• Antihyperthyroid agent
• Antineoplastic

Sodium iodide I 131, (SOE-dee-um EYE-oh-dyed) also called radioactive iodine or radioiodide, is a radiopharmaceutical (ray-dee-oh-far-ma-SOO-ti-kal) . Radiopharmaceuticals are radioactive agents, which may be used to diagnose some diseases by studying the function of the body's organs or to treat certain diseases.

Sodium iodide I 131 is used to treat an overactive thyroid gland and certain kinds of thyroid cancer. It is taken up mainly by the thyroid gland. In the treatment of hyperactive thyroid gland, radiation from the radioactive iodine damages the thyroid gland to bring its activity back down to normal. Larger doses of radioiodide are usually used after thyroid cancer surgery to destroy any remaining diseased thyroid tissue or to destroy thyroid cancer that has spread to other tissues.

When very small doses are given, a measure of the radioactivity taken up by the gland helps your doctor decide whether your thyroid gland is working properly. Also, an image of the organ on paper or a computer printout can be provided.

The information that follows applies only to the use of sodium iodide I 131 in treating an overactive or cancerous thyroid gland.

Sodium iodide I 131 is to be given only by or under the direct supervision of a doctor with specialized training in nuclear medicine or radiation oncology. It is available in the following dosage forms:

Oral
• Capsules (U.S. and Canada)
• Solution (U.S. and Canada)

In deciding to use a medicine, the risks of taking the medicine must be weighed against the good it will do. This is a decision you and your doctor will make. For sodium iodide I 131, the following should be considered:

Diet—If you eat large amounts of iodine-containing foods, such as iodized salt and seafoods, or cabbage, kale, rape (turnip-like vegetable), or turnips, the iodine contained in these foods will reduce the amount of this radiopharmaceutical that your thyroid gland will accept. Avoid these foods for at least 2 to 4 weeks before the treatment with radioiodide.

Pregnancy—Sodium iodide I 131 should not be used during pregnancy. This is to avoid exposing the fetus to radiation. Also, it may cause the newborn baby to have an underactive thyroid gland. A pregnancy test should be done prior to treatment with radioactive iodine if pregnancy is a possibility. Be sure you have discussed this with your doctor.

Breast-feeding—Sodium iodide I 131 passes into the breast milk and may cause unwanted effects, such as underactive thyroid, in the nursing baby. If you must receive this radiopharmaceutical, it will be necessary for you to stop breast-feeding several weeks before treatment. Be sure you have discussed this with your doctor.

Children—Sodium iodide I 131 has been used in children and has not been shown to cause different side effects or problems than it does in adults. However, vomiting may be more difficult to manage in younger children.

Older adults—Sodium iodide I 131 has been used in older people and has not been shown to cause different side effects or problems in older people than it does in younger adults.

Other medicines—Other medicines/tests

Tell your doctor if you have had an x-ray test recently for which you were given a radiopaque agent that contained iodine. The iodine contained in the radiopaque agent will reduce the amount of sodium iodide I 131 that your thyroid gland will accept. Also, tell your doctor if you are taking any other prescription or nonprescription (over-the-counter [OTC]) medicine.

• Diarrhea or
• Vomiting—The radioactive iodine will be present in the diarrhea and vomit and will put you and others at a higher risk of radiation contamination; also, some of the dose will be lost, making the treatment less effective

• Kidney disease—Kidney disease may cause the radioiodide to stay in the body longer than usual, which may increase the risk of side effects

• If you have heart disease and are receiving sodium iodide I 131 to treat an overactive thyroid—The radiation may worsen the thyroid condition if antithyroid medicine and/or beta-blockers, such as propranolol, are not given before and after treatment

Your doctor may have special instructions for you to get ready for your treatment. If you have not received such instructions or you do not understand them, check with your doctor ahead of time.

