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Aluminum and magnesium containing antacids, calcium carbonate, simethicone, or sucralfate and iron-containing compounds may decrease the absorption of Synthroid; so you should not take these medications within four hours of taking Synthroid. Agents that bind cholesterol such as cholestyramine have the same effect and should also not be taken apart from the Synthroid.
Antidepressants may increase the effects of Synthroid, and the dosage may need to be adjusted.
In regard to foods and herbs, while nothing is off limits in most cases, it is reasonable to limit intake of goitrogenic foods (for example, asparagus, cabbage, peas, turnip greens, broccoli, spinach, brussel sprouts, lettuce, soybeans). Soybean flour (infant formula), cottonseed meal, walnuts, and dietary fiber may also decrease absorption of Synthroid from the GI tract.
While this is not a complete list, it gives you an idea of interactions you may need to consider. For these reasons it is recommended that thyroid medication be taken on an empty stomach 30-60 minutes before meals or at least three hours afterward.
Medically reviewed by John A. Daller, MD; American Board of Surgery with subspecialty certification in surgical critical care
U.S. National Library of Medicine