Radioactive iodine (I-131) is a form of iodine that emits radiation. It is a well-established tool in nuclear medicine for both diagnostic purposes and treatment of specific thyroid conditions. This article focuses on “high-dose” I-131, which refers to therapeutic amounts used to address more significant thyroid issues.
Medical Applications of High-Dose Radioactive Iodine
High-dose radioactive iodine primarily treats differentiated thyroid cancer, such as papillary or follicular types. Following surgery to remove the thyroid gland, high-dose I-131 destroys any remaining normal or cancerous thyroid tissue, a process known as remnant ablation.
The treatment also eliminates thyroid cancer cells that may have spread to lymph nodes or distant sites like the lungs and bones. While radioactive iodine can also treat severe hyperthyroidism, larger doses are typically reserved for cancer treatment.
Understanding High Doses and Treatment Delivery
A “high-dose” of radioactive iodine refers to quantities significantly greater than those used for diagnostic imaging. For therapeutic purposes in thyroid cancer, doses can range from 30 millicuries (mCi) to over 200 mCi. Remnant ablation might use 30 to 100 mCi, while more extensive or metastatic disease may require 100 to 200 mCi or higher.
The precise dose of I-131 is determined by medical professionals based on the specific type and extent of the thyroid disease, the patient’s individual characteristics, and the physician’s assessment. Radioactive iodine is most commonly administered orally, either as a capsule or a liquid. This allows the body to absorb the radioisotope, which then travels through the bloodstream to target thyroid cells.
Preparing for and Recovering from Treatment
Preparation for high-dose radioactive iodine treatment involves several steps. Patients are required to follow a low-iodine diet for one to two weeks before treatment. This diet, which limits daily iodine intake to less than 50 micrograms, helps deplete the body’s natural iodine stores, encouraging thyroid cells to absorb more radioactive iodine. Foods to avoid include iodized salt, seafood, dairy products, and certain processed foods.
Another preparation step involves increasing thyroid-stimulating hormone (TSH) in the blood. A high TSH level promotes the uptake of iodine by thyroid cells. This can be achieved by temporarily stopping thyroid hormone medication (leading to hypothyroidism) or by receiving injections of recombinant human TSH (Thyrogen).
After treatment, radiation safety precautions are essential to minimize exposure to others. Patients typically undergo an isolation period, ranging from a few days to about a week, depending on the dose and local guidelines. During this time, maintain distance from others, especially pregnant women and young children, and practice strict hygiene. Short-term side effects can include neck tenderness, dry mouth, changes in taste, and nausea.
How Radioactive Iodine Works and Ongoing Care
Radioactive iodine works by leveraging the thyroid gland’s natural ability to absorb iodine from the bloodstream. Once swallowed, I-131 is absorbed into the body and selectively concentrated by any remaining thyroid cells, whether normal or cancerous. The radiation emitted by I-131, primarily beta particles, then acts locally to destroy these targeted cells, with minimal impact on other body tissues.
After high-dose radioactive iodine treatment, long-term follow-up is necessary to monitor treatment effectiveness and overall health. This ongoing care typically involves regular blood tests to check thyroid hormone levels, including TSH and thyroglobulin, which can indicate remaining thyroid tissue or cancer. Imaging scans may also be performed periodically to assess for any recurrence.
Most individuals who undergo high-dose radioactive iodine therapy will require lifelong thyroid hormone replacement therapy. This is because the treatment often leads to the permanent destruction of thyroid cells, necessitating external hormone supplementation to maintain normal bodily functions.