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 whic h the levels of thyroid-stimulating hormone (TSH) influence the uptake of F DG by thyroid carcinoma tumors Methods: Ten patients with follicular (n = 7 ) or papillary (n = 3) thyroid carcinoma underwent FDG PET during TSH suppr ession (<0.05 <mu>U/mL) and TSH stimulation (>22 muU/mL) within an average interval of 42 d (range, 28-73 d). The findings were evaluated by visual cr iteria. In addition, a tumor-to-background ratio (TBR) was determined for 1 7 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 +/- 2.53 (mean+/-F SD) to 5.84 +/- 4.84, corresponding to an average increase of 63.1% (P < 0.001). Determination of absolute counting rates ind icated that this increase was the result of a decrease in FDG metabolism in the background together with an increase in the tumor tissue. No relations hip was found between the presence or absence of iodine storage capacity (5 versus 12/17 lesions) and increase in FDG accumulation. Seven of 10 patien ts had additional iodine-positive metastases that showed no accumulation of FDG. Conclusion: Most locally recurrent and metastatic follicular and papi llary thyroid carcinomas exhibited a significant increase in FDG uptake on TSH stimulation. In 3 of 10 patients, TSH stimulation resulted in either de tection of new lesions or classification of the FDG uptake pattern as typic al for malignancy. These findings suggest that FDG uptake in recurrent and metastatic thyroid carcinoma depends on the TSH level. Therefore, we recomm end that PET examinations be performed in patients with thyroid carcinoma u nder TSH stimulation and follow-up examinations be performed under identica l TSH conditions to prevent erroneous interpretation.