1. Amiodarone
Amiodarone, a medication commonly prescribed for arrhythmias, contains a high iodine content—approximately 40 percent by weight. This excess iodine can interfere with thyroid hormone synthesis in two main ways: by inhibiting the conversion of T4 to the active hormone T3, and by dampening the thyroid-stimulating hormone (TSH) signal that normally prompts the thyroid gland to produce hormones.
This dual interference can lead to decreased thyroid hormone levels, resulting in hypothyroidism. The risk is particularly elevated in individuals with preexisting thyroid dysfunction or autoimmune thyroid disease. Given the long half-life of amiodarone, thyroid abnormalities may develop weeks to months after starting the medication.
2. Hyperthyroidism Medications
Medications used to treat hyperthyroidism, such as methimazole, propylthiouracil (PTU), potassium iodide, and radioiodine therapy, work by suppressing thyroid hormone production. While effective for controlling excessive thyroid activity, these treatments may overshoot, causing the thyroid hormone levels to drop too low.
The reduction in hormone output is often due to the shrinking of thyroid tissue or direct inhibition of hormone synthesis. Patients undergoing these treatments require regular monitoring of thyroid function tests to detect the transition from hyperthyroidism to hypothyroidism, which may necessitate levothyroxine replacement therapy.
3. Lithium
Lithium, a mood stabilizer prescribed mainly for bipolar disorder, is well documented to impact thyroid function. It can inhibit thyroid hormone release and alter iodine uptake by the gland, leading to hypothyroidism. Women over 45 and those with a family history of thyroid disease are at particular risk.
The onset of lithium-induced hypothyroidism can be gradual, often developing over months or years of treatment. Because lithium is critical for psychiatric stability, thyroid function tests should be regularly performed during therapy, and any hypothyroidism managed with hormone replacement as needed.
