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Levothyroxine may increase the effect of certain drugs which inhibit blood clotting (coumarin derivatives) and reduce the effect of drugs which lower the blood sugar.
Being treated simultaneously with colestyramine and levothyroxine, there should be an interval of 4 or 5 hrs between intake of the 2 drugs as colestyramine inhibits the uptake of levothyroxine in the body.
Rapid IV administration of phenytoin may raise plasma levels of free levothyroxine and liothyronine, and in isolated cases may favour the development of heart rhythm disturbances.
Levothyroxine may be displaced from its plasma protein-binding by salicylates, dicoumarol, high-dosed furosemide (250 mg), clofibrate and other substances.
