Synthroid can interact with many other medications and foods. Several drugs can interfere with Synthroid being absorbed if they are taken around the same time. They should be separated from Synthroid by at least four hours. Examples of these drugs include antacids, iron, calcium, colestipol, sucralfate, simethicone, raloxifene, orlistat, cholestyramine, and sodium polystyrene. Some types of food can also interfere with the absorption of Synthroid and should not be eaten with it. They include soybean flour (in soybean formula for infants), cotton seed meal, fiber, and walnuts.
Other drugs may change the amount of Synthroid your body needs. Blood tests should be taken to assess your levothyroxine levels if these drugs are taken with Synthroid. They include some antidepressants, some seizure medications, estrogens, and rifampin.
Some of the Synthroid-drug interactions can result in significant changes for the other drugs. Synthroid can make some drugs less effective, so you will need to increase the dose of these drugs. They include diabetes medications (pills and insulin), theophylline, and digoxin. By contrast, Synthroid can make other drugs more effective. An example of this is warfarin, which may lead to serious bleeding if the dose of warfarin is not reduced. As well, combining some antidepressants and Synthroid can increase the risk of heart problems, such as irregular heart rhythms.