Click here for Frequently Asked Questions on Radionucleotide Scans.
Radioactive isotopes, such as 123-I or 99-Tc may be taken up by the thyroid, and used to visualize the functional anatomy of the thyroid. Regions of the thyroid that are functioning and actively incorporating the isotope will be detected with a counter, forming an image of the thyroid that reflects the ability of specific regions of the thyroid to trap the isotope.Regions of the thyroid that are not functioning normally, or are filled with fluid, will not take up the isotope, and will appear as 'cold' or hypofunctioning regions on the thyroid scan.Imaging studies should be avoided in pregnant women or in women who may be uncertain as to whether they are pregnant. When doubt arises, a pregnancy test is recommended prior to the scan.
Quantification of the amount of iodine that a thyroid gland can take up may be obtained from an iodine uptake test. This test does not generate a picture of the thyroid, but involves ascertaining the percentage of radioactive iodine localized to the thyroid after a small amount of the isotope is administered. In most centers, the amount of radioactive iodine taken up is assessed a few hours after it is administered, and again 24 hrs later. This test is commonly obtained to differentiate various forms of hyperthyroidism, such as Grave's Disease, versus thyroiditis.
Are these imaging studies using radioactive materials safe for children?
There have been no long term reports of increased thyroid problems following use of 131-I. In fact, there have been studies showing no increased incidence of thyroid cancer after diagnostic scanning in children. See Thyroid cancer after diagnostic administration of iodine-131 in childhood. Radiat Res. 2001 Jul;156(1):61-70.
I have renal failure and I am on dialysis-Can I still have a diagnostic test or treatment with radioactive iodine?
Although a pre-test or pre-treatment assessment should be done to estimate dosing, these types of tests and treatments can be safely administered to patients with kidney failure on dialysis. See Nuclear medicine studies in the dialysis patient. Semin Dial. 2002 Jul-Aug;15(4):269-76.
Is any preparation required for the scan?
Try and avoid inadvertent exposure to exogenous iodine in the form of supplements, contrast dye, kelp, or some drugs. Similarly, the utility of the scan my be compromised in patients taking thyroid hormone
I had a PET scan for cancer surveillance "light up my thyroid"-is this common?
Posittron emission tomography (PET) scanning employs the used of radiolabelled glucose molecules to indirectly assess the metabolism of cells. The premise is that cancer cells have activated metabolism, willmore readily take up the glucose isotope, and the cells can be imaged by the radiation they emit. In some instances, the thyroid gland diffusely may take up large amounts of F18-fluorodeoxyglucose, in the absence of cancer involvement, often in the setting of autoimmune thyroid diseases such as Hashimoto's thyroitis. This has been described in women with a history of breast cancer that had positive thyroid PET scans after treatment. Factors Associated with the Development of New Onset Diffuse Thyroid F18-Fluorodeoxyglucose Uptake After Treatment of Breast Cancer in Patients Without a History of Thyroid Disease or Thyroid Dysfunction Thyroid. 2011 Dec 5. [Epub ahead of print]. In contrast, focal localization of F18-fluorodeoxyglucose to a very small area of the thyroid should prompt investigation as to the possibility of an incidental thyroid cancer Thyroid Incidentalomas on Fluorine-18-Fluorodeoxyglucose Positron Emission Tomography-Computed Tomography: Incidence, Malignancy Risk, and Comparison of Standardized Uptake Values Can Assoc Radiol J. 2011 Dec 2
Analysis of a large number of patients with PET scans demonstrates that incidental unexpected detection of a thyroid abnormality on PET imaging occurs in about 1/40 scans, or 2.5%. Of these, about 1/3 are ultimately diagnosed as thyroid cancer. Hence, although the majority of these incidentalomas are benign, the detection of thyroid uptake on PET scans warrants a careful assessment. Diagnostic and Clinical Significance of F-18-FDG-PET/CT Thyroid Incidentalomas. J Clin Endocrinol Metab. 2012 Aug 17.