Clinical Significance of Diffuse 18F-FDG Uptake in Residual Thyroid Gland after Unilateral Thyroid Lobectomy

Song, Hee Sung; Lee, Su Jin; Yoon, Seok Ho; Lee, Jandee; Soh, Euy Young; An, Young Sil; Yoon, Joon Kee

Abstract

PURPOSE
We investigated the clinical significance of diffuse uptake in remaining thyroid after unilateral lobectomy for thyroid cancer.
METHODS
A total of 144 thyroid cancer patients who underwent 18F-FDG PET/CT after lobectomy were enrolled in the present study. The PET/CT images were evaluated for the presence of diffuse 18F-FDG uptake with maximum SUV (SUVmax) >2.0 in the residual thyroid and placed into one of two groups: with diffuse uptake and without diffuse uptake group. Clinical, laboratory, and PET/CT parameters in both groups were compared. Correlations between SUVmax of thyroid and available parameters were analyzed.
RESULTS
Forty-two of 144 patients (29.2%) had diffuse thyroid uptake (mean SUVmax: 3.2+/-1.1). All patients with diffuse uptake and 96 (94.1%) without diffuse uptake were receiving thyroxine therapy (P=0.09). Thyroid function tests showed that most patients were euthyroid status (78.6 vs. 85.3%, P=0.36). TgAb levels were significantly higher in patients with diffuse uptake (338.0+/-664.6 vs. 57.3+/-46.4, P<0.0001). Mean attenuation values in the diffuse uptake group were significantly lower (72.2+/-15. vs. 97.0+/-16.0, P<0.0001). An inverse correlation was found between SUVmax and mean attenuation values of residual thyroid in all patients (r=-0.57, P<0.0001) and subgroup with diffuse uptake (r=-0.31, P<0.05).
CONCLUSION
In this study, diffuse 18F-FDG uptake in the residual thyroid after unilateral lobectomy was a relatively frequent finding and may be associated with chronic thyroiditis. This uptake is not influenced by thyroid status or thyroxine therapy. The 18F-FDG uptake is inversely correlated with mean attenuation value of thyroid.