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An iodine-131 (I-131) scan is a noninvasive radionuclide imaging procedure used to visualize the functionality of thyroid tissue, detect metastatic thyroid cancer, and assess the remaining thyroid tissue after thyroidectomy. The thyroid gland plays a vital role in producing essential thyroid hormones necessary for survival and regulating energy metabolism. The ingested iodine, critical for thyroid hormone production, is selectively absorbed by thyroid follicular cells and facilitated by sodium iodide symporters (NIS).[1] Thyroid-stimulating hormone (TSH) controls the function of the NIS, allowing the body to absorb the circulating iodine in the form of iodide salt. When iodide enters follicular cells, it undergoes oxidation by the thyroid peroxidase enzyme to convert iodide into iodine. Organification is the subsequent biochemical reaction that integrates iodine into thyroglobulin, thereby synthesizing the thyroid hormones thyroxine (T4) and triiodothyronine (T3). Iodine molecules can be labeled with radiotracers and utilized for both diagnostic and therapeutic applications in diverse thyroid conditions, including Graves disease, toxic multinodular goiter, and thyroid cancer. The 2 most frequently used radiotracers for thyroid tissue scans are I-123 and I-131. The primary differences between the 2 radiotracers are: I-123 has a half-life of approximately 13 hours, decays by gamma emission, and delivers superior image quality. I-123 is expensive and less readily available, and it is primarily used for imaging of the thyroid gland in cases of benign thyroid disease, such as thyroid nodules and multinodular goiter. Although I-123 can be used for imaging and monitoring thyroid cancer, its reduced sensitivity in detecting pulmonary metastases, cost considerations, and limited availability make I-131 the preferred choice.[2] I-131 has a half-life of approximately 8.2 days, decays through beta emission, and is more cost-effective and widely accessible.[2] 1-131 is primarily utilized for imaging in cases of thyroid cancer.

I-123 has a half-life of approximately 13 hours, decays by gamma emission, and delivers superior image quality. I-123 is expensive and less readily available, and it is primarily used for imaging of the thyroid gland in cases of benign thyroid disease, such as thyroid nodules and multinodular goiter. Although I-123 can be used for imaging and monitoring thyroid cancer, its reduced sensitivity in detecting pulmonary metastases, cost considerations, and limited availability make I-131 the preferred choice.[2] I-131 has a half-life of approximately 8.2 days, decays through beta emission, and is more cost-effective and widely accessible.[2] 1-131 is primarily utilized for imaging in cases of thyroid cancer. I-131 is utilized to treat differentiated thyroid cancer after thyroidectomy, toxic adenoma, toxic multinodular goiter, and Graves disease. The ingestion of radioiodine induces thyroid cell death through the emission of beta particles. The use of I-131 in both the treatment and diagnosis of thyroid cancer exemplifies theranostics, which is a concept that integrates diagnostic imaging and therapeutic intervention for a particular condition by using the same radiolabeled molecule, either in a modified form or with varying radiolabeled agent doses.[2][3] A whole-body pre-radioiodine assessment, post-radioiodine treatment evaluation, and ongoing surveillance for thyroid cancer are all valid applications for an I-131 scan. Pre-radioiodine treatment scans are conducted less frequently. After a thyroidectomy, most patients usually retain some remnant thyroid tissue in their bodies. They may then be given a therapeutic dose of I-131 and undergo a post-radioiodine scan. Pretreatment I-131 scans should be considered when surgical procedures and neck ultrasonography fail to assess the extent of residual disease clearly. In addition, the scans may be warranted when residual disease may influence the decision regarding the dose or necessity of radioiodine treatment.

Numerous complications associated with an I-131 scan stem from either the preparatory steps preceding the scan or prior treatment with I-131 before post-ablative scans. Some complications include: Hypothyroidism-associated symptoms include fatigue, depression, weight gain, constipation, muscle aches, and decreased concentration. Rapid growth of persistent or metastatic thyroid cancer with elevated TSH levels, potentially leading to airway obstruction or bone pain after rhTSH administration, in rare cases. Secondary malignancies due to I-131 treatment. Transient oligospermia and decreases in ovarian function as a result of I-131 treatment. Nausea, salivary gland inflammation, or dry mouth due to the therapy with I-131.[13] Bone marrow suppression with higher accumulated doses of iodine.[14] Mild anemia, low white blood count, or thrombocytopenia that is not typically clinically significant. Mild, transient hyponatremia and hypokalemia due to the induced hypothyroid state.[15] Increased risk of thyroid cancer and increased mortality from breast cancer after iodine therapy for Graves disease.[16][17][18] Moderate-to-severe thyroid eye disease that can worsen with I-131 treatment and is a relative contraindication. Stunning of the normal thyroid tissue that can diminish the effectiveness of therapeutic I-131 doses.