- Thyroid gland imaging studies with radionuclides:
- Provide both structural and functional information and can be very useful in determining the etiology of biochemical hyperthyroidism:
- In contrast, thyroidal nuclear imaging is not recommended in the evaluation of a patient with hypothyroidism
- Provide both structural and functional information and can be very useful in determining the etiology of biochemical hyperthyroidism:
- For nuclear imaging, scans using radioactive iodine isotopes are most preferred:
- Because these directly reflect the active accumulation (trapping) of iodine by the thyroid follicular cell and covalent attachment (organification) of iodine to thyroglobulin
- The preferred radionuclide for diagnostic nonthyroid cancer imaging:
- Is 123I, because this isotope emits only gamma rays that pass through tissue without significant cellular damage:
- In contrast, 131I emits both gamma rays for imaging, as well as damaging beta particles:
- So it can be used for the treatment of hyperthyroidism and thyroid cancer:
- To destroy iodine-avid thyroid tissue
- So it can be used for the treatment of hyperthyroidism and thyroid cancer:
- In contrast, 131I emits both gamma rays for imaging, as well as damaging beta particles:
- For diagnostic imaging:
- 123I is administered orally:
- With the measurement of iodine uptake and gamma scintigraphy images:
- Obtained 4 hours and / or 24 hours later
- With the measurement of iodine uptake and gamma scintigraphy images:
- Measured thyroidal uptake depends on the activity of NIS and overall iodine status as determined by the amount of circulating nonradioactive iodine:
- When there is an excess of nonradioactive iodine:
- The measured radioactive iodine uptake is reduced due to the competition between radioactive and nonradioactive iodine uptake by the thyroid follicular cells:
- Sources of excess nonradioactive iodine include kelp, seaweed, seafood, iodine-rich medications and agents (amiodarone, saturated solution of potassium iodide [SSKI], Lugol’s solution, povidone iodine, tincture of iodine, iodoform gauze), and radiographic contrast media used commonly in computed tomography (CT) scans and gallbladder studies
- The measured radioactive iodine uptake is reduced due to the competition between radioactive and nonradioactive iodine uptake by the thyroid follicular cells:
- When there is an excess of nonradioactive iodine:
- 123I is administered orally:
- Is 123I, because this isotope emits only gamma rays that pass through tissue without significant cellular damage:
- An alternate radionuclide is technetium99m pertechnetate (99mTc):
- Which is administered intravenously
- Images are obtained much more rapidly than 123I:
- Usually on the order of 30 to 60 minutes after the administration of the radionuclide tracer
- Although 99mTc will be trapped by the thyroid follicular cells:
- There is no iodine moiety for attachment to thyroglobulin, and therefore does not as accurately mimic the thyroidal uptake of iodine as radioiodine nuclides:
- Thus 123I thyroid scans have 5% to 8% fewer false negative results than 99mTc scans:
- However, because 99mTc scans are easier, faster, more readily available and less expensive to perform, they have largely replaced 123I scans at some institutions
- Thus 123I thyroid scans have 5% to 8% fewer false negative results than 99mTc scans:
- There is no iodine moiety for attachment to thyroglobulin, and therefore does not as accurately mimic the thyroidal uptake of iodine as radioiodine nuclides:
- Studies of direct comparison of radioiodine and 99mTc thyroid scans have been highly concordant in patients without nodules and in those with cold nodules:
- One study reported that of 273 patients with thyroid nodules, only two had increased uptake with pertechnetate and no uptake with radioiodine:
- However, if the results of the 99mTc scan are not in agreement with the clinical picture, an 123I scan should be performed
- One study reported that of 273 patients with thyroid nodules, only two had increased uptake with pertechnetate and no uptake with radioiodine:
- Although nuclear scans are useful in the differential diagnosis of biochemical hyperthyroidism:
- Other radiologic modalities (e.g., ultrasonography, CT, and magnetic resonance imaging [MRI]) provide information regarding structural anatomy of the thyroid and provide no functional data
- The primary role of thyroid ultrasound is in the initial evaluation of thyroid nodules:
- As recommended by the American Thyroid Association and the American Association of Clinical Endocrinologists
- Although thyroid ultrasound does not have a role in the initial evaluation of biochemical thyroid dysfunction:
- It may demonstrate changes that are consistent but are not necessarily diagnostic of chronic lymphocytic thyroiditis, subacute granulomatous thyroiditis, and postpartum thyroiditis
- Some individuals with subclinical hypothyroidism and sonographic features suggestive of chronic thyroiditis:
- Are at significant risk for developing overt hypothyroidism requiring thyroid hormone replacement therapy
Rodrigo Arrangoiz MS, MD, FACS, FSSO 