroxine administration. Methods: This was an online survey of 1,000 patients aged 19 to 90 years who self-reported a diagnosis of hypothyroidism by their clinician. Participants were randomly selected from a proprietary panel of research subjects who had previously consented to take surveys on various health-related topics. The referenced panel was developed and is maintained by Harmon Research Inc., a U.S.-based research firm. Pregnant patients were excluded. Eligible subjects had been taking levothyroxine monotherapy for >1 year. Survey questions were reviewed by board-certified endocrinologists. The survey was initiated in December 2023 and completed in January 2024. Results: Thirty-six percent of patients reported administering levothyroxine outside of FDA-approved labelling. This includes 18% who take levothyroxine ≤30 minutes before eating, 7% inconsistently at different times throughout the day, 2% during their meal and 9% less than 2 hours after eating. Use of vitamins/supplements known to interfere with levothyroxine is high. Calcium (30%) and iron (15%) were among the various supplements reportedly taken by patients concomitantly with levothyroxine, despite FDA-approved labelling stating that these supplements should be separated from levothyroxine by at least 4 hours. In the study 54% of participants reported taking antacids (15%) or acid reducers (39%) frequently. These medications are classified as “interfering drugs” in FDA-approved levothyroxine labelling. Most patients (72%) recalled physician instructions to take levothyroxine in the morning ≥30 minutes before breakfast with water. However, 55% did not recall instructions regarding the consumption of interfering vitamins/supplements; 60% did not recall instructions regarding the use of proton pump inhibitors (PPIs) or certain prescription medications with levothyroxine. Patients who reported difficulty adhering to administration guidance were more likely to report having stopped their prescribed medication for >1 month (25% vs. 10%; P≤0.05). Conclusions: Many patients are not following levothyroxine administration guidance in the FDA-approved labelling. Improved communication to patients may enhance levothyroxine administration adherence and improve clinical outcomes.