# Radioactive Iodine Therapy: A Patient Guide URL: https://thethyroidologist.com/blog/radioactive-iodine-guide/ Published: 2025-11-09 Author: The Thyroidologist Editorial Team Medical reviewer: Thyroid Specialist > How radioactive iodine treatment works, who it is for, what the day involves, and the safety precautions afterwards. Radioactive iodine is a treatment that uses a small, carefully measured dose of radioactive iodine to reduce thyroid tissue. Because the thyroid naturally takes up iodine, the treatment is concentrated exactly where it is needed. It is used mainly for two situations: an overactive thyroid, and thyroid cancer after surgery. ## How it works The thyroid absorbs iodine to make hormone. Radioactive iodine is taken up in the same way, and its radiation gradually destroys thyroid cells. Because the dose is concentrated in the thyroid, other parts of the body receive very little. ## Who it is for ### Overactive thyroid Radioactive iodine is a common treatment for Graves' disease and toxic nodules. It is effective, avoids surgery, and is usually a single treatment. It is generally not used in active thyroid eye disease, because it can worsen eye problems, and it is not used in pregnancy or breastfeeding. ### Thyroid cancer After surgery for differentiated thyroid cancer, radioactive iodine is sometimes given to destroy any remaining thyroid tissue and treat residual disease. Not everyone needs it — the decision depends on the cancer's type and stage. ## Before treatment You may be asked to follow a low-iodine diet for a period beforehand, and to stop certain thyroid medications temporarily so that the thyroid takes up the iodine well. If you are having treatment for cancer, you may have a scan first. Pregnancy must be excluded before treatment. Tell your team about any recent iodine-containing contrast scans, as these affect timing. ## The day of treatment Radioactive iodine is usually given as a capsule or a drink. It is swallowed and is painless. You may be advised to stay in a designated area for a short time, and you will be given specific instructions about contact with others. ## After treatment ### For an overactive thyroid Symptoms usually improve over several weeks to months. Because the treatment reduces thyroid activity, many people become hypothyroid and need levothyroxine — this is expected and easily managed. Regular blood tests track the response. ### For thyroid cancer You may have a whole-body scan afterwards to see where the iodine has been taken up. Follow-up then continues with blood tests, including thyroglobulin, and periodic imaging. ## Safety precautions For a short period after treatment, you will be advised to limit close contact with others, especially pregnant women and young children, and to follow simple hygiene measures. The exact precautions and their duration depend on the dose and local rules — your team will explain them clearly. ## Side effects Most people tolerate treatment well. Possible short-term effects include tenderness in the neck, a dry mouth, nausea, or altered taste. These usually settle. Long-term, the main consequence is an underactive thyroid, which is treated with replacement hormone. ## The bottom line Radioactive iodine is a well-established, effective treatment that targets the thyroid precisely. Understanding the preparation, the day itself and the precautions afterwards removes much of the anxiety. Discuss your individual plan, including fertility and pregnancy considerations, with your specialist. ## References & further reading - [Radioactive iodine — American Thyroid Association](https://www.thyroid.org/radioactive-iodine/) - [Overactive thyroid (hyperthyroidism) — NHS](https://www.nhs.uk/conditions/overactive-thyroid-hyperthyroidism/) --- Part of The Thyroidologist — https://thethyroidologist.com The content on this website is for general educational purposes only and does not constitute medical advice, diagnosis, or treatment. Always consult a qualified healthcare professional about your specific condition.