# Radioactive Iodine Therapy and Thyroid Scintigraphy

> Radioactive iodine therapy and scintigraphy for NEET-PG Medicine covers uptake patterns in Graves disease, toxic nodules and thyroiditis, and I-131 precautions.

- Canonical URL: https://prepelephant.com/topics/neet-pg/medicine/radioactive-iodine-therapy-and-thyroid-scintigraphy
- Exam / course: NEET-PG · Subject: Medicine
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-14
- Last updated: 2026-10-14
- Reference basis: Williams Textbook of Endocrinology; Bailey and Love's Short Practice of Surgery
- How to cite: "Radioactive Iodine Therapy and Thyroid Scintigraphy", PrepElephant, https://prepelephant.com/topics/neet-pg/medicine/radioactive-iodine-therapy-and-thyroid-scintigraphy

## Direct answer

Radioactive iodine therapy uses iodine-131, a beta emitter taken up by thyroid follicular cells, to destroy overactive or malignant thyroid tissue. Thyroid scintigraphy, usually with technetium-99m pertechnetate or iodine-123, maps the pattern of uptake and separates Graves disease, toxic nodules and thyroiditis before treatment is chosen.

## What you must remember

- **I-131 therapy:** a beta emitter, with gamma emission used for imaging, that treats Graves disease, toxic nodular goitre and differentiated thyroid cancer.
- **Technetium scan:** pertechnetate is trapped by the gland but not organified, so it shows the uptake pattern without measuring hormone synthesis.
- **Diffuse high uptake:** Graves disease, with generally increased uptake across the whole gland.
- **Hot nodule:** a toxic adenoma, with suppressed uptake in the rest of the gland.
- **Low uptake:** thyroiditis or exogenous thyroxine, where the gland releases stored hormone without actively trapping iodine.
- **Cold nodule:** a non-functioning nodule with a higher malignancy risk, which needs cytology rather than radioiodine.
- **Pregnancy and breastfeeding:** radioiodine is contraindicated in pregnancy, and breastfeeding must stop before treatment.
- **Contrast:** iodinated contrast blocks thyroid iodine uptake for weeks, so uptake studies are delayed after contrast exposure.

## Reading the uptake pattern before treating

The pattern of uptake decides both the diagnosis and the therapy. A diffuse high uptake and a single hot nodule lead to different treatments and different expectations.

| Uptake pattern | Likely diagnosis | Implication |
|---|---|---|
| Diffuse, high uptake | Graves disease | Radioiodine or antithyroid drugs, with eye disease checked first |
| Single hot nodule, rest suppressed | Toxic adenoma | Radioiodine or surgery, since the rest of the gland is suppressed |
| Multiple hot nodules | Toxic multinodular goitre | Radioiodine or surgery, particularly for large goitres |
| Low uptake | Thyroiditis or exogenous thyroxine | Antithyroid drugs not needed, and the cause is treated |
| Cold nodule | Cyst, adenoma or carcinoma | Fine needle aspiration cytology, not radioiodine |

Radioiodine is given at a dose chosen to ablate or reduce function, and many patients with Graves disease become hypothyroid, which then needs lifelong levothyroxine. Active Graves orbitopathy can worsen after radioiodine, so steroid cover is considered in those patients. Patients should avoid pregnancy for several months after treatment, and the scan interpretation depends on the timing of any recent contrast or iodine exposure.

## Where the exam tries to catch you

Most errors come from applying a radioiodine answer to a pattern that calls for a different test.

- **Trap:** Using scintigraphy to rule out thyroid cancer. — **Correct:** A cold nodule needs cytology, since a scan cannot exclude malignancy.
- **Trap:** Giving I-131 to a pregnant woman with Graves disease. — **Correct:** Radioiodine is contraindicated in pregnancy, so antithyroid drugs are used and pregnancy is avoided during therapy.
- **Trap:** Choosing I-131 for thyroiditis. — **Correct:** Thyroiditis shows low uptake and is often self-limiting, so radioiodine is not indicated.

## Frequently asked questions

### What does low uptake on a thyroid scan suggest?

Low uptake suggests thyroiditis or exogenous thyroxine, because the gland is releasing stored hormone without actively trapping iodine. Radioiodine treatment is not indicated for this pattern, since the thyrotoxicosis is usually transient or drug-induced.

### Why is radioactive iodine contraindicated in pregnancy?

Radioactive iodine crosses the placenta and is taken up by the fetal thyroid, which can cause permanent fetal hypothyroidism. Pregnancy must be excluded before treatment, and women are usually advised to avoid conception for several months after therapy.

### Why should iodinated contrast be avoided before a radioiodine uptake study?

Iodinated contrast carries a large iodine load that blocks thyroid iodine uptake for several weeks. This invalidates the uptake measurement, so the study or treatment should be delayed after contrast exposure.

### Which thyroid condition commonly becomes hypothyroid after radioiodine therapy?

Graves disease commonly becomes hypothyroid after radioiodine therapy, because the gland is destroyed to a variable degree. Lifelong levothyroxine is then needed, and this is an accepted outcome of treatment rather than a failure.