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]
Nuclear & Electronics Product Page
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-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.
Preparation
Pregnancy and breastfeeding are contraindications, as a developing fetus can start absorbing radioactive iodine at approximately 10 weeks of gestation. Patients are advised to follow a low-iodine diet for approximately 7 to 14 days before the procedure. This dietary regimen excludes iodized salt, dairy products, egg yolks, and seafood while emphasizing the consumption of fresh vegetables, fruits, meats, and noniodized or kosher salt. The dietary restrictions are intended to improve the efficacy of I-131 therapy and the accuracy of pre- and post-therapy scans using I-131. Nevertheless, a recent comparative study did not reveal any significant difference in the response to radioactive iodine therapy for Graves disease between patients who adhered to a low-iodine diet and those who did not.[2]
Patients undergoing treatment with antithyroid medications for hyperthyroidism are advised to discontinue these medications for 5 to 7 days to enhance iodine uptake. However, in severe hyperthyroidism, where discontinuing the medications could be detrimental, the discontinuation period may be reduced to 3 days.
Elevating TSH levels enhances iodine uptake and optimizes imaging. The optimal TSH level for the test or potential treatment is 30 mIU/L or higher.[2]
There are 2 ways to achieve elevated TSH levels, as mentioned below.
Thyroid hormone withdrawal is recommended for patients with advanced and high-risk thyroid cancer. Patients who have undergone total or near-total thyroidectomy should discontinue thyroid replacement therapy for a period of 4 to 6 weeks before the procedure to induce pituitary TSH secretion. This recommendation is relevant for patients receiving thyroid hormones either as replacement therapy or suppressive doses. Due to its shorter half-life, reducing the discontinuation period to 2 weeks when using triiodothyronine may be feasible. [2]
Thyrotropin alfa, a recombinant TSH (rhTSH) molecule, increases TSH levels without discontinuing thyroid hormone replacement, thus preventing hypothyroidism symptoms. [2] The dosing regimen involves 2 subcutaneous injections of 0.9 mg each, administered to patients 24 hours apart. The American Thyroid Association (ATA) approves this method for remnant ablation after total thyroidectomy in patients with differentiated thyroid cancer who do not exhibit distant metastasis. [2]
Procedures
In a pre-ablation or surveillance I-131 scan, patients ingest a small dose of radioactive iodine in the form of capsules or liquid, and the scan is typically performed 24 to 72 hours later. Patients undergo post-therapy I-131 scans 3 to 7 days after an I-131 ablation or adjuvant therapy dose.
The imaging procedure for I-131 involves specific patient preparations and positioning to ensure accurate results, as mentioned below.
The patient must fast or abstain from eating for at least 4 hours before and up to 1 hour after taking the iodine dose.
The patient must recline in a supine position on a movable examination table to conduct this procedure. Before each series of images, they are instructed to consume a substantial amount of water to eliminate any potential physiological activity in the pharynx or esophagus.
A high-energy collimator γ-camera equipped with I-131 sodium iodide is used to capture both anterior and posterior planar images, as well as spot images of the nasopharynx, thyroid bed, or chest.
A medium-energy collimator can be chosen instead of a low-energy collimator while using I-123, as it can improve image quality. The scanning procedure is typically conducted at the 18- to 24-hour intervals.
A single-photon emission computed tomography (SPECT) scan can examine the area from the skull base to the diaphragm and focus on any specific concerns found during the whole-body I-131 scan. This enhances the ability to detect both regional and distant metastasis.
Serum thyroglobulin and anti-thyroglobulin antibody assays are obtained simultaneously. [4]
An I-131 scan serves as a valuable diagnostic tool for examining various thyroid disorders, with primary applications including:
Distinguishing hyperthyroidism from other types of thyrotoxicosis, although I-123 is more frequently utilized.
Visualizing ectopic thyroid tissue.
Visualizing remnants of normal or cancerous thyroid tissue and detecting thyroid cancer recurrence or distant metastasis.
Restaging thyroid cancer typically occurs 6 to 12 months after thyroidectomy and radioactive iodine ablation therapy.
Assisting in the calculation of the appropriate I-131 therapy dosage.
Determining the safest dosage of I-131 therapy for vital organs, particularly in cases where patients have undergone multiple prior treatments. This process is known as dosimetry.
Determining the functionality of thyroid nodules.
Evaluating congenital thyroid abnormalities, with the preference for I-123 due to its lower radiation exposure. [5]
The potential diagnoses associated with thyroid-related conditions are as follows:
Graves disease
Thyrotoxicosis due to Hashimoto disease
Thyroiditis
Presence of remnant thyroid tissue in the thyroid bed after total thyroidectomy
Recurrence of differentiated thyroid cancer
Metastasis of differentiated thyroid cancer
Toxic multinodular goiter
Toxic adenoma
Ectopic thyroid tissue
Excess exogenous thyroid hormone replacement
Physiological Uptake
Thyroid tissue exhibits a significant affinity for iodine uptake. Extra-thyroidal tissues such as the stomach, salivary glands, breast (particularly during lactation), and the urinary tract also contain NIS, resulting in physiological iodine uptake.[6] As radioiodine is metabolized in the liver and subsequently excreted through the urinary tract, it is common to observe visualization of the liver, gallbladder, kidneys, bladder, and ureters during imaging.
