Potential Side Effects of I-131 Therapy for Thyroid Cancer: What Patients Should Know

Radioactive iodine (I-131) therapy has been a cornerstone in the treatment of thyroid cancer for over 70 years. While it involves relatively high doses of radiation, it is generally considered safe and effective when administered under medical supervision. However, like any medical treatment, it can lead to certain side effects. Most are mild and temporary, typically appearing within a few days after treatment. Understanding these potential reactions helps patients prepare and manage symptoms more effectively.

Common Short-Term Side Effects

Sialadenitis – Inflammation of the Salivary Glands
One of the most frequently reported side effects is discomfort or swelling in the salivary glands, particularly the parotid glands located near the jaw. This occurs because the salivary glands also absorb small amounts of radioactive iodine. Patients may experience pain, dry mouth, or facial swelling. Staying well-hydrated, using sour candies to stimulate saliva flow, and practicing good oral hygiene can help reduce the risk. In most cases, symptoms resolve on their own within a few days. For more severe discomfort, over-the-counter pain relievers or prescribed treatments can provide relief.

Thyroid Bed Inflammation
If any thyroid tissue remains after surgery, it can become inflamed following I-131 exposure—a condition known as radiation thyroiditis. This may cause tenderness, pain, or swelling in the neck area. These symptoms are usually mild and transient. Applying warm compresses and taking anti-inflammatory medications as advised by a physician can help ease discomfort. Serious complications are rare, and most patients recover without intervention.

Gastrointestinal Reactions

Nausea and Vomiting
A minority of patients undergoing I-131 therapy report gastrointestinal symptoms such as nausea, and in some cases, vomiting. These typically occur within the first 24 to 72 hours post-treatment. The severity varies from person to person. For those affected, doctors often recommend anti-nausea medications (antiemetics), which are highly effective. Eating light, bland meals and staying hydrated can also support recovery. Symptoms usually subside within a few days, and long-term digestive issues are uncommon.

Blood and Bone Marrow Considerations

Temporary Bone Marrow Suppression
I-131 can temporarily affect bone marrow function, leading to a short-term decrease in blood cell counts—particularly white blood cells and platelets. This condition, known as myelosuppression, is usually mild and reversible. Routine blood tests after treatment help monitor these levels. In rare cases where counts drop significantly, supportive care such as growth factor injections or dietary adjustments may be recommended. Full recovery of blood parameters typically occurs within weeks to months.

Special Precautions for Advanced Cases

Risk of Pulmonary Complications in Metastatic Disease
Patients with widespread lung metastases require special attention. High-dose I-131 can accumulate in metastatic lung lesions, increasing the risk of radiation-induced pneumonitis or long-term pulmonary fibrosis. To minimize this risk, physicians carefully calculate the optimal dosage and may use fractionated dosing strategies. Close monitoring with imaging and pulmonary function tests is essential during follow-up. Preventive measures, including corticosteroids in select cases, may be considered to protect lung health.

Personalized Treatment Planning Is Key

Successful I-131 therapy relies heavily on individualized planning. Factors such as tumor size, extent of metastasis, overall health, and prior treatments influence the approach. Modern protocols emphasize precision dosimetry—using advanced imaging and calculations to deliver the maximum therapeutic benefit while minimizing harm to healthy tissues. This tailored strategy improves outcomes and reduces the likelihood of serious side effects.

In conclusion, while I-131 therapy for thyroid cancer is largely safe, awareness of potential side effects empowers patients to seek timely care and support. With proper preparation, monitoring, and management, most individuals tolerate treatment well and continue on a positive recovery path.