Thyroid
Diffuse Thyroid Uptake:
- The normal thyroid gland typically demonstrates little or no FDG uptake.
- Mild diffuse thyroid uptake is considered normal.
- Moderate to intense diffuse thyroid uptake can be normal, or can reflect thyroiditis. In such cases, we typically report, “Diffuse intense thyroid uptake is noted, which can reflect thyroiditis.”
Incidental Focal Thyroid Uptake:
- An incidental focus of thyroid uptake is seen in up to 4% of scans.
- Up to 33% of FDG-avid thyroid nodules will represent a primary thyroid malignancy (the nodule itself may be difficult to visualize on the non-contrast co-registered CT images).
- Therefore, any focus of uptake above background metabolic activity warrants further clinical assessment, typically ultrasound and biopsy.
Caveat:
- Beware the intensely avid thyroid nodule partially masked by diffuse intense thyroid uptake. While the diffuse uptake may be normal or may reflect thyroiditis, the underlying hot nodule may represent an incidental primary thyroid cancer.
- Parathyroid adenomas can also be hypermetabolic (we do not report this in our differential unless there is a high clinical suspicion for a parathyroid adenoma).
Case 1
Case 1 (Image 2)
Axial images demonstrate normal mild uptake of FDG in the thyroid gland. The CT appearance of the gland is also normal.
Case 2
Case 2 (Image 2)
Axial images demonstrate moderate FDG uptake in the thyroid gland. The CT appearance of the gland is normal.
Case 3
Case 3 (Image 2)
Axial images demonstrate diffuse intense FDG uptake in the thyroid gland. The CT appearance of the gland is normal.
Case 4
Case 4 (Image 2)
Axial images confirm the uptake to represent a hypodense left lobe thyroid nodule, containing a punctate calcification. As ≈ 30% of hypermetabolic thyroid nodules represent a primary thyroid cancer, such nodules require ultrasound evaluation (biopsy is usually necessary). In this case, biopsy revealed a primary thyroid cancer.
Case 7
Case 7 (Image 2)
Axial images also demonstrate this diffuse intense uptake. The CT image suggests a slightly enlarged and nodular morphology of the gland.
Case 7 (Image 3)
Given the rather bulbous appearance of the thyroid isthmus, the intensity of the MIP image was manually decreased. This decreased intensity reveals a hypermetabolic nodule masked by the diffuse intense uptake. Biopsy revealed a primary thyroid cancer.
Case 7 (Image 4)
The hypermetabolic nodule within the isthmus is evident once the intensity of the gland is manually decreased. Caveat: Always manually decrease image intensity in diffusely avid thyroid glands to exclude a hidden hypermetabolic nodule.
