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Drug facts

The drug radioiodine is the radioactive form of the element iodine. There are a number of radioactive isotopes of iodine such as Iodine-131, Iodine-123, Iodine-124 and Iodine-125, however Iodine-131 is most widely used in hospitals. The active ingredient of radioiodine is sodium iodide. It has been used in the medical field since the 1950’s and it is safe for human use when the necessary preparation and precautions are carried out correctly. It is used to treat thyroid diseases such as overactive thyroid (Graves disease, goitre) and thyroid cancers, in addition to surgery and thyroid medications (such as thyroxine and carbimazole).

Radioiodine treatment is only given by a Nuclear Medicine Department in the hospital. Iodine-131 has a half-life of 8 days, which means its radioactivity decays into half in 8 days. It emits both beta and gamma radiation. It is available in 2 forms: solution or capsules. In most cases, low doses (5-30mCi) are given as outpatient treatment. In some thyroid cancer cases higher doses (50-150mCi) are given and hospitalization in an isolation ward is required (up to 1 week).

Radioiodine therapy is individualised and each patient will receive different doses and different total number or doses. Additionally, some thyroid cancer patients might be required to go for a scan using the gamma camera of the Nuclear Medicine Department. This scan is for diagnostic purposes and enables the doctor to visualise the thyroid gland and detect if the cancer has spread to other parts of the body. Other than that, the patient might be required to go for a thyroid uptake study before the treatment, which helps the doctor to decide on an accurate dose for the treatment. In this study, a small amount of radioiodine solution is given to the patient and a special machine is used to calculate how much of radioiodine is taken up by the thyroid.

How does the drug work

Just like the element iodine, radioiodine accumulates in the thyroid tissue. Small amounts are taken up by the salivary glands, gastric tissues and it is also excreted in breast milk. It is not taken up by other tissues, unless a cancerous thyroid tissue has spread to other parts of the body. The beta radiation emitted by radioiodine then kills the hyperactive thyroid tissue or cancer cells. As radioiodine only targets thyroid tissue, it is a form of targeted therapy. The gamma radiation emitted by radioiodine allows the thyroid gland and thyroid cancer cells to be seen by the gamma camera.

Before you use this medicine

The doctor will provide a list of preparation steps and a counselling session for you before starting radioiodine treatment. The things you must do include:

  1. You must inform the doctor:
    • all the medications you are currently taking, including herbal and traditional medicine
    • if you have any drug allergies, and
    • if you have difficulty swallowing
  2. Pregnant women must not take radioiodine and all female patients of childbearing age might be required to do a urine pregnancy test before treatment.
  3. You will be asked to stop your thyroid medications and herbal medicines up to 1 month before treatment.
  4. You will be asked to stop eating seafood 1 week before treatment.
  5. You must fast 2 hours or wait 2 hours after food before taking radioiodine.
  6. The doctor will inform you if you have to be hospitalized and if other tests are required (such as blood test, urine test, gamma camera scan and thyroid uptake study).
  7. Your next appointment will be given before you take radioiodine. If you have not been given an appointment, you must inform the doctor or nurse.
  8. The doctor will also prescribe you thyroid medications (if needed) and tell you when to start taking the medicines. You must collect the medicines from pharmacy before taking radioiodine.
  9. If you have any questions, you have to ask the doctor or nurse before taking radioiodine as you will be asked to leave the department after taking radioiodine.

How to use this medicine

The doctor will decide the dose of radioiodine for each patient. Both radioiodine solution and capsules will be given in a special shielded pot in the Nuclear Medicine Department.

Radioiodine solution is a clear, colourless, odourless and tasteless liquid. The solution is given in water (around 5-10ml) in a small bottle. It is taken by drinking through a plastic straw, followed by some plain water.

As for the capsule, it is given through a plastic tube so that the patient does not touch the capsule, followed by plain water. In both cases, you should drink the solution or take the capsule carefully to avoid spillage.

After taking radioiodine, you will be required to leave the department or sent to the ward, so any questions should be asked before the treatment.