Dosing—

The doses of radiopharmaceuticals will be different for different patients and for the different types of treatments. The amount of radioactivity of a radiopharmaceutical is expressed in units called becquerels or curies. The usual adult dose of radioiodide to treat an overactive thyroid gland is 148 to 370 megabecquerels (4 to 10 millicuries). The usual amount of radioiodide to treat cancer of the thyroid is much larger, 1.1 to 7.4 gigabecquerels (30 to 200 millicuries). The dose you receive depends on your size and age. The amount of radiation received by specific areas of the body to treat a disease is many times higher than that received from any diagnostic test, such as x-rays and nuclear medicine scans. Doses may need to be repeated, depending on the kind of disease you have and how your body is responding to treatment.

There are no special precautions when this medicine is used in very small doses to help study the function of the thyroid. However, if you are receiving sodium iodide I 131 for an overactive thyroid or cancer of the thyroid, your doctor may tell you to follow some or all of these guidelines for 48 to 96 hours after receiving the medicine, to help reduce the chance of contaminating other persons:

• Do not kiss anyone, or handle or use another person's eating or drinking utensils, toothbrush, or bathroom glass.
• Do not have sex.
• Do not sit close to others, especially pregnant women, and do not hold children in your lap for long periods of time.
• Sleep alone.
• Wash the tub and sink after each use (including after brushing teeth).
• Wash your hands after using or cleaning the toilet.
• Use a separate towel and washcloth.
• Wash your clothes, bed linens, and eating utensils separately.
• Sodium iodide I 131 is passed in the urine. To prevent contamination of your home, flush the toilet twice after you urinate.

To increase the flow of urine and lessen the amount of radioactive iodine in your body, drink plenty of liquids and urinate often.

If you were treated with sodium iodide I 131 for an overactive thyroid, your doctor may want to check the level of thyroid hormone in your blood every 2 to 3 months during the first year, and once a year thereafter. This is to make sure that your thyroid has not become underactive.

Studies have not shown that sodium iodide I 131 increases the chance of cancer or other long-term problems. When used to treat an overactive thyroid gland, sodium iodide I 131 may cause the patient to have an underactive thyroid gland after treatment. The thyroid gland may become underactive even several years after treatment for hyperthyroidism. Before receiving this medicine, be sure you have discussed its use with your doctor.

Along with its needed effects, a medicine may cause some unwanted effects. Although not all of these side effects may occur, if they do occur they may need medical attention.

When this medicine is used in very small doses to help study the function of the gland, side effects are rare. However, check with your doctor as soon as possible if any of the following side effects occur after treatment for overactive thyroid or cancer of the thyroid:

• After treatment of overactive thyroid
  • Symptoms of an underactive thyroid
    • Changes in menstrual periods;  clumsiness;  coldness;  drowsiness;  dry, puffy skin;  headache;  listlessness;  muscle aches;  thinning of hair (temporary)—may occur 2 to 3 months after treatment;  unusual tiredness or weakness;  weight gain 
    
    

• Rare
  • After treatment of overactive thyroid
    • Excessive sweating;  fast heartbeat;  fever;  palpitations;  unusual irritability;  unusual tiredness 
    
    
  • After treatment of cancer of the thyroid
    • Black, tarry stools;  blood in urine or stools;  cough or hoarseness;  fever or chills;  lower back or side pain;  painful or difficult urination;  pinpoint red spots on skin;  unusual bleeding or bruising 
    
    

Other side effects may occur that usually do not need medical attention. These side effects may go away during treatment as your body adjusts to the medicine. However, check with your doctor if any of the following side effects continue or are bothersome:

• Less common
  • After treatment of overactive thyroid or cancer of the thyroid
    • Neck tenderness or swelling;  sore throat 
    
    
  • After treatment of cancer of the thyroid
    • Loss of taste (temporary);  nausea and vomiting (temporary);  tenderness of salivary glands 
    
    

Other side effects not listed above may also occur in some patients. If you notice any other effects, check with your doctor.