USMLE (United States Medical Licensing Examination) Step 1 Practice Exam

What is the role of 123-I uptake in evaluating thyroid function?

1. Detect hormone levels

2. Evaluate gland size

3. Evaluate synthetic activity of the thyroid gland

4. Measure blood flow to the thyroid

The correct answer is: Evaluate synthetic activity of the thyroid gland

In the evaluation of thyroid function, the uptake of 123-I (iodine-123) plays a crucial role in assessing the synthetic activity of the thyroid gland. This radioactive isotope is administered to the patient, and its uptake by the thyroid gland indicates how well the gland is functioning in producing thyroid hormones. The thyroid gland uses iodine to synthesize thyroid hormones, and the amount of iodine absorbed reflects the gland's activity. A higher uptake of 123-I typically suggests increased synthetic activity, which can be seen in conditions such as hyperthyroidism. Conversely, a low uptake may indicate an underactive thyroid state or thyroiditis. This method is particularly useful for distinguishing between different types of thyroid disorders, such as differentiating hyperfunctioning nodules from those that are non-functioning or diagnosing conditions such as Graves' disease. Other methods for evaluating thyroid function might include measuring hormone levels, but this does not directly evaluate the gland's activity; rather, it measures the end products of the thyroid's synthetic activity. Additionally, assessing gland size provides anatomical information but does not inform about functional capacity. Blood flow measurement to the thyroid is not typically assessed with 123-I uptake and would require different imaging modalities. Overall, the 123-I uptake provides direct insight