Since commercially available levothyroxine products may not be bioequivalent, it is recommended to stay with the same manufacturer when possible. Thyroid hormone absorption can be adversely affected by many drugs and supplements, including cholestyramine, ferrous sulfate, sucralfate, calcium, and antacids containing aluminum hydroxide. It is best to take levothyroxine on an empty stomach first thing in the morning and to separate consumption of interfering substances by at least a half-hour and ideally by three to four hours. Always assess patients for compliance with doses and screen for interfering substances prior to increasing the dose of levothyroxine for an elevated TSH.
Suggested Reading
AACE. Medical guidelines for clinical practice for the evaluation and treatment of hyperthyroidism and hypothyroidism. www.aace.com/publications/guidelines. Accessed July 11, 2011.