Radioiodine (I-131) therapy exploits the thyroid gland's unique ability to absorb iodine — delivering targeted radiation directly and selectively to thyroid cancer cells while sparing the rest of the body. It is the most site-specific systemic cancer treatment available, and is planned in close coordination with thyroid surgery.
Radioiodine therapy uses radioactive iodine (Iodine-131 or I-131) to destroy thyroid tissue. The thyroid gland is the only organ in the body that actively absorbs and concentrates iodine — a property it uses to produce thyroid hormones. Differentiated thyroid cancer cells retain this ability, making them uniquely targetable.
When a patient swallows I-131 (as a capsule or liquid), it is absorbed into the bloodstream and selectively taken up by thyroid tissue and differentiated thyroid cancer cells — wherever they are in the body. The radioactive iodine then emits beta radiation from within these cells, destroying them while causing minimal damage to surrounding normal tissue.
This makes RAI one of the most precise systemic cancer treatments available — it can treat thyroid cancer cells even in distant sites such as the lungs or bones, as long as those cells retain iodine-uptake ability. The treatment is given as a single oral dose and is generally well tolerated.
The success of radioiodine therapy depends directly on the quality of thyroid surgery that precedes it. A properly performed total thyroidectomy — removing all normal thyroid tissue — maximises the uptake of I-131 into residual cancer cells, making the treatment far more effective.
I plan each thyroidectomy with the RAI strategy already in mind. The extent of surgery, the decision on central neck dissection, and the post-operative TSH target are all coordinated before the patient even meets the nuclear medicine physician.
— Dr. Vinod T. Gore, MBBS MS FAIS FIAGES FARIS (Edin)The therapy exploits one of the most selective biological properties in medicine — the thyroid's unique ability to absorb iodine from the bloodstream.
The patient swallows I-131 as a capsule. It is absorbed from the gut and enters the bloodstream — circulating throughout the body.
Only thyroid cells and differentiated thyroid cancer cells actively absorb the iodine — concentrating the radioactivity precisely where it is needed.
I-131 emits beta radiation within the thyroid cells — damaging their DNA irreparably and causing them to die. The range of beta radiation is just 1–2mm, sparing adjacent tissue.
A whole-body scan 5–7 days after treatment maps all areas of iodine uptake — confirming ablation and detecting any distant metastases not seen on prior imaging.
Not all thyroid cancers respond to radioiodine. Response depends on whether cancer cells retain the sodium-iodide symporter (NIS) — the mechanism that allows iodine uptake.
The most common thyroid cancer (80–85% of cases). Differentiated cells retain iodine-uptake ability. Highly responsive to RAI — excellent prognosis with surgery + RAI in intermediate and high-risk disease.
Second most common differentiated thyroid cancer. Retains iodine-uptake ability — responds well to RAI, particularly useful for treating haematogenous metastases to lungs and bones.
A variant of follicular cancer. Hürthle cells have reduced sodium-iodide symporter expression — many are poorly iodine-avid or completely RAI-resistant. Response is unpredictable.
Medullary thyroid cancer (MTC) arises from C-cells (parafollicular cells) — which do not produce thyroid hormone and do not absorb iodine. RAI has no role. Anaplastic thyroid cancer is undifferentiated and also non-iodine-avid.
RAI is used in four distinct clinical situations — each with a specific goal. The indication always follows thyroid surgery and is guided by ATA risk stratification.
Destroys residual normal thyroid tissue remaining after total thyroidectomy. This improves the sensitivity of follow-up thyroglobulin monitoring and whole-body scanning for recurrence detection.
Standard after total thyroidectomy for intermediate and high-risk differentiated thyroid cancer.
Remnant AblationTargets and destroys iodine-avid distant metastases — including lymph node, lung, and bone metastases — that retain iodine-uptake ability. Can achieve remission even in stage IV disease.
Pulmonary micrometastases, bone metastases from follicular thyroid cancer.
Therapeutic IntentI-131 used to reduce overactive thyroid tissue in Graves' disease, toxic multinodular goitre, or toxic adenoma — destroying enough thyroid tissue to render the patient euthyroid or hypothyroid.
Alternative to long-term antithyroid drugs or surgery for Graves' disease.
Non-cancer UseA low-dose I-131 scan maps all iodine-avid tissue in the body — used before therapeutic dosing to assess residual thyroid tissue and detect metastases, and after treatment to confirm ablation.
Pre-therapy diagnostic scan; post-therapy confirmation scan at day 5–7.
Diagnostic / StagingRAI therapy involves careful preparation over several weeks. Proper preparation — especially TSH stimulation and low-iodine diet — is critical to maximising treatment effectiveness.
TSH must be elevated (TSH >30 mIU/L) to stimulate iodine uptake by cancer cells. Achieved by either stopping thyroid hormone tablets for 3–4 weeks (hypothyroid withdrawal) or rhTSH injections (Thyrogen) for 2 days — more comfortable and preferred where available.