When to take

The doctor will decide how often each patient needs to take radioiodine. The follow-up appointment may be 3 or 6 months later or more. The doctor will decide each time if you need more radioiodine.

How long you take

Depending on the disease progression and treatment efficacy, the doctor will decide how many doses each patient needs. Some patients may only need a few doses but others may need more doses and treatment might continue for years.

When using this medicine

All patients that take radioiodine will emit radiation from their body and must follow strictly to the doctor’s instruction and counselling. You must follow the given instructions for 1 week after treatment. The doctor will give you sick leave from work or school for that period. These are some of the steps you must do to reduce radiation to the surrounding and family members:

  1. Keep a distance from other people for at least 1 meter (3 feet).
  2. Do not go near children, pregnant women and babies.
  3. Do not sleep in the same bed with other people.
  4. Do not go to crowded places such as parties, gatherings and shopping malls.
  5. After using the bathroom, flush the toilet at least 2-3 times.
  6. Use separate towels, bed sheets, cutlery and plates.
  7. Wash clothes separately from others.
  8. Do not do any blood or urine tests for 1 week after treatment.
  9. Do not do other diagnostic procedures (such as ultrasound scan, barium test) for 5 days after treatment.
  10. Avoid sexual intercourse for 1 month (or less, depending on the dose).
  11. For females, do not get pregnant 6 months after treatment.
  12. For breastfeeding women, stop breastfeeding as instructed by the doctor. Use formula feed instead.
  13. Drink a lot of water to flush radioiodine from the body.
  14. Eat a low sodium diet and avoid food high in iodine (such as seafood, seaweed and iodised salt).
  15. Take sweets or drinks containing citric acid to stimulate saliva excretion and prevent swelling of salivary glands.
  16. Follow the doctor’s instruction in how to take thyroid medications (to omit for 5 days to 1 week after treatment).

For hospitalised patients, their radiation levels will be monitored and they might be discharged earlier if the radiation level drops to a safe level. After discharge, the patient still needs to follow the given instructions for 1 week.

Possible side effects

Early side effects (within hours, days or weeks): Skin rash or itching or flushing, swelling of the face, difficulty in breathing, nausea, vomiting, diarrhoea, abdominal pain, swelling of the thyroid, swelling of the salivary glands (which may cause pain, some loss of taste and a dry mouth), overactive thyroid (signs such as increased appetite, palpitations, feeling restless/anxiety, weight loss or sweating).

Late side effects (within weeks, months or years): Underactive thyroid (signs such as feeling tired, loss of energy, feeling cold, a slow heart rate, muscle weakness, cramps, dry flaky skin, hair loss and weight gain), underactive parathyroid (signs such as ‘pins and needles’, weakness, muscle spasms, muscle twitches or cramps all over, tingling, vibrating, burning, numbness, trouble concentrating, feeling dizzy or irritable, sensitivity to noise, muscles paralysis or fits/seizures).

Other late side effects:

Bone marrow depression (bruising more easily and bleeding for longer), may be more at risk for stomach and blood cancers, a small increase in the risk of developing bladder and breast cancer.


  1. Iodine-131, Wikipedia.
  2. Theracap131 37MBq-5.55GBq capsules, hard (PL 00221/0102). Medicines and Healthcare products Regulatory Agency.
  3. Sodium Iodide (131I) 74MBq & 925MBq solution for injection (PL 00221/0113). Medicines and Healthcare products Regulatory Agency.
  4. Package leaflet for Theracap131TM 37MBq-5.55GBq capsules, hard. GE Healthcare.
  5. Technical Leaflet: Summary of Product Characteristics – oral sodium (131I) iodide solution.(December 2008) CIS bio international, France.
  6. Summary of Product Characteristics for Sodium Iodide (I131) Capsules T, hard capsules.(30 March 2011) Mallinckrodt Medical BV, The Netherlands.
Last Reviewed : 05 June 2013
Writer : Ng Yee Lian