Radioactive Iodine Uptake Test
The radioactive iodine uptake (RAIU) test measures the percentage of a small radioactive iodine dose taken up by the thyroid over a set period — usually 6 and 24 hours.
It is the key test for distinguishing causes of thyrotoxicosis.
Last medically reviewed: 1 November 2025
How it works
You swallow a tiny dose of radioactive iodine (I-123 or I-131). The thyroid — which needs iodine to make hormone — absorbs some of it. A probe over the neck measures uptake at intervals.
The radiation dose is very small — comparable to or less than a chest X-ray.
What the results mean
High uptake suggests hormone overproduction — Graves’ disease (diffuse high uptake) or toxic nodules (patchy uptake). Low uptake suggests thyroiditis (hormone leaking, not produced), factitious thyrotoxicosis, or recent iodine load.
This distinction is critical: Graves’ needs antithyroid drugs or ablation; thyroiditis needs only symptom control.
Preparation and limitations
Preparation involves avoiding iodine-rich foods, certain medications (amiodarone, potassium iodide), and contrast dye for weeks before testing. Your doctor will give specific instructions.
Recent iodine exposure (contrast CT, seaweed supplements) can falsely lower uptake. Pregnancy and breastfeeding are contraindications.
Frequently asked questions
Is the radioactive iodine dangerous?
No — the dose is extremely small, similar to a chest X-ray. It is not the same as therapeutic radioactive iodine used to treat hyperthyroidism.
Why was my uptake low?
Low uptake in thyrotoxicosis suggests thyroiditis (hormone leaking) rather than overproduction — a different condition with different management.
Can I eat normally before the test?
You will need to avoid iodine-rich foods and certain medications for a period before testing — follow your doctor’s preparation instructions carefully.
References & further reading
This page is based on established medical knowledge and guidelines from authoritative sources. Always consult a qualified clinician for personal medical advice.
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