Levothyroxine is the generic form of Synthroid, a synthetic thyroid hormone. It is a replacement for a hormone that is naturally produced by your thyroid gland to regulate the body’s energy and metabolism. Levothyroxine is used to treat hypothyroidism, enlargement of the thyroid, thyroid cancers, pituitary disorders and low thyroid hormone production due to surgery, radiation and medication use. This drug may interact with other medications, as well as certain foods and beverages.
Timing of Meals
Consumption of levothyroxine with certain foods can significantly reduce its absorption. Thyroid hormones levels can decrease if you take your medication with foods -- or foods that contain -- soybean flour, cotton seed meal, walnuts, dietary fiber, calcium or calcium fortified juices. These foods should be avoided within several hours of taking your medication. In addition, thyroid hormone levels may increase if you fast while on this medication. To avoid such fluctuations, levothyroxine should be administered on a consistent schedule, with regard to time of day and relation to meals, per your doctor's orders.
Levothyroxine can impair the efficacy of oral hypoglycemic agents and insulin, substances used for diabetics for blood glucose control. It may cause hypoglycemia, glucose intolerance, new-onset diabetes mellitus or exacerbation of preexisting diabetes, therefore interfering with your blood glucose control. It is important for your physician to closely monitor your blood glucose levels if you are taking drugs for hypoglycemia.
Multivitamins with Minerals
Supplements, especially those containing calcium or iron, are problematic with levothyroxine. Taking calcium at the same time as levothyroxine can have a significant impact on thyroid hormone levels. According to the Marin Medical Society, the administration of levothyroxine with calcium can reduce hormone levels by 40 percent. This causes a decreased oral bioavailability of levothyroxine, drug binding and alterations in gastric pH, thus the intended pharmacologic effects of this drug are reduced. It is recommended that you take levothyroxine at least four hours before or after other supplements, such as iron or multivitamin with minerals. But check with your doctor to see if you should continue your supplement regimen.
Antacids, when taken with levothyroxine, may decrease the oral bioavailability of this thyroid hormone medication. While the mechanism of interaction is unknown, it is thought to involve nonspecific absorption of levothyroxine in the gastrointestinal tract, thus reducing the effects of levothyroxine. The worst offenders are antacids containing magnesium, aluminum, calcium and iron. It is recommended that take antacids at least four hours apart from when you take levothyroxine. Otherwise, you may require an increase dosage of levothyroxine during antacid use.
Taking iron, especially as a dietary supplement, has the potential to decrease the bioavailability and pharmacologic effects of levothyroxine. This interaction is thought to be a result of binding to form highly acidic pH level, creating insoluble complexes that are poorly absorbed in the gastrointestinal tract. Therefore, your thyroid hormone levels will be elevated and not effective. If you separate levothyroxine from iron by at least two to four hours, you may be able to avoid this interaction. Your thyroid hormone levels, however, should be closely monitored.
Estrogen interacts with levothyroxine by increasing the concentration of thyroid-binding globulins in your blood. These substances are proteins that attach to thyroid hormone so that it can circulate throughout your body. Consequently, there may be a reduction in unbound or free thyroxine available for hormone production. Increased thyroid hormone concentrations result. Individuals secrete thyroxine to compensate for this effect; however, people with hypothyroidism lack the mechanism to adapt. If you are on estrogen, your physician should request blood tests to check your levothyroxine levels every 12 weeks and adjust the thyroid dosage accordingly. If you experience fatigue, cold intolerance, constipation, weight gain, depression, hair loss, joint or muscle pain, dry skin, abnormal menstrual periods, hoarseness or any other clinical manifestations of hypothyroidism, you should contact your physician immediately.
- Marin Medical Society; Food-Drug and Supplement-Drug Interactions; William Elliot, M.D.; Fall 2009.
- PDR Health: Synthroid.
- Drugs.com: Levothyroxine Food & Lifestyle Interactions.
- eMedTv: Levothyroxine Drug Interactions; Kristi Monson, Ph.D. and Arthur Schenstadt, M.D.; August 5, 2008.
- Drugs.com; Levothyroxine Drug Interactions.