Decreased absorption w/ cholestyramine, Ca, Al, Mg, Fe supplements, polystyrene sulfonates, sucralfate, lanthanum, bile acid sequestrants (eg, colestipol), anion/cation exchange resins (eg, kayexalate, sevelamer), proton pump inhibitors & orlistat (if possible, separate doses for 4 hr). Soy-containing compd & high-fiber diet. Dose requirements may be altered due to increased thyroid hormone metabolism & displacement from plasma proteins by anticonvulsants (eg, carbamazepine, phenytoin). Increased metabolism & excretion by enzyme inducers (eg, rifampicin & barbiturates). Partial inhibition of peripheral T4 to T3 transformation by propranolol, amiodarone, lithium, iodide, oral contrast agents, propylthiouracil, glucocorticoids. Increased dosage requirements by tyrosine kinase inhibitors (eg, imatinib, sunitinib). Reduced serum levels by sertraline. May increase serum conc of levothyroxine-binding globulin w/ OCs, oestrogen, tamoxifen, clofibrate, methadone, 5-FU. May increase thyroid hormone requirements w/ HMG-CoA reductase inhibitors eg, simvastatin & lovastatin.