Infected and inflamed tissues, cystic lesions, and ectopic thyroid tissue can also take up and trap I-131. Ectopic thyroid tissue, visualized on an I-131 scan, results from embryological remnants or malformations during the development of the thyroid gland. Typical examples include a lingual or sublingual thyroid, a thyroglossal duct, or a mediastinal thyroid gland.[6][7][8]
Pathological Uptake
After a thyroidectomy, any radioiodine uptake that deviates from the anticipated physiological distribution observed on a post-therapy I-131 scan is indicative of either remnant thyroid tissue or metastatic disease. Conducting a whole-body scan after therapy is crucial to influence the treatment plan based on the results.[9]
Increased blood flow and the presence of NIS in tumors enable iodine uptake in conditions such as breast cancer, gastric adenocarcinoma, or bronchial adenocarcinoma.[6] External contamination by body fluids or, in rare cases, iodine ingestion can mimic metastatic involvement. Typically, this pattern is readily recognized by experienced healthcare professionals and is correlated clinically with the patient's medical history and laboratory results.[10] These observations typically involve superficial areas, and acquiring lateral and oblique images helps to identify the lesion. Furthermore, using SPECT can provide valuable information regarding the anatomical location of the uptake for further assessment.
Patients with hyperthyroidism with elevated or normal radioiodine uptake indicate Graves disease or thyrotoxicosis due to Hashimoto disease. Individuals exhibiting nearly absent radioiodine uptake may have thyroiditis, an excessive or intentional thyroid replacement, or ectopic hyperthyroidism, such as metastatic follicular thyroid cancer or struma ovarii.
Thyroid stunning refers to inhibiting the uptake or retention of I-131 after administering a diagnostic dose. Stunning is a transient phenomenon and occurs less frequently with I-123. Therefore, I-123 is the preferred radiotracer for imaging benign thyroid conditions. Stunning occurs due to the reduction in the number of functioning thyroid cells caused by radiation emitted during the test.[11]
When a small dose of I-131 is administered before a larger treatment dose, it may lead to a phenomenon called "stunning" of thyroid cells, potentially diminishing the treatment's efficacy. Radioactive iodine therapy is approved and deemed safe for both adults and children.[12]
Certain medications can disrupt iodine uptake in the thyroid. To prepare for scanning, the use of the following medications for the specified duration should be discontinued before the procedure:
Methimazole for 3 to 5 days
Propylthiouracil for 3 to 5 days
Bromides for 1 week
Mercurials for 1 week
Nitrates for 1 week
For more information, please visit I-131 radiotherapy.
Perchlorate for 1 week
Salicylates, in large doses, for 1 week
Sulfonamides for 1 week
Thiocyanate for 1 week
Iodine-containing cough medicines and vitamins for 2 weeks
Iodine solution (Lugol) for 2 to 3 weeks
Iodine-containing topical agents for 2 to 3 weeks
Kelp for 2 to 3 weeks
T3 for 2 to 3 weeks
Levothyroxine (T4) for 4 to 6 weeks
Thyroid extracts or desiccated thyroid extracts for 4 weeks
Intravenous iodinated contrast materials for 4 to 6 weeks
Oil-based iodinated contrast materials for 3 to 6 months
Amiodarone for 3 to 6 months
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.
In the interest of patient safety and education, adhering to specific guidelines following I-131 ablation or adjuvant therapy is crucial, some of which are mentioned below.
Patients should practice self-isolation following I-131 ablation or adjuvant therapy, considering their dose and patient-specific characteristics, according to nuclear medicine recommendations.
Patients intending to travel by air should be aware that they may trigger radiation detectors for approximately 2 to 3 months and should carry an official letter confirming this.
Patients are advised to refrain from becoming pregnant for 6 months following radioactive iodine treatment.
Patients should cease breastfeeding for at least 6 weeks before undergoing the test or treatment to prevent increased uptake and radiation absorption in breast tissue.
Patients are recommended to discontinue breastfeeding following I-131 administration. Subsequent pregnancies may allow for the resumption of breastfeeding. However, after I-123 administration, breastfeeding can safely resume after a waiting period of 72 hours.
An I-131 scan is an important imaging tool for diagnosing and treating thyroid disorders. Performing pre- and post-therapy scans in patients with differentiated thyroid cancer enables disease surveillance and assists clinicians in determining the most suitable treatment approach. Furthermore, it is essential to identify potentially false-positive and false-negative imaging findings during interpretation and correlate them with the patient's characteristics.
Disclosure: Sahzene Yavuz declares no relevant financial relationships with ineligible companies.
