Amiodarone is a medication that is used to treat arrhythmias, which are irregular types of heartbeats. It is known for having a high rate of efficacy, but also tending to cause side effects more frequently than less potent medications. A link between amiodarone and hypothyroidism, or underactivity of the thyroid gland, has been shown in research. Other thyroid dysfunctions, including hyperthyroidism, or overactivity of this gland, may also result from taking this medication.
The link between amiodarone and hypothyroidism is partially due to its molecular composition. This medication contains iodine, which can influence thyroid function. In over 25 percent of patients taking this medication, some type of thyroid dysfunction can occur, sometimes several months after the last dose has been taken.
There is a more common association between amiodarone and hypothyroidism in places where individuals receive a diet with ample iodine. Areas where individuals receive insufficient iodine generally show this drug to be linked with other thyroid disorders. Other risk factors can predict hypothyroidism, as well. Pre-existing thyroid conditions exist in around 68 percent of individuals that develop this dysfunction after taking amiodarone. Females are around 50 percent more likely to develop hypothyroidism than males after taking this drug.
A specific type of thyroid dysfunction called Hashimoto thyroiditis seems to contribute to the link between amiodarone and hypothyroidism. This dysfunction seems to increase the effect of iodine on the thyroid gland. Often, the presence of iodine can decrease this gland's ability to produce thyroid hormone. People with Hashimoto thyroiditis tend to develop hypothyroidism over time, and the presence of amiodarone may speed up this process.
Certain actions of this drug may also lead to the association between amiodarone and hypothyroidism. It may directly damage some of the tissue comprising the thyroid gland. Additionally, amiodarone may inhibit the passage of certain hormones into this gland that encourage its function. Finally, the drug may directly compete for receptor space within the gland, preventing it from properly operating.
Other medications for arrhythmia do not seem to show the same tendency to cause hypothyroidism. This may be due to their differences in structure, and lack of iodine. One study showed that 25.8 percent of people taking amiodarone developed hypothyroidism detectable by measuring enzymes. Patients taking a placebo or sotalol, a different arrhythmia treatment, only showed enzyme levels associated with hypothyroidism in 6.6 percent of cases. Individuals taking amiodarone showed obvious signs of hypothyroidism in 5 percent of cases, but only 0.3 percent of people taking sotalol had comparable symptoms, demonstrating how strongly this drug is linked to thyroid underactivity.