Abstract
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Introduction: F18-fluorodeoxyglucose (F18-FDG) positron emission tomography is a powerful tool most commonly utilized in oncologic imaging. F18-FDG is a glucose analog, preferentially absorbed by hypermetabolic tissues, the presence of which suggests the possibility of neoplasm. However, F18-FDG uptake is not specific to malignancy and is also seen in different physiologic, inflammatory, or infectious processes. A commonly detected abnormality is the presence of diffuse radiotracer uptake throughout the thyroid, which has been associated with an increased risk of thyroid dysfunction. The primary goal of this retrospective study was to determine the percentage of people in the study population who demonstrate diffuse F-18 FDG thyroid uptake and thyroid dysfunction, compare radiotracer uptake to a randomly selected population without thyroid dysfunction, and determine if there are different degrees of radiotracer uptake for different causes of thyroid dysfunction.
Methods: In this retrospective study, the Froedtert and MCW imaging database was used to identify patients with diffuse thyroid F18-FDG uptake on PET from May 2016 through May 2021. A total of 78 PET reports were identified, belonging to a total of 45 different patients. The imaging database was also used to randomly identify 10 patients with normal thyroid uptake during this same period. 11 patients were excluded from the study due to either inaccurate characterization of the thyroid related imaging findings or insufficient clinical data.
Multiple data points were collected from each PET and the patients’ electronic health records were reviewed for evidence of thyroid dysfunction. The 34 patients with diffusely increased radiotracer uptake in the thyroid were grouped into those with documented thyroid dysfunction and those without any apparent evidence of thyroid dysfunction. These groups were used to calculate the prevalence of thyroid dysfunction in the sample population. The ratio of maximum thyroid SUV to background liver maximum SUV was measured in all 31 patients with documented underlying thyroid dysfunction, and this was compared to a randomly selected control group of patients with no evidence of thyroid dysfunction.
Results: 34 patients were included in the study analysis, 31 of whom were determined to have a documented history of thyroid dysfunction. The average thyroid to liver uptake ratio in the population of individuals with thyroid dysfunction (M=2.22, SD=0.21) was significantly higher than the average uptake ratio in the control population without thyroid disease (M=0.48, SD 0.11), with a statistically significant difference in the means of the two populations; t(33)=10.07, p<0.01). The average uptake ratios in the different subgroups of patients with thyroid dysfunction were calculated, but the small number of patients in the different subgroups precluded any statistically significant results.
It was noted that using a thyroid to liver uptake ratio of 1 as a cutoff to suggest the possibility of thyroid dysfunction would accurately characterize the presence or absence of thyroid dysfunction in 41/44 patients in this study.
Conclusions: Subjects with diffusely increased radiotracer uptake in the thyroid have a high rate of underlying thyroid dysfunction, calculated at 91% in our sample population. The population of patients with underlying thyroid dysfunction demonstrated a statistically significant elevation in the ratio of thyroid to liver radiotracer uptake compared to the control group without thyroid dysfunction. The data from this study suggest that using a cutoff of thyroid to liver uptake ratio of 1.0 could be utilized to suggest underlying thyroid dysfunction, which applied retrospectively would have accurately characterized 93% of cases being assessed in this study.
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