Low-iodine diet for 2 weeks before treatment — no iodised salt, seafood, dairy, or iodine supplementsRadioiodine is given as a single oral capsule or liquid — swallowed in a few seconds. The dose is calculated based on risk stratification: low activity (1.1 GBq) for ablation in low-risk disease; higher activities (3.7–7.4 GBq) for high-risk or metastatic disease. Painless and quick.
Hospital admission for 2–5 days required at doses above regulatory thresholds for radiation safetyA whole-body post-treatment scan is performed at day 5–7 — mapping all areas of iodine uptake to confirm ablation and detect any previously unknown metastases. Thyroid hormone replacement is restarted. TSH-suppressed thyroxine therapy then continues long-term to reduce recurrence risk.
Thyroglobulin (Tg) monitoring every 6–12 months is the primary surveillance tool post-RAIThe American Thyroid Association (ATA) risk stratification system guides RAI decisions after surgery. Not all thyroid cancer patients need RAI — the decision depends on tumour size, histology, extrathyroidal extension, and nodal/distant metastases.
Intrathyroidal, well-differentiated papillary thyroid cancer. No vascular invasion. No lymph node involvement. Complete surgical resection. No distant metastases.
RAI usually NOT recommendedMinor extrathyroidal extension, aggressive histology (tall cell, columnar), vascular invasion, lymph node metastases (N1b or multiple N1a), or tumour >4 cm.
RAI generally recommendedGross extrathyroidal extension, incomplete surgical resection, distant metastases present, post-operative serum thyroglobulin suggestive of distant metastatic disease.
RAI strongly recommendedAfter receiving RAI, patients emit low-level radiation and must follow precautions to protect family members and the public — particularly pregnant women and young children.
Duration of precautions depends on the dose given. For standard ablation doses (1.1 GBq), precautions typically last 3–5 days. For higher therapeutic doses, hospital isolation may be required for 2–5 days before precautions at home continue for 1–2 weeks. Your nuclear medicine team will give you a personalised radiation safety card.
Sleep alone for the first 3–5 days after treatment. Avoid sharing a bed with your partner or sleeping in the same room as young children.
Avoid close prolonged contact with children under 5 and pregnant women for 5–10 days. Brief contact (a few minutes) is generally acceptable after the first 1–2 days at standard doses.
Drink plenty of fluids to speed iodine excretion in urine. Flush the toilet twice after use. Wash hands frequently. Use separate utensils for the first 2–3 days.
Avoid prolonged close contact in crowded public spaces for 3–5 days. You can drive your own vehicle. Airport security scanners can detect I-131 for up to 3 months — carry your radiation safety card.
Most patients can return to work within 3–5 days for standard ablation doses, provided the job does not involve close sustained contact with pregnant colleagues or young children.
RAI is absolutely contraindicated in pregnancy. Breastfeeding must be stopped 6–8 weeks before treatment and cannot be resumed for that child. Pregnancy should be deferred for 6–12 months post-RAI.
RAI is generally well tolerated. Most side effects are mild and temporary. Late effects are dose-dependent and more relevant with high cumulative doses for metastatic disease.
Mild nausea in the first 24 hours after the dose — more common with higher activities. Antiemetics given prophylactically. Usually resolves within a day.
Acute — Day 1–2Painful swelling and tenderness of the parotid and submandibular glands — from I-131 uptake by salivary gland tissue. Lemon sweets, massage, and hydration reduce risk. Occurs in 10–30%.
Acute — Days 3–7Long-term reduction in saliva production from cumulative salivary gland damage. More common with high or repeated doses. Artificial saliva, sialogogues, and good oral hygiene help.
Late — Cumulative DosesDry eyes from lacrimal gland uptake of iodine — less common. Lubricating eye drops help. More likely with higher doses or repeated treatments.
Late — Higher DosesTemporary swelling and discomfort in the neck from inflammation of residual thyroid tissue or lymph nodes taking up I-131. Usually mild, lasting a few days. NSAIDs or steroids if significant.
Acute — Days 3–10Temporary mild reduction in blood counts — more relevant with very high cumulative doses used for metastatic disease. Monitored with blood tests. Rarely clinically significant at standard ablation doses.
High Cumulative DosesHypothyroidism after RAI is expected and treated. After thyroid ablation, patients are placed on lifelong thyroxine (T4) replacement — with TSH kept suppressed in high-risk disease to further reduce recurrence risk. This is a known, planned outcome — not a complication.
The effectiveness of radioiodine therapy is directly determined by the completeness and quality of the preceding thyroid surgery. Dr. Gore plans each thyroidectomy with the post-operative RAI strategy already integrated.
If you have been diagnosed with thyroid cancer or advised radioiodine therapy, consult Dr. Gore to understand how surgery and RAI work together — and what the right approach is for your specific cancer.