Which test uses radioactive iodine to create an image of the thyroid gland?

• A. Ultrasound of the thyroid
• B. Thyroid scan
• C. CT scan of the neck
• D. MRI of the neck

Answer: (B)

The main idea is imaging thyroid function using a radioactive tracer. In a thyroid scan, a small amount of radioactive iodine (typically I-123) is given and the thyroid tissue takes it up. A gamma camera then captures the distribution of radioactivity, producing an image that shows both the size and shape of the gland and how actively different parts are functioning. This helps identify diffuse changes, hot or cold nodules, and overall uptake patterns.

Ultrasound images the thyroid with sound waves and provides anatomy without radiation. CT uses X-rays to create cross-sectional pictures, sometimes with contrast, but it doesn’t depend on iodine uptake by the thyroid. MRI uses magnetic fields to image soft tissues without radioactive tracers. Because they don’t rely on radioactive iodine uptake, these tests don’t produce images of thyroid function in the same way as a thyroid scan.