Radioactive Iodine Treatment for Thyroid Disease

How radioactive iodine treatment works

The thyroid gland takes up iodine from the bloodstream to make thyroid hormones. Radioactive iodine, usually iodine-131, behaves in the same way. Once inside thyroid cells, the radiation it emits damages those cells over time. This reduces hormone production in an overactive gland or destroys remaining thyroid tissue that may harbour cancer cells after surgery.

Because other tissues absorb very little iodine, the rest of the body receives only limited radiation exposure. According to Cleveland Clinic, this selective uptake is what makes the treatment both effective and relatively well tolerated for most people. The process is gradual; hormone levels and gland size change over weeks to months rather than overnight.

The same principle underlies diagnostic radioactive iodine scans, but the amount used for treatment is higher. Treatment is always given by a specialist nuclear medicine or endocrinology team in a controlled setting.

When this treatment is considered

Radioactive iodine is used in two main situations: certain forms of hyperthyroidism and selected cases of differentiated thyroid cancer.

For hyperthyroidism, it is often offered when antithyroid tablets have not controlled the condition adequately, when side effects of tablets occur, or when a person prefers a definitive treatment rather than long-term medication. It is commonly used for Graves’ disease and for overactive (“toxic”) nodules or multinodular goitre. NICE guidance in the United Kingdom positions radioactive iodine as a first-line definitive option for many people with Graves’ disease, while noting important exceptions such as active thyroid eye disease or plans for pregnancy in the near future.

After surgery for papillary or follicular thyroid cancer, radioactive iodine may be given to destroy any remaining normal thyroid tissue and any microscopic cancer cells that take up iodine. It is not used for medullary or anaplastic thyroid cancers because those cells do not absorb iodine. Decisions rest on the cancer’s stage, risk features and the person’s overall health.

Understanding the role of iodine and thyroid health helps explain why this treatment is possible and why dietary iodine intake is carefully managed beforehand.

Preparing for treatment

Preparation aims to maximise uptake of the radioactive iodine by the thyroid and to ensure safety. A low-iodine diet is usually advised for one to two weeks beforehand so that the thyroid is “hungry” for iodine. This involves avoiding iodised salt, seafood, dairy products high in iodine, and certain supplements. Detailed lists are provided by the treating centre.

Antithyroid medicines may be stopped for a period before treatment so that they do not block iodine uptake. In some cancer cases, recombinant TSH injections are used instead of stopping thyroid hormone replacement. Blood tests confirm that hormone levels and other markers are suitable. A pregnancy test is mandatory for anyone who could be pregnant, because radioactive iodine can harm a developing foetus.

People are also given clear written instructions about radiation precautions that will apply after the dose is taken. These practical steps form part of the wider picture of what happens after radioactive iodine.

What happens on the day

The radioactive iodine is usually swallowed as a small capsule or, less often, as a liquid. The process itself is quick. Some centres give an anti-sickness tablet beforehand. After swallowing the dose, people are asked not to eat or drink for a short time so that absorption is complete.

Depending on the amount given and local regulations, some people go home the same day while others stay overnight or longer in a specialised room. The nuclear medicine team measures radiation levels and advises when it is safe to leave. Most of the radioactivity is excreted in the urine over the following days.

According to MedlinePlus, the treatment is designed so that thyroid cells receive the highest radiation dose while other organs receive much less.

Aftercare, isolation and daily life

After treatment, residual radiation means that people need to limit close contact with others for a period that varies according to the dose and local guidelines. Typical advice includes sleeping alone, keeping a distance from pregnant women and young children, avoiding prolonged public transport, and practising careful hygiene when using the toilet. Drinking extra fluids helps flush the radioactivity from the body.

These precautions usually last a few days to a couple of weeks. The treating team supplies precise, written instructions tailored to the individual. Returning to work or school depends on the type of job and the residual radiation level. Full details about recovery and practical adjustments appear in the guide to life after radioactive iodine treatment.

Most people feel well enough to resume normal activities once the isolation period ends, although fatigue can linger for a short time.

Possible short-term and longer-term effects

Many people experience few or mild short-term effects. Some notice neck tenderness, a dry mouth, altered taste, or mild nausea in the first few days. Salivary glands can become temporarily swollen because they also take up a small amount of iodine. These symptoms usually settle with simple measures such as frequent sips of water or sugar-free sweets.

A temporary rise in thyroid hormone levels can occur in the first week or two as stored hormone is released from damaged cells. Doctors sometimes continue beta-blockers during this window to ease symptoms such as a rapid heartbeat.

Longer term, the most common outcome after treatment for hyperthyroidism is underactivity of the thyroid (hypothyroidism). This develops gradually over months and is managed with daily thyroid hormone replacement tablets. Regular blood tests guide the correct dose. Dry mouth or changes in taste persist in a minority of people.

Mayo Clinic notes that side effects tend to be related to the amount of radioactive iodine given and that higher amounts used after cancer surgery may produce more noticeable salivary or taste changes.

• Temporary neck discomfort or swelling
• Dry mouth or reduced saliva
• Altered sense of taste or smell
• Mild nausea or tiredness
• Later development of hypothyroidism requiring replacement therapy

Serious complications are uncommon when the treatment is given under specialist supervision.

Follow-up care and hormone replacement

Follow-up begins within weeks of treatment. Blood tests check free T4, T3 and TSH to track the response. For hyperthyroidism, the goal is usually to reach a stable, treated state; many people ultimately need lifelong levothyroxine. For cancer patients, thyroglobulin levels and whole-body scans may be used later to monitor for any remaining tissue.

Once hormone replacement is started, doses are adjusted according to symptoms and blood results. People are encouraged to take tablets consistently, usually on an empty stomach, and to tell their doctor about other medicines or supplements that might affect absorption. Information about iodine and thyroid health remains relevant because excessive iodine intake can still influence residual thyroid tissue or interfere with monitoring.

Ongoing care is shared between the endocrinology or nuclear medicine team and the person’s general practitioner.

Pregnancy, fertility and other special situations

Radioactive iodine must never be given during pregnancy or breastfeeding. Pregnancy is deferred for a period after treatment (commonly six months or longer, according to local advice) so that residual radiation has cleared and thyroid hormone levels are stable. Breastfeeding is stopped permanently after treatment because iodine concentrates in breast milk.

Men may be advised to avoid conception for several months because temporary reductions in sperm count can occur. Fertility usually recovers. Thyroid eye disease associated with Graves’ disease can occasionally worsen after radioactive iodine; specialists may recommend steroids or alternative treatments when eye disease is active.

People who care for young children or work closely with them receive extra guidance on isolation timing. The treatment team discusses all of these points before consent is given.

Talking with your healthcare team

Radioactive iodine is one option among several. A careful discussion weighs the benefits against personal circumstances such as age, family plans, the presence of eye disease, previous neck surgery, and individual preference. Questions about the expected time away from work, the likelihood of needing hormone replacement, and the exact isolation rules are best answered by the treating centre, because practice varies slightly between hospitals.

Anyone experiencing new or worsening symptoms after treatment—such as difficulty swallowing, marked neck swelling, or signs of under- or over-activity of the thyroid—should contact their specialist team promptly. Regular review ensures that hormone levels stay in the target range and that any late effects are recognised early.

Further practical information on recovery appears in the dedicated page covering after radioactive iodine treatment.