Does Amiodarone (antiarrhythmic medication) affect hypothyroidism or hyperthyroidism more?

From the Guidelines

Amiodarone is more likely to cause hypothyroidism than hyperthyroidism, with hypothyroidism being two to four times more common than hyperthyroidism in patients receiving long-term amiodarone therapy. This antiarrhythmic medication contains high amounts of iodine and can disrupt normal thyroid function through several mechanisms, including the inhibition of peripheral conversion of T4 to T3 and direct toxic effects on thyroid cells 1.

Key Points to Consider

• Hypothyroidism occurs in patients taking amiodarone due to its high iodine content and toxic effects on the thyroid gland.
• The prevalence of hypothyroidism versus hyperthyroidism can be influenced by the geographic location and iodine status of the patient population, with hypothyroidism being more common in iodine-sufficient areas and hyperthyroidism occurring more frequently in iodine-deficient regions.
• Patients taking amiodarone should have baseline thyroid function tests (TSH, free T4) before starting treatment and regular monitoring every 3-6 months thereafter to detect any thyroid dysfunction early.
• Treatment of amiodarone-induced hypothyroidism typically involves levothyroxine supplementation, while hyperthyroidism may require the withdrawal of amiodarone, antithyroid medications, prednisone, or surgical thyroidectomy 1.

Clinical Implications

• The long half-life of amiodarone (approximately 100 days) and its storage in adipose tissue mean that thyroid effects can persist for months after discontinuation.
• In hypothyroid patients with a strong clinical indication for amiodarone, the drug may be continued with appropriate thyroid hormone supplementation, highlighting the need for careful management and monitoring of these patients 1.

From the FDA Drug Label

Amiodarone can cause either hypothyroidism or hyperthyroidism. Thyroid function should be monitored prior to treatment and periodically thereafter, particularly in elderly patients, and in any patient with a history of thyroid nodules, goiter, or other thyroid dysfunction Hypothyroidism has been reported in 2 to 4% of patients in most series, but in 8 to 10% in some series Hyperthyroidism occurs in about 2% of patients receiving amiodarone, but the incidence may be higher among patients with prior inadequate dietary iodine intake Amiodarone-induced hyperthyroidism usually poses a greater hazard to the patient than hypothyroidism because of the possibility of thyrotoxicosis and/or arrhythmia breakthrough or aggravation, all of which may result in death.

Amiodarone affects both hypothyroidism and hyperthyroidism, but hyperthyroidism is considered to pose a greater hazard due to the risk of thyrotoxicosis and arrhythmia. The incidence of hypothyroidism is reported to be higher, occurring in 2 to 4% of patients in most series, but up to 8 to 10% in some series, whereas hyperthyroidism occurs in about 2% of patients receiving amiodarone 2.

From the Research

Amiodarone's Effect on Thyroid Function

Amiodarone, an antiarrhythmic medication, can affect thyroid function, leading to both hypothyroidism and hyperthyroidism. The evidence suggests that:

• Amiodarone-induced hypothyroidism is more common in iodine-sufficient areas, while amiodarone-induced hyperthyroidism is more common in iodine-deficient regions 3.
• The incidence of amiodarone-induced thyroid dysfunction can range from 15-20% of cases, with both hypothyroidism and hyperthyroidism occurring 4.
• In pediatric and young adult patients, the prevalence of amiodarone-induced thyroid dysfunction is significant, with 17.3% developing subclinical hypothyroidism and 13.7% developing hypothyroidism, while 2.1% developed hyperthyroidism 5.

Comparison of Hypothyroidism and Hyperthyroidism

The studies suggest that:

• Hypothyroidism is more common than hyperthyroidism in patients taking amiodarone, with 25.8% of amiodarone-treated patients developing subclinical hypothyroidism and 5.0% developing overt hypothyroidism, compared to 5.3% developing hyperthyroidism 6.
• Amiodarone-induced hypothyroidism can range in severity from mild symptoms to severe myxedema, while amiodarone-induced hyperthyroidism can be more difficult to diagnose and treat 7.
• The treatment options for amiodarone-induced hypothyroidism and hyperthyroidism differ, with levothyroxine replacement used for hypothyroidism and antithyroid drugs used for hyperthyroidism 7, 4.

Risk Factors and Monitoring

The evidence suggests that:

• Older males are at a higher risk of developing hypothyroidism while taking amiodarone, with 30.8% developing hypothyroidism compared to 6.9% of controls 6.
• Regular monitoring of thyroid function is necessary for patients taking amiodarone, with recommendations for weekly intervals for the first 5 weeks after initiation 5.
• The majority of thyroid dysfunction occurs within the first 35 days of treatment, highlighting the need for early monitoring and detection 5.