Patient Guide · I-131 Therapy
Your doctor has offered youradioactive-iodine treatment.Here's what it involves.
Radioactive iodine (also called I-131) is a special form of iodine, swallowed as a capsule. Because your thyroid gland absorbs iodine naturally — the way it absorbs iodine from food — the treatment travels right to your thyroid and works there, with the rest of your body absorbing very little. It\'s used for two things: an overactive thyroid (your doctor may call this Graves\' disease or a toxic thyroid nodule), and thyroid cancer after surgery to remove the thyroid. This page walks you through what to do before, what the treatment day feels like, and the simple steps that protect your family for the first week.
Important: this page is educational. It does not replace the specific advice of your nuclear-medicine physician, endocrinologist and treating team, who know your history, imaging and dose plan. Always follow their instructions in preference to anything on this page.
What is I-131 therapy?
Iodine that finds your thyroid, wherever it is
A radioactive form of iodine, given as a capsule
Iodine-131 is a radioactive form of iodine, the same mineral your body uses to make thyroid hormone. When you swallow the capsule, your thyroid absorbs it naturally — the way it absorbs iodine from your food. The radioactive iodine then delivers a treatment dose right where it's needed: your thyroid, or (after thyroid surgery) any remaining thyroid or cancer cells. The rest of your body absorbs very little.
The radiation comes from inside your body
You may have heard of "radiotherapy" where a machine outside the body aims radiation at a tumour. This treatment is different — the radiation comes from inside you, wherever the iodine has settled. For a few days after treatment, you carry a small amount of radiation in your body. The after-care steps below are about protecting your family during those few days.
Different from chemotherapy
Chemotherapy usually causes hair loss and severe nausea. This treatment does not cause hair loss and rarely causes severe nausea. Most people feel largely normal within days.
Who receives I-131
Two main indications in Indian centres
Overactive thyroid
Adults whose thyroid produces too much hormone. The most common reasons are a condition called Graves' disease, or an overactive nodule (lump) in the thyroid. The treatment quietens the overactive thyroid so hormone levels come back to normal, usually within 6 to 12 weeks.
Activity + setting
A small dose, given as a single capsule. Usually you go home the same day.
Typical course
Most people need only one treatment. A small number may need a second dose after 6 to 12 months if levels stay high.
Thyroid cancer, after surgery
Adults who have had the thyroid removed by an operation for thyroid cancer. This treatment finishes the job — it destroys any remaining thyroid tissue and any tiny cancer cells that may have spread. It is used for the two most common types of thyroid cancer (called papillary and follicular).
Activity + setting
A larger dose than for overactive thyroid. Some patients go home the same day; others stay one or two nights in a special hospital room designed for this treatment.
Typical course
Usually a single treatment after surgery. More treatments are given only if follow-up scans show it's needed.
How to prepare
The weeks before your treatment
A special "low-iodine" diet for 1 to 2 weeks before
Your team will give you a food list. The idea is to reduce iodine in your diet so your thyroid absorbs the treatment iodine more eagerly. Avoid iodised salt, seafood, seaweed, dairy products, egg yolks, and any multivitamin with iodine. Meat, rice, vegetables, fruit and non-iodised salt are all fine.
Tell your team about all medicines you take
Some medicines, cough syrups, antiseptics (like Betadine) and CT scan dyes contain iodine and can block the treatment for weeks. Also mention any heart medicine — one particular heart medicine affects treatment for many months. Bring your full medicine list to your consultation. Do not stop any medicine on your own; the team will guide you.
Pausing thyroid hormone tablets (for thyroid cancer patients only)
If you're being treated for thyroid cancer, your team may ask you to stop your thyroid hormone tablets for about 3 to 6 weeks before treatment. This makes the treatment work better. During those weeks you may feel tired, cold and slow-thinking — this is temporary and reverses once you restart the tablets. Some centres offer an injection alternative that avoids these symptoms; ask if it's available for you.
Pregnancy + breastfeeding
This treatment must not be given during pregnancy or breastfeeding. Women who could become pregnant will have a pregnancy test within 48 hours of treatment. If you are breastfeeding, you must stop before treatment; you cannot restart for that baby.
Drink water, avoid food just before + after
The capsule is small — swallowed with water. Don't eat for 2 hours before or 1 hour after. In the days after, drink plenty of water. This helps the unused iodine clear from your body through urine.
What to bring
If you're going home the same day, bring someone to drive you. If you're staying overnight in the hospital's special therapy room, bring loose comfortable clothing, phone + charger, snacks and something to keep you occupied — you'll be in the room by yourself for a night or two.
The treatment day
Usually a short visit; sometimes a short admission
- 1
Check-in + final review
You arrive on an empty stomach (nothing to eat for 2 hours). The team checks your blood tests, confirms you've followed the diet, and confirms the dose. If anything is off, the treatment may be rescheduled — don't worry, it's common.
