What are the thyroid effects of amiodarone?

From the Guidelines

Amiodarone can cause both hypothyroidism and hyperthyroidism due to its high iodine content and direct toxic effects on the thyroid gland, with hypothyroidism being two to four times more common than hyperthyroidism. The thyroid effects of amiodarone are significant, and patients on this medication require close monitoring of their thyroid function. According to the study 1, hypothyroidism is more common than hyperthyroidism, and the drug may be continued with appropriate thyroid hormone supplementation in hypothyroid patients with a strong clinical indication for amiodarone. The study 1 also highlights the importance of monitoring thyroid function in patients on amiodarone, as the drug's high iodine content and structural similarity to thyroxine can impact thyroid function and metabolism. Furthermore, the 2014 AHA/ACC/HRS guideline 1 recommends careful consideration of the risks and benefits of treating patients with atrial fibrillation and a known history of thyroid disease with amiodarone, and close monitoring of these patients. Some key points to consider when managing patients on amiodarone include:

• Baseline thyroid function tests (TSH, free T4, free T3) before starting therapy
• Monitoring every 3-6 months during treatment
• Early recognition of thyroid dysfunction
• Hypothyroidism may require levothyroxine supplementation while continuing amiodarone
• Hyperthyroidism management is more complex and may necessitate thionamides, corticosteroids, or even thyroidectomy in severe cases. Overall, the thyroid effects of amiodarone are a significant concern, and close monitoring and management are essential to minimize the risk of morbidity and mortality.

From the FDA Drug Label

Amiodarone inhibits peripheral conversion of thyroxine (T 4) to triiodothyronine (T 3) and may cause increased thyroxine levels, decreased T 3 levels, and increased levels of inactive reverse T 3 (rT 3) in clinically euthyroid patients. Because of its release of inorganic iodine, or perhaps for other reasons, amiodarone can cause either hypothyroidism or hyperthyroidism. 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

The thyroid effects of amiodarone include:

• Hypothyroidism: reported in 2 to 4% of patients, managed by amiodarone dose reduction and/or thyroid hormone supplement
• Hyperthyroidism: occurs in about 2% of patients, poses a greater hazard due to the possibility of thyrotoxicosis and/or arrhythmia breakthrough or aggravation Key points to consider:
• Thyroid function should be monitored prior to treatment and periodically thereafter
• Amiodarone can cause increased thyroxine levels, decreased T 3 levels, and increased levels of inactive reverse T 3 (rT 3) in clinically euthyroid patients 2

From the Research

Thyroid Effects of Amiodarone

The thyroid effects of amiodarone are a significant concern due to its high iodine content and prolonged biological half-life. Some key points to consider include:

• Amiodarone can cause both hypothyroidism and hyperthyroidism in patients, with an estimated 15-20% of patients developing some form of thyroid dysfunction 3, 4, 5.
• Amiodarone-induced hypothyroidism (AIH) is typically treated with levothyroxine replacement therapy, and amiodarone discontinuation is not always necessary 4, 6, 5.
• Amiodarone-induced thyrotoxicosis (AIT) can be divided into two main types: type 1, which is a form of true iodine-induced hyperthyroidism, and type 2, which is a drug-induced destructive thyroiditis 4, 6, 5.
• Type 1 AIT is usually treated with thioamides, while type 2 AIT is best treated with oral glucocorticoids 4, 6, 5.
• Mixed or indefinite forms of AIT may require a combination of thioamides, potassium perchlorate, and steroids 4, 6.

Diagnosis and Management

Diagnosis and management of amiodarone-induced thyroid dysfunction can be challenging, and regular monitoring of thyroid function tests is essential for early detection 3, 6, 5. Some key considerations include:

• Screening for thyroid disease before starting amiodarone and periodic monitoring of thyroid function tests are advocated 6.
• Thyroidectomy may be an option for patients with severe cardiac dysfunction or those who are resistant to medical therapy 4, 7.
• Radioiodine treatment is usually not feasible due to the amiodarone-related iodine load, which decreases thyroidal radioiodine uptake 4, 7.

Types of Thyroid Dysfunction

The types of thyroid dysfunction caused by amiodarone include:

• Hypothyroidism: typically treated with levothyroxine replacement therapy 4, 6, 5.
• Hyperthyroidism: can be divided into two main types, type 1 and type 2, which require different treatment approaches 4, 6, 5.
• Mixed or indefinite forms: may require a combination of thioamides, potassium perchlorate, and steroids 4, 6.