Radioiodine therapy is a targeted Nuclear Medicine treatment that uses radioactive iodine, usually iodine-131 (I-131), to destroy thyroid cells. The thyroid naturally absorbs iodine to produce thyroid hormones. When I-131 is swallowed as a capsule or liquid, it enters the bloodstream and is taken up mainly by thyroid tissue, where it releases radiation over a short distance.
Radioiodine may be used to treat an overactive thyroid gland or selected differentiated thyroid cancers that retain the ability to absorb iodine. In thyroid cancer care, it is generally given after surgery to destroy remaining thyroid tissue, treat suspected microscopic disease or target iodine-avid cancer that has spread elsewhere in the body.
Radioiodine is not required for every patient with thyroid cancer, and it does not effectively treat cancers that do not absorb iodine. The treatment dose, preparation, admission requirements and follow-up plan depend on whether it is being used for a benign thyroid condition or thyroid cancer.
- Graves’ disease causing hyperthyroidism
- A toxic thyroid nodule or multinodular goitre
- Hyperthyroidism that has returned after antithyroid medication
- Papillary thyroid cancer or follicular thyroid cancer that absorbs iodine
- Selected other differentiated thyroid cancers with iodine-avid tissue
- Remaining thyroid tissue after thyroidectomy when radioactive iodine ablation is clinically indicated
- A risk of microscopic residual thyroid cancer after surgery
- Iodine-avid thyroid cancer remaining after surgery, that has returned, or that has spread to lymph nodes, lungs, bones or other sites
How Radioiodine Therapy Is Performed
Before Treatment
The Nuclear Medicine physician reviews the diagnosis, pathology findings, surgery details, imaging, previous treatment and medical history.
Thyroid-function tests and other relevant blood tests are performed.
For thyroid cancer, thyroglobulin and anti-thyroglobulin antibody levels may be measured.
Kidney function, blood counts or other organ-function tests may be assessed, particularly when a higher treatment activity is planned.
Recent use of iodine-containing contrast, supplements, medicines or antiseptics is reviewed because excess iodine may reduce treatment uptake.
A low-iodine diet may be recommended for a specified period, particularly before treatment for thyroid cancer.
For thyroid cancer treatment, the thyroid-stimulating hormone (TSH) level usually needs to be raised to encourage iodine uptake.
TSH may be raised by temporarily stopping thyroid hormone medication or by administering recombinant human TSH injections.
If thyroid hormone is stopped, temporary symptoms of hypothyroidism may occur, including tiredness, feeling cold, constipation, reduced concentration or low mood.
Antithyroid medicines may need to be stopped and restarted at specified times when treating hyperthyroidism.
The patient is instructed when to stop eating and drinking before treatment and when normal intake can resume.
A diagnostic uptake study or scan may be performed in selected patients.
The Nuclear Medicine team calculates or selects the treatment activity according to the condition and treatment goal.
Written radiation-safety instructions are discussed before treatment so that arrangements can be made at home.
During Treatment
The patient attends a designated Nuclear Medicine treatment area.
I-131 is administered by mouth, usually as one or more capsules and occasionally as a liquid.
The capsule is swallowed whole with water according to the team’s instructions.
The administration itself usually takes only a few minutes.
The patient may need to remain in the treatment area temporarily while radiation levels are assessed.
Treatment for hyperthyroidism is commonly provided as an outpatient procedure.
Higher-activity treatment for thyroid cancer may require admission to a specially designated room, depending on the administered activity and applicable regulations.
Contact with staff and visitors may be limited while radiation levels are higher.
A post-treatment whole-body scan may be performed several days later in patients treated for thyroid cancer to show where the radioiodine has accumulated.
After Treatment
The patient receives personalised written instructions covering hydration, diet, medicines, activity, travel, follow-up and radiation safety.
Drinking fluids and emptying the bladder regularly may be advised to help remove excess radioiodine.
Careful toilet hygiene and handwashing are required because a portion of the radioiodine leaves the body through urine and other bodily fluids.
Temporary precautions may include maintaining distance from others, limiting close or prolonged contact and sleeping separately.
Close contact with children and pregnant people must be avoided for the period specified by the Nuclear Medicine team.
Food, utensils, towels and personal items may need to be handled according to the team’s instructions.
Return to work, public transport and long-distance travel may need to be delayed temporarily.
A treatment certificate or letter may be provided because sensitive radiation detectors at airports or other locations may detect residual radioactivity.
Thyroid hormone replacement is started or resumed according to the treatment plan.
Antithyroid medicines may be restarted at a specified time when treating hyperthyroidism.
Blood tests are arranged to monitor thyroid function and assess the response.
Patients treated for hyperthyroidism may take several weeks or months to experience the full treatment effect.
Follow-up after thyroid cancer treatment may include thyroglobulin testing, neck ultrasound, radioiodine imaging or other scans.
Additional radioiodine or another treatment may be considered if iodine-avid disease remains or returns.
Potential Benefits of Radioiodine Therapy
- Targeted treatment based on the thyroid’s natural ability to absorb iodine
- Treatment delivered orally without a surgical incision
- Destruction of overactive thyroid tissue in selected patients with hyperthyroidism
- Treatment of thyroid tissue remaining after thyroidectomy when clinically indicated
- Treatment of iodine-avid microscopic disease that may not be visible or removable surgically
- Treatment of iodine-avid thyroid cancer in more than one part of the body
- Limited radiation exposure to many tissues that do not concentrate iodine
- A short administration procedure
- Outpatient treatment for many patients
- The ability to repeat treatment when clinically appropriate
- Integration with thyroid surgery, thyroid hormone therapy and other cancer treatments
- Post-treatment imaging that may reveal the distribution of iodine-avid tissue
Side Effects of Radioiodine Therapy
- Tiredness, nausea or reduced appetite
- Mild neck pain, tenderness or swelling
- Temporary worsening of thyroid-related symptoms
- Inflammation of the remaining thyroid tissue
- Hypothyroidism following treatment for hyperthyroidism
- Dry mouth
- Swelling or tenderness of the salivary glands
- Changes in taste or smell
- Sore throat
- Dry or watery eyes
- Temporary changes in menstrual cycles, sperm count, overall fertility
- Reduced blood counts after higher-activity or repeated treatment
- Inflammation or scarring of the lungs in rare patients with extensive iodine-avid lung metastases
- Rare severe worsening of hyperthyroidism or thyroid storm
- Rare longer-term salivary-gland or tear-duct problems
- A small increased risk of certain second cancers after higher cumulative exposure
- Persistent or recurrent disease if the targeted tissue does not absorb sufficient iodine