Disclosure: Yana Puckett declares no relevant financial relationships with ineligible companies.
Why will I receive radioiodine treatment?
You will receive radioiodine because you and your doctor have agreed that this is the most appropriate treatment for your thyroid cancer. Special precautions will be taken to protect you and the hospital staff caring for you, to prevent or limit the amount of exposure to a radioactive source. You may have some concerns about feeling isolated or alone during this procedure. This handout will tell you about the special precautions that you and the hospital staff must use during your treatment. It will also give you some instructions to follow when you go home.
Why do I need special care?
Iodine-131 (I-131) is radioactive. Radiation can be measured around your body and I-131 is present in your skin oil and sweat. Whatever touches your skin will pick up a small amount of radiation. The I-131 leaves your body through your urine primarily. Small amounts of I-131 may also leave through your saliva, sweat and feces. The amount of radioiodine remaining in your thyroid tissue is responsible for the desired medical effect. Radioiodine disappears by itself over a period of time.
How does radioiondine work?
Radioiodine is collected by the thyroid gland. The radiation given off by this form of iodine decreases the function of the thyroid cells and inhibits their ability to grow. This is the desired medical effect of this treatment. Some of the other tissues in your body will receive some incidental radiation. This small amount of radiation has NOT been shown to produce any adverse effect.
How long does radioiodine stay in your body?
The radioiodine from your treatment will remain in your body temporarily. Most of the radioiodine not collected by your residual thyroid tissue will be eliminated during the first two days after your treatment. The amount that remains in your thyroid tissue is responsible for the desired medical effect. A radiation safety specialist will come to your room one to two times a day to measure your radiation levels. These radiation levels will be less everyday. On discharge from the hospital, a minimal amount of radiation remains in your body and will go away over the next several weeks.
Where will I stay when I have my iodine therapy?
You may be admitted to the hospital. If this is necessary, you will be in a private room with a private bathroom. Special precautions are taken to protect you and the hospital staff caring for you. Most patients, however, are able to return home immediately after administration of the iodine, and will need to follow specific radiation safety precautions at home.
If you are admitted to the hospital, your room will have a protective covering on the floor. There will also be plastic covers on some of the furniture, door handles, light switch, telephone and TV remote. This is to protect others who will use the room after you, and facilitate cleanup. While you are in this special room you will be allowed visitors only for a short time period. You can call your family and friends on the telephone.
If you are treated as an outpatient, family members should not come closer than six feet from you the first day and three feet the second day. Further precautions will most likely not be necessary after this, but this will be determined by the radiation safety specialists with a specific measurement. At home, please cover items that are frequently shared, such as a TV remote control or computer keyboard or wear disposable gloves.
Your doctors and nurses will come into your room to take care of you. The staff will always be available to you to assist you whenever needed. However, the staff will spend as little time as possible in your room. This will help to limit the staff's exposure to radiation.Your doctors and nurses will come into your room to take care of you. The staff will always be available to you to assist you whenever needed. However, the staff will spend as little time as possible in your room. This will help to limit the staff's exposure to radiation.
How can I reduce radiation exposure to others?
There are three basic principles to remember:
Distance: The greater the distance you are from others, the less radiation they will receive. Try not to remain in close contact with others for longer than is necessary.
Time: Radiation exposure to others depends on how long you remain close to them. You should try to minimize the time spent in close contact with others.
Hygiene: good hygiene minimizes the possibility that other people will be contaminated with the radioiodine that leaves your body. Since most of the radioiodine leaves your body in your urine, good toilet hygiene and careful, thorough washing of your hands will reduce the possibility of contamination.
How do I apply these principles?
In the hospital:
Meals will be served on disposable plates with disposable utensils.
All trash and linens should be placed in the appropriate receptacles in your room.
Flush the toilet three times after you use it.
If you feel like you are going to vomit, try to go into the bathroom. Vomit into the toilet. Call your nurse right away.
Plan to wear a hospital gown and slippers as you will be confined to your room. Do not bring a suitcase full of personal items. Anything that is taken into your room will need to be checked by the radiation safety office before it can be removed. This is to keep the radioactive material from going into public areas. Some items may need to be retained if they absorb too much radioactivity from perspiration.
At home:
Observe the following recommendations for the first week you are home. You can always check with your doctor if you have any questions.
Sleep alone for the first few (3 - 4) days. Avoid kissing or sexual intercourse for three to four days after treatment. Do not sleep together for an entire night until a week after treatment.
Avoid prolonged physical contact, particularly with children and pregnant women; limit to 15 minutes. Do not resume care of an infant or breastfeeding until your doctor tells you that you can. Stay 3 to 6 feet away from other people, except when contact is necessary.
Do not prepare or serve food to other people. Do not wash, dry and put away dishes without using vinyl gloves. Wash your hands with soap and plenty of water each time after you go to the toilet.
Want more information on master oscillator power amplifier? Feel free to contact us.