May reduce effect of antidiabetics. May intensify anticoagulant effect of coumarin derivatives. May influence effect of levothyroxine w/ PIs (eg, ritonavir, indinavir, lopinavir). Elevated fT4 & fT3 fraction & increased hepatic metabolization w/ phenytoin. Elevated fT4 fraction w/ salicylates, dicoumarol, furosemide (in high doses of 250 mg) & clofibrate. Hypothyroidism &/or reduced hypothyroidism control w/ orlistat. May decrease absorption w/ sevelamer. May decrease efficacy w/ tyrosine-kinase inhibitors (eg, imatinib, sunitinib). Inhibit absorption w/ ion exchange resins (eg, cholestyramine, colestipol). Potentially decreased effect w/ Al (eg, antacids, sucralfates), Fe containing drugs & Ca salts. Propylthiouracil, glucocorticoids, β-sympatholytics, iodine-containing contrast media & amiodarone inhibit peripheral conversion of T4 to T3. May trigger hyperthyroidism & hypothyroidism w/ amiodarone. Decreased efficacy & increased TSH level w/ sertraline, chloroquine/proguanil. Increased hepatic clearance w/ hepatic enzyme inducers (eg, barbiturates, carbamazepine). Women on estrogen-containing contraceptives or postmenopausal under hormone-replacement therapy may have increased need for levothyroxine. May lower uptake w/ soy products.