- 2
Swallow the capsule
The capsule sits inside a small protective container. You swallow it in one go with water — no chewing. The capsule has no taste. You may be asked to sit quietly for 20 to 30 minutes afterwards.
- 3
Radiation check
Before you leave (or before you move to the overnight room), a safety officer measures the amount of radiation coming from you. This measurement tells the team when it's safe for you to go home, or when family visits are OK.
- 4
Go home or stay overnight
If your dose is small, you go home the same day with a written care sheet and a therapy card. If your dose is larger (usually for cancer treatment), you stay 1 to 3 nights in a special hospital room that's built to contain the radiation. Once the reading drops enough, you go home.
- 5
Follow-up scan (thyroid cancer patients only)
After thyroid cancer treatment, a whole-body scan is taken 3 to 10 days later. It shows where the iodine settled — helpful for planning follow-up. It's just a picture — no extra treatment is given.
Radiation-safety after your treatment
Protecting your family for the first week
For a few days after treatment you carry small amounts of I-131 in your body — mostly in what remains of your thyroid, and briefly in urine + sweat + saliva. These simple steps for the first 3 to 7 days after treatment keep the dose to your family well below any medical limit. Your team will give you the specific number of days that apply to your dose; the guidance below is the general template.
Distance from family (first 3 to 7 days)
Stay about 1 metre away from family — usually 3 days for overactive-thyroid patients, up to 7 days after thyroid cancer treatment. Sleep in a separate bed. Avoid long close contact (long hugs, holding a child on your lap). The more distance and the less time close to them, the smaller their exposure.
Pregnant women + young children
Stay farther away (about 2 metres, for 7 days) from pregnant women and children under 2 years. No pregnant visitors to your bedroom during this time. If you look after a small child and there's no one else, tell the team before treatment — they can help plan.
Bathroom
Most of the iodine leaves your body through urine in the first 2 to 3 days. Sit down to use the toilet to avoid splashes. Flush twice. Wash hands well. Use a bathroom of your own if you can. If any urine gets on your skin, wash it off straight away with soap and water. Wash any stained clothing on its own once.
Sweat, saliva + shared items
Small amounts of iodine come out in sweat and saliva for the first few days. Don't share drinks, cutlery, plates or towels. Wash your own dishes separately for 3 to 4 days. Best to avoid kissing during this period.
Sleeping + intimacy
Sleep alone for the number of days your team tells you. Avoid intimate contact (kissing, sexual activity) for about 7 days. Both men and women should use reliable contraception during the treatment and for 6 to 12 months afterwards — the treatment can affect fertility for a while.
Airports + work
You may set off very sensitive radiation alarms (at airports, courts, embassies) for weeks to months afterwards, even when the amount in your body is tiny and no risk. Your team will give you a card that explains this — carry it with you. You can go back to work when your team says, usually 3 to 7 days.
Call the team if…
You get lasting neck pain or swelling, a high fever, trouble swallowing, severe nausea that won't settle, unexplained bruising or bleeding, or anything else that worries you. Your team will give you a 24-hour phone number — use it. Most side effects are mild and settle in a few days.
Worth knowing · Supply quality
Not all I-131 supply is the same
The I-131 you receive travels through a regulatory + logistics chain — from producer to importer to your hospital dose calibrator. Two quality signals are worth asking about, because they affect the accuracy and traceability of the dose you actually receive.
Where the iodine comes from + how it's measured
Every batch that reaches your hospital comes with paperwork from India's atomic energy regulator (AERB) and a certificate showing the exact strength on a specific date and time. Your hospital re-measures the strength before giving it to you. If you're curious, you can ask your team to show you this measurement — takes 30 seconds and confirms what you're getting.
Capsule vs liquid form
Modern practice is a capsule — safer for you and for the staff (no risk of spills, exact dose in each capsule). Some centres still use a liquid form; it works the same way but is trickier to handle. If your centre uses capsules, that's the newer, safer approach.
The technical specification and regulatory framework for the I-131 supplied through Saxsons Group is on theclinical evidence libraryfor your treating team to review — EANM 2022 guideline, ATA 2015 management guideline, IAEA SRS 63 discharge criteria.
Questions to ask your care team
Eight questions worth writing down
Take this list to your pre-treatment consultation. You will be given a lot of information at once — a written list of the questions that matter to you helps make sure nothing slips through.
- ❓What is the exact dose you're planning for me?
- ❓For thyroid cancer: what type and stage do I have, and why did you pick this particular dose?
- ❓Will I go home the same day, or do I need to stay overnight? If overnight, how many nights?
- ❓How long should I stay away from my spouse, children, parents — and by what distance?
- ❓Which of my current medicines need to be stopped, and when should I restart them?
- ❓Will I have a scan after treatment? When?
- ❓When can I go back to work? Are there any parts of my job (working with children, elderly, air travel) I need to avoid for a while?
- ❓What symptoms should make me call the team? What number do I use after clinic hours?