Hyperthyroidism is characterized by an overactive thyroid gland that produces excessive amounts of the hormones thyroxine (T4) and triiodothyronine (T3). This hormonal excess accelerates the body’s metabolism, leading to a state known as thyrotoxicosis. While autoimmune disorders like Graves’ disease are the most frequent cause, many prescription and over-the-counter medications can inadvertently trigger this hormonal imbalance.
Prescription Drugs That Contain High Levels of Iodine
The thyroid gland relies on iodine to synthesize its hormones, and an excessive iodine load can disrupt its regulatory system. Certain medications contain high concentrations of iodine, which can trigger the Jod-Basedow effect in susceptible individuals. This involves the overstimulation of autonomously functioning thyroid tissue, such as pre-existing nodules, causing the uncontrolled production and release of thyroid hormone.
The antiarrhythmic medication amiodarone is the most prominent example, as it is nearly 37% iodine by weight, delivering many times the recommended daily intake. Amiodarone can cause hyperthyroidism through two distinct mechanisms, collectively called Amiodarone-Induced Thyrotoxicosis (AIT). Type 1 AIT represents the Jod-Basedow effect, where excess iodine fuels new hormone production, typically in a thyroid gland with underlying pathology.
In contrast, Type 2 AIT is a destructive thyroiditis, caused by the drug’s direct toxic effect on thyroid cells. Damage to the thyroid follicles causes a “leak” of pre-formed hormones into the bloodstream, leading to temporary thyrotoxicosis. High iodine content is also problematic in radiological contrast agents used for imaging, as a single administration can expose the body to a massive iodine load, potentially unmasking hyperthyroidism in patients with undiagnosed thyroid autonomy.
Medications That Directly Elevate Thyroid Hormone Levels
The direct introduction of too much thyroid hormone into the body causes exogenous hyperthyroidism. This mechanism is straightforward dose-dependent toxicity, where hormone levels become elevated due to an external source rather than the thyroid gland overproducing hormones.
This scenario most commonly occurs in patients treated for hypothyroidism who receive an excessive dose of replacement therapy. Medications such as levothyroxine (synthetic T4) or liothyronine (synthetic T3) can cause hyperthyroidism if the dosage is not carefully monitored. Natural desiccated thyroid (NDT) products, which contain both T4 and T3, also carry this risk of over-replacement.
Unregulated dietary or weight-loss supplements can also cause exogenous hyperthyroidism. Some of these products are contaminated with or intentionally include unlabeled amounts of T3 and T4 hormones. Patients seeking to boost metabolism can unknowingly ingest enough thyroid hormone to induce thyrotoxicosis, especially if the supplement contains the highly active T3 hormone.
Immunomodulators and Other Drugs Causing Thyroid Inflammation
Some medications induce hyperthyroidism through a destructive, autoimmune-like process called thyroiditis. This mechanism involves an inflammatory attack on the thyroid gland, which damages follicular cells and causes the uncontrolled release of stored thyroid hormones. The resulting hyperthyroid phase is usually temporary but can be severe.
Immune checkpoint inhibitors (ICIs), such as nivolumab and pembrolizumab, used in cancer immunotherapy, are frequent culprits. These drugs “release the brakes” on the immune system to attack cancer cells, but this heightened activity can mistakenly target the body’s own tissues, including the thyroid. The resulting thyroiditis often presents as a transient period of hyperthyroidism, typically within the first few months of treatment.
Following the initial hormone release, the damaged tissue is often depleted and unable to produce sufficient hormones, leading to a subsequent state of hypothyroidism. Certain targeted therapies, like multi-targeted tyrosine kinase inhibitors, can also induce thyroiditis through direct cytotoxicity. Additionally, the mood stabilizer lithium can rarely induce hyperthyroidism by triggering an autoimmune response or causing destructive thyroiditis.
Recognizing the Onset of Drug-Induced Hyperthyroidism
Recognizing the symptoms of drug-induced hyperthyroidism allows for timely intervention. Patients may experience symptoms related to an accelerated metabolism, including a rapid or irregular heart rate, unexplained weight loss, and excessive sweating or heat intolerance. Neurological signs like hand tremors, anxiety, and nervousness are also common manifestations.
Diagnosis begins with a blood test to check the thyroid-stimulating hormone (TSH) level, which is typically suppressed when T4 and T3 hormone levels are high. Patients must provide a complete medication history to the treating physician, including all prescription drugs and over-the-counter supplements. This information allows the clinician to determine if a medication is the likely cause of the hormonal imbalance.