Drug interactions with Thyroid Hormones (Levothyroxine)

1. Sympathomimetics- concurrent use may increase effects of thyroid preparation; thyroid preparations enhance effects of sympathomimetics. Thyroid hormones increase risk of coronary insufficiency in pts with existing coronary artery disease.

***IF THE PT IS EUTHYROID AND THE DOSE IS STABLE, THERE IS NO LIKELY ENHANCEMENT OF EPINEPHRINE’S ACTION!!!

2. Anticoagulants (warfarin) – increase of thyroid dose may require decrease in dose of warfarin because of risk of increase in prothrombin time (INR). Pts on both meds should have their INR checked regularly (*When med consult, you need to know INR and bleeding time)

3. tricyclic antidepressants (TCAs): thyroid hormones increase risk of cardiovascular toxicity of TCA’s; TCAs increase risk of thyroid toxicity; arrhythmias, CNS stimulation most common; probably due to increased sensitivity of adrenergic receptors.

4. Hepatic enzyme (p450) inducers (phenytoin, barbiturates): increased metabolic degradation of levothyroxine. Increase in thyroid dose may be needed.

Category: Pharmacology Notes