Thyroid nodules may also be diagnosed via radionuclide thyroid scan, whole-body radioactive iodine scan, positron emission tomography (PET scan) or iodine-131 single photon emission computed tomography (SPECT).

• Using either technetium 99 mTc pertechnetate or I123
• Radionuclide scan is contraindicated during pregnancy
• Thyroid scintigraphy is useful to determine the functional status of a nodule. It is specifically indicated in patients with thyroid nodule and a low serum TSH to determine if the nodule has autonomous functioning.
• In scintigraphy, iodine radioisotopes (more commonly used; usually I-123) or technetium pertechnetate (99Tc), are injected and then the radioisotope uptake time by the thyroid gland is measured.^[1][2]

• High radioisotope uptake=Hot nodule:
  • Hyperfunctioning nodules
• Low radioisotope uptake=Cold nodule:
  • Most benign nodules
  • Most malignant thyroid nodules

• Post therapy whole-body iodine scanning is typically conducted approximately 1 week after RAI therapy to visualize metastases.

• Diagnostic whole body scanning (DxWBS), either following thyroid hormone withdrawal or recombinant hormone TSH (rhTSH), 6–12 months after remnant ablation may be of value in the follow-up of patients with high or intermediate risk of persistent disease, but should be done with I-123 or low activity I-131.^[3]

• Improved diagnostic accuracy of indeterminate thyroid nodules
• In patients with thyroid PET incidentaloma, the incidence of primary thyroid malignancy is very high ^[4]
• There is insuffiecient evidence to recommend to or against routine clinical use
• Usage indications:
  • Simple disease localization in thyroglobulin (Tg) positive, RAI scan–negative patients
  • Initial staging and follow-up of high-risk patients with poorly differentiated thyroid cancers unlikely to concentrate RAI in order to identify sites of disease that may be missed with RAI scanning and conventional imaging.
  • Initial staging and follow-up of invasive or metastatic Hurthle cell carcinoma.
  • As a powerful prognostic tool for identifying which patients with known distant metastases are at highest risk for disease-specific mortality.
  • As a selection tool to identify those patients unlikely to respond to additional RAI therapy.
  • As a measurement of post treatment response following external beam irradiation, surgical resection, embolization, or systemic therapy.

Larson SM, Robbins R 2002 Positron emission tomography in thyroid cancer management. Semin Roentgenol 37:169–174. 316. ^[5]

False-positive 18FDG-PET findings can be due to:

• Inflammatory lymph nodes
• Suture granulomas
• Increased muscle activity

Therefore, cytologic or histologic confirmation is required before one can be certain that an 18FDG-positive lesion represents metastatic disease.

• May provide superior lesion localization after remnant ablation, but it is still a relatively new imaging modality