Effect may be influenced by PIs (eg, ritonavir, indinavir, lopinavir); phenytoin. May reduce effect of antidiabetics. May increase risk of haemorrhage w/ coumarin derivatives. Intensified effect w/ salicylates, dicumarol, furosemide, clofibrate. Possible decreased absorption of thyroid hormones w/ PPIs. Reduced control of hypothyroidism when used concomitantly w/ orlistat. Absorption may be decreased w/ sevelamer. Efficacy may be decreased w/ tyrosine kinase inhibitors (eg, imatinib, sunitinib). Absorption may be inhibited by ion exchange resins (eg, cholestyramine or cholestipol). Potentially decreased effect w/ Al, Fe & Ca salts. May inhibit the peripheral conversion of T
4-T
3 w/ propylthiouracil, glucocorticoids, β-sympatholytics, iodine-containing contrast media; amiodarone. Decreased efficacy & increased serum TSH level w/ sertraline, chloroquine/proguanil. Increased hepatic clearance by hepatic enzyme induction (eg, barbiturates, carbamazepine). Oestrogen-containing contraceptives. Decreased intestinal absorption w/ soy-containing compd. Falsely decreased or increased thyroid immunoassay test results w/ biotin.