Is Thyroid Function Scan Related to X-rays?
No, thyroid function scans (radioiodine uptake studies and thyroid scintigraphy) are not related to X-rays—they use radioactive isotopes (typically iodine-123 or technetium-99m) that emit gamma radiation, not X-radiation. 1, 2
Key Distinction Between Imaging Modalities
Nuclear Medicine Scans (NOT X-rays)
- Thyroid scintigraphy and radioiodine uptake studies use radiopharmaceuticals (I-123, I-131, or Tc-99m pertechnetate) that are administered to the patient and emit gamma radiation detected by a gamma camera 3, 4
- These studies provide functional imaging that measures thyroid activity and iodine uptake, not structural anatomy 3, 2
- I-123 is preferred over I-131 for diagnostic imaging due to superior imaging quality 1
X-ray Based Imaging (Different Technology)
- CT scans use X-radiation to create cross-sectional images based on differential tissue attenuation of the X-ray beam 3
- CT provides structural imaging but cannot assess thyroid function 5
- Plain X-rays are not used for thyroid evaluation 3
Clinical Applications of Radioiodine Scans
Primary Indications
- Differentiating causes of thyrotoxicosis when laboratory tests are ambiguous (Graves' disease vs. toxic adenoma vs. toxic multinodular goiter vs. thyroiditis) 1
- Planning radioactive iodine therapy by calculating appropriate I-131 doses for hyperthyroidism treatment 1
- Identifying autonomously functioning nodules in toxic adenoma 1
- Confirming goiter consists entirely of thyroid tissue when associated with thyrotoxicosis 1
When NOT Indicated
- No role in hypothyroidism workup as all causes show decreased uptake 5, 1
- Not recommended for evaluating thyroid nodules in euthyroid patients as most nodules are "cold" and most cold nodules are benign 5, 1
- Not useful as first-line imaging for structural thyroid evaluation 1
Important Clinical Caveat
External radiation (X-rays) to the neck can cause thyroid dysfunction and cancer, but this is completely separate from diagnostic thyroid nuclear medicine scans 6. Therapeutic external radiation doses exceeding 26 Gy frequently produce hypothyroidism, and neck irradiation increases thyroid cancer risk 15- to 53-fold, particularly when administered at a young age 6. However, diagnostic radioiodine scans use much lower radiation doses and serve a different purpose 2.