Levothyroxine, the synthetic form of thyroxine, is routinely used as the primary form of thyroid hormone replacement in the management of hypothyroidism, a condition that affects up to 7% of the general population.[1] In 2017, levothyroxine was the most commonly prescribed medication in the United States, surpassing those used for diabetes, blood pressure, and heart disease.[2]
Levothyroxine use is monitored with thyroid blood tests, the results of which depend partly on the stability of circulating thyroid hormone levels after oral ingestion. Fluctuating thyroid test results require more frequent adjustments in levothyroxine dose. In turn, constantly adjusting a patient's levothyroxine dose requires greater follow-up monitoring, which is associated with increased use of healthcare resources, higher costs, and lost economic productivity due to lost time and wages.[3]
Thus, there has been substantial interest in ensuring that patients are given the appropriate instructions for taking their levothyroxine, to allow the medication to be absorbed as consistently as possible.
Current Guidelines
Levothyroxine is absorbed in the small intestine and is 70%-80% bioavailable in the euthyroid individual.[4]Peak absorption is achieved at approximately 2 hours after oral ingestion but can be delayed to 3-4 hours if it is ingested simultaneously with interfering medications, supplements, or some foods/drinks.
