Clinical Significance of Diffusely Increased 18F-FDG Uptake in the Thyroid Gland

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FIGURE 1.
Normal, low ¹⁸F-FDG uptake in thyroid of 37-y-old woman referred for restaging of previously treated tongue cancer. Thyroid is not normally visible on 3-dimensional maximum-intensity-projection images.
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FIGURE 2.
Diffusely increased ¹⁸F-FDG uptake in thyroid of 47-y-old man referred for evaluation of neck lymphadenopathy. Patient was taking thyroxine replacement therapy for hypothyroidism. On ultrasound examination, thyroid gland was prominent and heterogeneous. Fine-needle aspiration biopsy of thyroid revealed chronic thyroiditis. Serum TSH was 0.2 mIU/L (reference range, 0.3–5.0 mIU/L), free thyroxine was 1.6 ng/dL (reference range, 0.8–1.8 ng/dL), and TPO antibodies were elevated at 6,160 IU/mL (reference range, <40 IU/mL), suggesting diagnosis of lymphocytic thyroiditis. Shown are transaxial CT image (A), transaxial PET image (B), transaxial fused PET/CT image (C), and 3-dimensional maximum-intensity-projection image (D).
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FIGURE 3.
Thyroid-related findings in 133 patients with diffuse thyroid ¹⁸F-FDG uptake.
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FIGURE 4.
Plot of serum TSH vs. SUV_max in 21 patients with no prior history of thyroid disease. P value was found to be 0.089.

TABLE 1

Patients Undergoing Fine-Needle Aspiration Biopsy After PET

		SUV_max
Patient no.	Reason for PET/CT	Right lobe	Left lobe	Sonography findings	Cytology report	Thyroid status
1	Restaging of lung cancer	6.5	5.6	Diffusely heterogeneous parenchyma	Hashimoto's thyroiditis	Was already taking thyroxine
2	Evaluation of pigmented choroidal mass	9	11.2	Heterogeneous parenchyma; increased vascularity	Hashimoto's thyroiditis	Was already taking thyroxine
3	Evaluation of neck lymphadenopathy	8.4	7.4	Mildly prominent, heterogeneous thyroid gland	Chronic thyroiditis	Was already taking thyroxine
4	Restaging of non-Hodgkin's lymphoma	16.8	15.1	Enlargement with diffusely heterogeneous parenchyma	Hashimoto's thyroiditis	Was already taking thyroxine

TABLE 2

Results of the 19 Patients Subsequently Found to Have Thyroid Disease/Abnormality

	SUV_max
Patient no.	Right lobe	Left lobe	TSH (mIU/L)	Free thyroxine (ng/dL)	TPO antibody (IU/mL)	Ultrasound report
1	11.3	11.1	70.0	0.5	—	—
2	5.7	4.0	—	—	—	Hashimoto's thyroiditis
3	6.5	6.3	23.1	0.6	—	Nodular goiter (coarsened echotexture, nodularity)
4	4.8	6.3	27.5	0.7	1,073	—
5	11.4	12.0	12.8	0.8	298	—
6	9.9	12.9	159	0.7	2,680	Hashimoto's thyroiditis
7	5.1	4.6	32.3	0.7	5,820	—
8	4.7	4.2	8.0	—	510	—
9	9.2	7.8	5.4	1.5	655	Likely Hashimoto's thyroiditis
10	9.0	11.2	2.5	1.9	33	Hashimoto's thyroiditis
11	5.6	6.8	7.1	1.5	7,520	Hashimoto's thyroiditis
12	6.5	6.3	1.8	0.9	272	—
13	5.2	5.8	4.9	—	—	—
14	8.1	3.3	56.6	0.6	—	Markedly heterogeneous parenchyma
15	4.7	4.9	64.1	0.8	3,680	—
16	12.9	14.2	2.5	1.0	120	Hashimoto's thyroiditis
17	5.7	5.6	6.0	—	—	Autoimmune thyroid disease
18	4.5	5.3	6.1	1.0	546	—
19	3.6	3.9	7.3	—	—	—

Reference values: TSH, <5 mIU/L; free thyroxine, 0.8–1.8 ng/dL; TPO antibody, <40 IU/mL.