Table 2.

Substances that interact with levothyroxine

Drugs and supplements that decrease the effect of levothyroxine

Reduced levothyroxine absorption

Iron

Calcium carbonate

Cholestyramine

Aluminum

Cimetidine

Sucralfate

Iodine (includes kelp tablets)

Selenium

Magnesium

Zinc

Soya

Fiber

Caffeine

Antacids

Increased levothyroxine clearance

Phenytoin

Carbamazepine

Phenobarbital

Rifampicin

Increased levothyroxine binding

Estrogen hormone replacement therapy

Drugs affected by levothyroxine

Drug effect enhanced by levothyroxine

Warfarin

Amitriptyline

Drug effect decreased by levothyroxine

Propranolol