amiodarone and levothyroxine

Amiodarone inhibits type 1 and type 2 5'-deiodinase enzymes, reducing the conversion of levothyroxine (T4) to the active triiodothyronine (T3). It also interferes with thyroid hormone synthesis and release, and can cause thyroid dysfunction through its iodine content.

In patients with hypothyroidism on levothyroxine replacement, amiodarone can reduce T3 production, leading to persistent or worsening hypothyroidism with elevated TSH. Conversely, amiodarone can also induce hyperthyroidism, requiring levothyroxine dose reduction or cessation. Clinical effects include fatigue, weight gain, or hyperthyroid symptoms, and may affect cardiac stability.

Monitor thyroid function tests (TSH, free T4, free T3) at baseline before starting amiodarone, then every 1-3 months during the first year of therapy, and periodically thereafter. Adjust levothyroxine dose based on TSH and free T4 levels. Consider alternative antiarrhythmic agents if thyroid dysfunction is difficult to manage.

The interaction is well-established through clinical experience and observational studies. Amiodarone is known to affect thyroid function and levothyroxine requirements, as reflected in product monographs and clinical guidelines.