Oroxine Drug Interactions

Interactions Decreasing Levothyroxine Absorption: Cholestyramine, calcium-, aluminium-, magnesium-, iron supplements, polystyrene sulfonates, sucralfate, lanthanum, bile acid sequestrants (e.g. colestipol), anion/cation exchange resins (e.g. kayexalate, sevelamer), and proton pump inhibitors decrease the absorption of levothyroxine. Separate the dosages of levothyroxine and the previously mentioned medicines as much as possible to avoid interaction in the stomach or small bowel.
Soy-containing compounds and high-fibre diets can decrease the intestinal absorption of levothyroxine. Therefore, a dosage adjustment of levothyroxine may be necessary, in particular at the beginning or after termination of nutrition with soy supplements.
Weight loss drug: Orlistat may decrease levothyroxine absorption which may result in hypothyroidism. To avoid this orlistat and levothyroxine should be administered at least 4 hours apart. Regular monitoring for changes in thyroid function is required.
Interactions Affecting Levothyroxine: 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 levothyroxine, resulting in increased levothyroxine requirements.
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.
Treatment with tyrosine kinase inhibitors (e.g. imatinib and sunitinib) was associated with increased levothyroxine dosage requirements in hypothyroid patients.
The concurrent use of sertraline can reduce serum levels of levothyroxine (with concomitant increased TSH levels).
Co-administration of oral contraceptives, as well as a number of other drugs, including oestrogen, tamoxifene, clofibrate, methadone, and 5-fluorouracil may increase serum concentration of thyroxine-binding globulin, and therefore increase levothyroxine dosage requirements.
Reports indicate that some HMG-CoA reductase inhibitors (statins), such as simvastatin and lovastatin, may increase thyroid hormone requirements in patients receiving levothyroxine therapy. It is unknown if this occurs with all statins. Close monitoring of thyroid function and appropriate levothyroxine dose adjustments may be necessary when levothyroxine and statins are co-prescribed.
A number of drugs may affect thyroid function tests and this should be borne in mind when monitoring a patient on levothyroxine therapy.
Interactions Affecting Other Drugs: Levothyroxine can increase the need for insulin or oral antidiabetics in patients with diabetes.
Lowering the dose of levothyroxine can cause hypoglycaemia if the insulin or oral antidiabetics dose remains unchanged.
Levothyroxine increases the effect of anticoagulants and it may be necessary to reduce the dose of anticoagulant if excessive hypoprothrombinaemia and bleeding are to be avoided.
Phenytoin levels may be increased by levothyroxine.
If co-administered with cardiac glycosides, adjustment of dosage of cardiac glycoside may be necessary.
Levothyroxine increases receptor sensitivity to catecholamines thus accelerating the response to tricyclic antidepressants. The effects of sympathomimetic agents are also enhanced.
Post-marketing cases have been reported indicating a potential interaction between ritonavir containing products and levothyroxine. Thyroid-stimulating hormone (TSH) should be monitored in patients treated with levothyroxine at least the first month after starting and/or ending ritonavir treatment.
Laboratory Test Interactions: A number of drugs may decrease serum concentration of thyroxine-binding globulin, and therefore decrease levothyroxine 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 levothyroxine therapy. Administration of acetylsalicylic acid together with levothyroxine 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.