TFour 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.
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.
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.
Cholestyramine given concurrently reduces the gastrointestinal absorption of thyroxine. A number of other drugs may decrease absorption of levothyroxine sodium, and hydroxide, bile acid sequestrants (e.g., colestipol), anion/cation exchange resins (e.g., kayexalate, sevelamer), sucralfate, calcium carbonate, and ferrous sulfate.
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.
A number of drugs may decrease serum concentration of thyroxine-binding globulin, and therefore decrease levothyroxine sodium dosage requirements, including androgens and anabolic steroids.
Treatment with some tyrosine kinase inhibitors (e.g., imatinib and sunitinib) was associated with increased levothyroxine sodium dosage requirements in hypothyroid patients. Treatments with amiodarone has been associated with multiple effects on thyroid function including increased levothyroxine sodium dosage requirements in hypothyroid patients. A number of drugs may affect thyroid function tests and this should be borne in mind when monitoring a patient on levothyroxine sodium therapy.