Thyvex-50/Thyvex-100 Drug Interactions

Interactions affecting other drugs: Levothyroxine increases the effect of anticoagulants (Warfarin), and it may be necessary to reduce the anticoagulation dosage if excessive, hypoprothrombinemia and bleeding are to be avoided.
Blood sugar levels are raised, and dosage of anti-diabetic agents may require adjustment.
Tricyclic anti-depressants (e.g., amitriptyline, imipramine, dosulepin) response may be accelerated because levothyroxine increases sensitivity to catecholamines; concomitant use may precipitate cardiac arrhythmias.
The effects of sympathomimetic agents (e.g., adrenaline or phenylephrine) are also enhanced.
Cardiac glycosides: If levothyroxine therapy is initiated in digitalised patients, the dose of digitalis may require adjustment. Hyperthyroid patients may need their digoxin dosage gradually increased as treatment proceeds because initially patients are relatively sensitive to digoxin.
NSAIDs: False low plasma concentrations have been observed with concurrent anti-inflammatory treatment such as phenylbutazone or acetylsalicylic acid and levothyroxine therapy.
Beta Blockers: levothyroxine (thyroxine) accelerates metabolism of propranolol, atenolol and sotalol.
General anaesthetics: Isolated reports of marked hypertension and tachycardia have been reported with concurrent ketamine administration.
Interactions affecting Levothyroxine: Amiodarone may inhibit the de iodination of thyroxine to triiodothyronine resulting in a decreased concentration of triiodothyronine, thereby reducing the effects of thyroid hormones.
Anti-convulsant, such as carbamazepine and phenytoin, enhance the metabolism of thyroid hormones and may displace them from plasma proteins.
Initiation or discontinuation of anti-convulsant therapy may alter levothyroxine dosage requirements.
Effects of Levothyroxine may be decreased by concomitant sertraline.
Absorption of levothyroxine (thyroxine) possibly reduced by antacids, proton pump inhibitors, calcium salts, cimetidine, oral iron, sucralfate, colestipol, polystyrene sulphonate resin and cholestyramine (administration should be separated by 4-5 hours).
Metabolism of levothyroxine (thyroxine) accelerated by rifampicin, barbiturates, and primidone. (May increase requirements for levothyroxine (thyroxine) in hypothyroidism).
Imatinib: Plasma concentration of levothyroxine (thyroxine) possibly reduced by imatinib.
Beta blockers may decrease the peripheral conversion of levothyroxine to triiodothyronine.
Lipid regulating drugs: Lovastatin has been reported to cause one case each of hypothyroidism and hyperthyroidism in two patients taking levothyroxine.
Sex Hormones: Oestrogen, oestrogen containing product (including hormone replacement therapy) and oral contraceptives may increase the requirement of thyroid therapy dosage. Conversely, androgens and corticosteroids may decrease serum concentrations of Levothyroxine binding globulins.
Anti-obesity drugs such as orlistat may decrease levothyroxine absorption which may result in hypothyroidism (monitor for changes in thyroid function).
A number of drugs may affect thyroid function tests, and this should be borne in mind when monitoring a patient on levothyroxine therapy.
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.
Drug-Food Interactions: Consumption of certain foods may affect levothyroxine sodium absorption thereby necessitating adjustments in dosing. Soybean flour (infant formula), cotton seed meal, walnuts, calcium and calcium-fortified orange juice, and dietary fibre may decrease the absorption of levothyroxine sodium from the gastrointestinal tract.
Drug-Laboratory Interactions: A number of drugs and foods are known to alter serum levels of TSH, T4 and T3 and may thereby influence the interpretation of laboratory tests of thyroid function. Changes in Thyroxine-Binding Globulin (TBG) concentration should be taken into consideration when interpreting T4 and T3 values. Drugs such as estrogens and estrogen-containing oral contraceptives increase serum TBG concentrations. TBG concentrations may also be increased during pregnancy, in infectious hepatitis and acute intermittent porphyria. Decreases in TBG concentrations are observed in nephrosis, severe hypoproteinemia, severe liver disease, acromegaly, and after androgen or corticosteroid therapy. Familial hyper- or hypo-thyroxine binding-globulinemias have been described. The incidence of TBG deficiency is approximately 1 in 9000. Certain drugs such as salicylates inhibit the protein-binding of T4. In such cases, the unbound (free) hormone should be measured.