Influence of Thyroid-Stimulating Hormone Levels on Uptake of FDG in Recurrent and Metastatic Differentiated Thyroid Carcinoma

The objective of this prospective study was to determine the extent to which the levels of thyroid-stimulating hormone (TSH) influence the uptake of FDG by thyroid carcinoma tumors Methods: Ten patients with follicular (n 5 7) or papillary (n 5 3) thyroid carcinoma underwent FDG PET during TSH suppression (,0.05 µU/mL) and TSH stimulation (.22 µU/mL) within an average interval of 42 d (range, 28–73 d). The findings were evaluated by visual criteria. In addition, a tumor-to-background ratio (TBR) was determined for 17 lesions that were visualized. Results: In 15 of 17 lesions with positive FDG uptake, TSH stimulation was associated with an increase in the TBR from 3.85 6 2.53 (mean 6 SD) to 5.84 6 4.84, corresponding to an average increase of 63.1 % (P, 0.001). Determination of absolute counting rates indicated that this increase was the result of a decrease in FDG metabolism in the background together with an increase in the tumor tissue. No relationship was found between the presence or absence of iodine storage capacity (5 versus 12/17 lesions) and increase in FDG accumulation. Seven of 10 patients had additional iodine-positive metastases that showed no accumulation of FDG. Conclusion: Most locally recurrent and metastatic follicular and papillary thyroid carcino-mas exhibited a significant increase in FDG uptake on TSH stimulation. In 3 of 10 patients, TSH stimulation resulted in either detection of new lesions or classification of the FDG uptake pattern as typical for malignancy. These findings suggest that FDG uptake in recurrent and metastatic thyroid carcinoma depends on the TSH level. Therefore, we recommend that PET examinations be performed in patients with thyroid carcinoma under TSH stimulation and follow-up examinations be performed under identical TSH conditions to prevent erroneous interpreta-tion. Key Words: FDG PET; thyroid-stimulating hormone; thyroid carcinom