Introduction

Thyrotoxicosis is a condition in which thyroid hormone levels become excessively high, creating a hypermetabolic state that affects multiple organs. Various medications, including amiodarone, iodine-containing compounds, lithium, and immune checkpoint inhibitors, can interfere with thyroid function, leading to hyperthyroidism. Recognising the role of thyroid dysfunction from drugs is essential for timely diagnosis and effective treatment.

Understanding thyroid function and its disruptions

The thyroid gland regulates metabolism by producing thyroid hormones (T3 and T4), which is controlled by thyroid-stimulating hormone (TSH). But some medications disrupt this balance, triggering medication-related thyroid disease by increasing hormone production, inflammation-induced thyroid damage, or immune system activation.

How the thyroid normally functions

The thyroid gland is located in the front of the neck, which produces triiodothyronine (T3) and thyroxine (T4) using iodine and tyrosine. The pituitary gland regulates hormone levels through TSH, ensuring a stable metabolic rate.

When some drugs interfere with this system, an excessive release of hormones leads to drug-induced hyperthyroidism, which presents with various systemic symptoms.

Medications that can trigger thyrotoxicosis

Amiodarone-induced thyrotoxicosis (AIT)

Amiodarone, a widely used antiarrhythmic medication, is a major cause of drug-induced hyperthyroidism due to its high iodine content. It affects the thyroid in two ways:

• Type 1 AIT: Occurs in people with pre-existing thyroid issues, causing excessive hormone production
• Type 2 AIT: Involves inflammation that leads to the release of stored thyroid hormones without increased synthesis.

Because amiodarone remains in the body for a long time, thyroid toxicity from medications may persist even after stopping the drug. Treating amiodarone-induced thyrotoxicosis requires balancing thyroid health with the cardiac benefits of the medication.

Iodine-induced hyperthyroidism

Excess iodine from contrast dyes, topical antiseptics, and supplements can lead to thyrotoxicosis, especially in individuals with existing thyroid conditions. Normally, the thyroid temporarily reduces hormone production in response to excess iodine (the Wolff-Chaikoff effect), but in some cases, this fails, leading to iodine-induced hyperthyroidism.

Lithium and tyrosine kinase inhibitors (TKIs)

Lithium, commonly prescribed for bipolar disorder, interferes with iodine processing in the thyroid, causing thyroid dysfunction from drugs. Similarly, tyrosine kinase inhibitors (TKIs), such as sunitinib and sorafenib, can lead to thyroiditis caused by drugs, causing a temporary hyperthyroid phase before potential long-term hypothyroidism.

Immune checkpoint inhibitors and thyroiditis

Certain cancer immunotherapy drugs, such as nivolumab and pembrolizumab, can trigger immune-related thyroiditis. These drugs alter immune function, leading to inflammation in the thyroid that often begins with hyperthyroidism but may later transition to hypothyroidism.

Recognising the symptoms of drug-induced thyrotoxicosis

The symptoms of drug-induced thyrotoxicosis are similar to primary hyperthyroidism and can include:

• Heart issues: Rapid heartbeat (tachycardia), irregular heart rhythms, and atrial fibrillation
• Nervous system effects: Tremors, anxiety, and difficulty sleeping
• Metabolic changes: Unexplained weight loss, increased appetite, and heat intolerance
• Eye-related symptoms: Some cases resemble Graves’ disease
• Muscle and bone problems: Weakness and increased bone turnover

In severe cases, thyroid storm from medications may develop, which is a life-threatening emergency marked by high fever, confusion, and dangerously fast heart rates.

How drug-induced thyrotoxicosis is diagnosed

Reviewing medical history and symptoms

A detailed medication history and symptom timeline help identify medication-related thyroid disease.

Laboratory tests

• Thyroid function tests (TSH, Free T3, Free T4): Usually show low TSH with high thyroid hormone levels.
• Thyroid autoantibody tests (TRAb, TPO-Ab): Help distinguish from Graves’ disease.
• Inflammatory markers (ESR, CRP): Elevated in thyroiditis-related cases.

Imaging for further evaluation

• Thyroid ultrasound with Doppler: Detects increased blood flow in Type 1 AIT.
• Radioactive iodine uptake scan: Differentiates Graves’ disease from drug-induced thyrotoxicosis.

Differentiating drug-induced thyrotoxicosis from other conditions

It’s important to distinguish drug-induced thyrotoxicosis from other causes of thyroid hormone imbalance due to drugs such as:

• Graves’ disease: Positive TRAb antibodies, high radioactive iodine uptake
• Toxic multinodular goitre: Caused by hormone-producing thyroid nodules
• Subacute thyroiditis: Viral-related, painful thyroid, low iodine uptake
• Factitious thyrotoxicosis: Caused by excessive intake of thyroid hormone supplements

Treatment and management strategies

Stopping the drug if possible

Discontinuing the offending medication can help resolve thyroid toxicity from medications. However, in amiodarone-induced thyrotoxicosis, stopping the drug is complex due to its role in heart health.

Controlling symptoms

• Beta-blockers (e.g., propranolol, atenolol): Help manage thyroid storm from medications by controlling heart rate
• Steroids (prednisone): Useful for thyrotoxicosis treatment related to inflammation

Treating the underlying thyroid dysfunction

• Antithyroid Drugs (Methimazole, PTU): First-line treatment for hyperthyroidism and Type 1 AIT by inhibiting hormone production. PTU is preferred in thyroid storm and first-trimester pregnancy
• Iodine Therapy (Lugol’s, SSKI): Temporarily suppresses thyroid hormone production before surgery. Used cautiously in iodine-induced hyperthyroidism
• Potassium Perchlorate: Used in amiodarone-induced thyrotoxicosis to block iodine uptake
• Cholestyramine: Enhances thyroid hormone excretion, helpful in refractory cases

Advanced treatment options

• Radioactive iodine (¹³¹I) therapy: Used when medications fail to control the condition
• Thyroidectomy (surgical removal): Considered for severe or recurrent cases

Long-term outlook and follow-up

The prognosis for drug-induced thyrotoxicosis varies based on the specific drug involved. Some cases resolve after stopping the medication, while others may lead to long-term hypothyroidism. Regular thyroid function monitoring is recommended for patients on medications known to affect the thyroid, such as amiodarone and TKIs.

Summary

Drug-induced thyrotoxicosis should always be considered in patients experiencing hyperthyroid symptoms while taking amiodarone, iodine-based drugs, lithium, or immune checkpoint inhibitors. Early detection, proper diagnostic evaluation, and individualised treatment are essential for managing this condition effectively. Healthcare providers should closely monitor medication-related thyroid disease to prevent complications and ensure better patient outcomes.

FAQs

Can this condition be prevented?

While it’s not always avoidable, regular thyroid function monitoring can help catch changes early, especially if you’re taking medications that affect the thyroid.

Will my thyroid go back to normal if I stop the medication?

In many cases, yes. Once the medication is discontinued, thyroid function often stabilises. However, some people may develop long-term hypothyroidism and need ongoing treatment.

Is amiodarone-induced thyrotoxicosis permanent?

Not necessarily, but it can last a while because amiodarone stays in the body for months. The type of thyrotoxicosis you have (Type 1 or Type 2) determines the treatment approach and how long symptoms may persist.

How often should my thyroid be checked if I’m on a high-risk medication?

If you're taking medications like amiodarone, lithium, or TKIs, your doctor will likely check your thyroid every 3 to 6 months—or more often if you start experiencing symptoms. Regular monitoring can help prevent complications.