Thyroxil Drug Interactions

Levothyroxine sodium increases the effect of anticoagulants, and it may be necessary to reduce the dose of anticoagulant if excessive hypoprothrombinemia and bleeding are to be avoided. Phenytoin levels may be increased by Levothyroxine sodium.
Interactions affecting thyroxine: Anticonvulsants such as carbamazepine and phenytoin enhance the metabolism of thyroid hormones and may displace them from plasma proteins. Initiation or discontinuation of anticonvulsant therapy may alter Levothyroxine sodium dose requirements.
Enzyme inducers like rifampicin and barbiturates increase the metabolism and excretion of thyroxine, resulting in increased Levothyroxine sodium requirements. If co-administered with cardiac glycosides, adjustment of dosage of cardiac glycoside may be necessary.
The effects of sympathomimetic agents are also enhanced.
Levothyroxine sodium increases receptor sensitivity to catecholamines thus accelerating the response to tricyclic antidepressants.
Interactions decreasing thyroxine absorption: Cholestyramine, calcium-, aluminum-, magnesium-, iron supplements, aluminum hydroxide, polystyrene sulfonates, sucralfate, lanthanum, bile acid sequestrants (e.g. colestipol), anion/cation exchange resins (e.g. kayexalate, sevelamer), calcium carbonate, and ferrous sulphate and proton pump inhibitors decrease the absorption of thyroxin. Separate the dosages of thyroxine and the previously mentioned medicines as much as possible to avoid interaction in the stomach or the small bowel. Soy-containing compounds and high-fiber diets can decrease the intestinal absorption of thyroxine. Therefore, a dosage adjustment of Levothyroxine sodium may be necessary, in particular at the beginning or after termination of nutrition with soy supplements. Co-administration of oral contraceptives, as well as a number of other drugs, including estrogen, tamoxifen, clofibrate, methadone, and 5-fluorouracil may increase serum concentration of thyroxine-binding globulin, and therefore increase Levothyroxine sodium dosage requirements. Reports indicate that some HMG-CoA reductase inhibitors (statins) may increase thyroid hormone requirements in patients receiving thyroxine therapy. It is unknown if this occurs with all statins. Close monitoring of thyroid function and appropriate thyroxine dose adjustments may be necessary when thyroxine and statins are co-prescribed. Treatment with some tyrosine kinase inhibitors (e.g., imatinib and sunitinib) was associated with increased Levothyroxine sodium dosage requirements in hypothyroid patients. Medicines that (partially) inhibit the peripheral transformation of T4 to T3 like propranolol, amiodarone, lithium, iodide, oral contrast agents, propylthiouracil and glucocorticoids lower the T3 level and therefore also the therapeutic effect. The concurrent use of sertraline can reduce serum levels of thyroxine (with concomitant increased TSH levels).
Interactions affecting other drugs: Thyroxine can increase the need for insulin or oral antidiabetics in patients with diabetes. Lowering the dose of thyroxine can cause hypoglycemia if the insulin or oral antidiabetics dose remains unchanged.
Laboratory test interactions: A number of drugs may decrease serum concentration of thyroxine-binding globulin, and therefore decrease thyroxine dosage requirements, including androgens and anabolic steroids.
False low plasma concentrations have been observed with concurrent anti-inflammatory treatment such as phenylbutazone or acetylsalicylic acid and thyroxine therapy. Administration of acetylsalicylic acid together with thyroxine results in an initial transient increase in serum free T4. Continued administration results in normal free T4 and TSH concentrations, and therefore, patients become clinically euthyroid. A number of drugs may affect thyroid function tests, and this should be borne in mind when monitoring a patient on Levothyroxine sodium therapy.