Diagnostic value of CT density in patients with diffusely increased FDG uptake in the thyroid gland on PET/CT images

OBJECTIVE. The purpose of this article is to determine the diagnostic significance of thyroid density as determined by CT in patients with a diffuse hypermetabolic thyroid on PET/CT images. SUBJECTS AND METHODS. One hundred twelve patients were enrolled in this study, and all underwent PET/CT and a thyroid function test between August 2007 and December 2008. The 56 study patients enrolled had visible ¹⁸F-FDG uptake in the thyroid, whereas the 56 control subjects had no visible FDG uptake. Maximum standardized uptake values (SUV _max), Hounsfield units, thyroid-stimulating hormone levels, free thyroxine levels, and visual assessments of thyroid CT densities (grades 0-2) were evaluated. Analyses were performed to identify variables that differed between patients and control subjects and to examine relationships between Hounsfield unit and SUV_max values, free thyroxine level, or thyroid-stimulating hormone level. RESULTS. Mean Hounsfield unit and SUV _max values were significantly different between the two groups (p < 0.001), and SUV_max values were found to be significantly different between grades. Free thyroxine and thyroid-stimulating hormone levels were also significantly different between thyroid density grades 0 and 2 (p < 0.001). A strongly negative relationship was found between Hounsfield units and SUV_max values (r = 0.75), whereas Hounsfield units were found to be weakly correlated with free thyroxine level (r = 0.36) and thyroid-stimulating hormone level (r = -0.33). CONCLUSION. CT density is closely related to FDG uptake, and low CT density coupled with high FDG uptake suggests a change in thyroid tissue composition and is probably helpful for assessing the disease process and thyroid function of patients whose thyroid show diffuse FDG uptake on FDG PET/CT.