levothyroid interactions

Levothyroxine has a narrow therapeutic index, and bioavailability is sensitive to a number of dietary factors. To ensure adequate and consistent absorption, levothyroxine should be administered on an empty stomach with water at least 30-60 minutes before breakfast. In addition, a number of medications, including over-the-counter drugs, interfere with the absorption of levothyroxine and must be administered at separate times to allow for proper uptake of the medication.

Common offenders include calcium and iron preparations. These supplements should be separated from levothyroxine by 3-4 hours. This patient is separating levothyroxine from calcium by only an hour, which is likely leading to decreased absorption of levothyroxine and subsequent under-replacement. She should be counseled to take her calcium later in the day, and TSH should be rechecked in 6 weeks.