Quick answer: these are two related nuclear medicine tests. The thyroid scan takes pictures of the thyroid gland to show its size, shape, position and any nodules. The radioactive iodine uptake (RAIU) test measures how much iodine your thyroid absorbs, which shows how active it is; it involves no imaging. You usually swallow a small capsule or liquid of radioactive iodine, then come back a few hours later and often again the next day for quick measurements and pictures. Each uptake measurement takes five minutes or less, and the scan 30 minutes or less. Tell your doctor about iodine, contrast and thyroid medications beforehand, and if you are or might be pregnant or are breastfeeding.
This is one of the gentlest tests in nuclear medicine: no tunnel, no noise, just a camera near your neck. The main things to get right are the preparation and coming back at the right time.
What the tests show
- Thyroid scan: an image of the gland, its size, shape and position, and whether there are areas that take up more or less tracer.
- Thyroid uptake: a number showing what percentage of the swallowed iodine the thyroid has absorbed at a given time, which reflects how active the gland is.
Doctors use them to:
- check whether the gland is working properly
- help diagnose an overactive thyroid (hyperthyroidism) and find its cause
- assess thyroid nodules and areas of inflammation
- check whether thyroid cancer has spread, usually with a whole-body scan in people who have or had thyroid cancer
- evaluate changes after medication, surgery or radiation
- plan radioactive iodine treatment for hyperthyroidism
How to prepare
Iodine in your body can compete with the tracer and make results inaccurate, so preparation focuses on iodine and thyroid medicines.
Tell your doctor if you:
- had a CT scan, x-ray, procedure or treatment with iodine-based contrast in the past two months
- take medications or supplements containing iodine, including some cough syrups, multivitamins and heart medications, or eat kelp or seaweed
- take thyroid medications. Your doctor may ask you to stop some of them for a period before the test. Do not stop any medication without instructions.
- have allergies to iodine, medications or anesthetics
- are pregnant, might be pregnant or are breastfeeding
You may also have blood tests for thyroid hormones in the days before, and you may be asked not to eat for several hours before swallowing the tracer, because food can affect the uptake measurement.
From the scanner room: [TU EXPERIENCIA: qué preguntas haces tú antes de dar la cápsula (contrastes recientes, medicación, suplementos) y qué errores de preparación ves más a menudo.]
The tracer
The most common tracer for thyroid imaging is radioactive iodine (I-123), given as a small capsule or liquid that you swallow. Tracers have little or no taste. For some exams, a different tracer is injected into a vein about 30 minutes before the pictures.
How the test works, step by step
Day 1
- You swallow the radioactive iodine capsule or liquid.
- A few hours later, often 4 to 6 hours after swallowing, you have the first uptake measurement. You sit in a chair while a probe, a small device that looks like a microphone, is placed in front of your neck for a few minutes. It does not touch you. Some centers also measure your thigh for comparison.
- Depending on the protocol, the thyroid scan may be done the same day.
Day 2 (often)
- About 24 hours after swallowing, you come back for a second uptake measurement, and in some centers the scan.
The scan: you lie on a table with your head tipped back and neck extended. The gamma camera takes pictures of the thyroid from three angles. You need to stay still for short periods. The camera gives off no radiation itself.
From the scanner room: [TU EXPERIENCIA: horarios reales en tu servicio (¿se hace en uno o dos días?, ¿a qué horas?) y trucos para que el cuello extendido no resulte incómodo.]
How long does it take?
| Part | Typical time |
|---|---|
| Swallowing the tracer | A few minutes |
| First uptake measurement | Often 4 to 6 hours later; 5 minutes or less |
| Second uptake measurement | Often at 24 hours; 5 minutes or less |
| Thyroid scan | 30 minutes or less |
Because measurements are spaced out, the test often involves two visits on consecutive days, but each visit is short.
What it feels like
The tests are painless. The only discomfort is lying still with your neck extended during the scan. If an injection is used, you feel a quick pin prick. Side effects are very rare.
After the test
You can usually return to normal activities right away. The small amount of tracer loses its radioactivity over time and leaves the body in urine and stool. Drink plenty of water to help flush it out.
Pregnancy and breastfeeding
These tests are not performed during pregnancy, because of the risk to the fetus, and they are not recommended during breastfeeding. Tell your doctor if either applies to you; they may suggest a different test or timing.
Understanding your results
A radiologist or nuclear medicine physician interprets the results and sends a report to your doctor. In general terms:
- High uptake suggests the thyroid is overactive, as in Graves’ disease or some nodular goiters.
- Low uptake may be seen with inflammation of the thyroid (thyroiditis), recent iodine exposure or certain medications.
- «Hot» nodules take up more tracer than the surrounding gland and are usually not cancer.
- «Cold» nodules take up less tracer. Most are not cancer either, but they usually need further evaluation, often with ultrasound and sometimes a biopsy.
Your doctor will combine the results with your blood tests and symptoms. See [link: Thyroid Ultrasound: What It Shows].
Tips from the technologist
[TU EXPERIENCIA: 2-3 consejos propios. Ideas: llevar una lista de suplementos aunque parezcan inofensivos, apuntar la hora exacta de la cápsula, no perder la cita del día siguiente.]
Frequently asked questions
Is the thyroid uptake test the same as radioactive iodine treatment?
No. The uptake test uses a very small diagnostic amount of radioactive iodine. Radioactive iodine treatment uses a much larger amount to treat an overactive thyroid or thyroid cancer. See [link: Radioactive Iodine Therapy: What to Expect and Precautions].
Do I need to stop my thyroid medication?
Sometimes. It depends on the medication and the reason for the test. Your doctor will tell you whether and when to stop. Never stop on your own.
Can I eat before a thyroid uptake test?
You may be asked not to eat for several hours before swallowing the tracer. Follow your center’s instructions.
Am I radioactive after the test?
Only slightly and briefly. Drink plenty of water to help clear the tracer.
Why did I have a CT with contrast and now have to wait?
Iodine-based contrast floods the thyroid with iodine, which can make the uptake test inaccurate for up to about two months.
This article is for general information and does not replace advice from your doctor or nuclear medicine team. Do not stop medications without medical instructions.