Absolute %uptake of quantitative SPECT/CT is more accurate than relative uptake ratio of planar scan for identification of Graves’ disease and destructive thyroiditis

Introduction: Although Technetium-99m uptake in the thyroid has been estimated by relative uptake ratio (thyroid-to-background ratio, T/B ratio), absolute measurement of %uptake in quantitative SPECT/CT has a potential to replace the T/B ratio. Here, the accuracy of the %uptake and T/B ratio for the diagnosis of Graves' disease and destructive thyroiditis was investigated.

Methods: Quantitative thyroid SPECT/CT (for %uptake) with planar thyroid scan (for T/B ratio) was performed in 723 patients from Jan 2020 to Sep 2022 at our hospital. Exclusion criteria were under 19 years of age (n=32), anti-thyroid drug medication or hyperthyroidism history (n=243), thyroid hormone medication or thyroiditis history (n=53), immune-check point inhibitor treatment (n=16), thyroid cancer or previous thyroid surgery (n=15), internal or external radiotherapy to thyroid (n=2), amiodarone (n=2), lithium (n=4), single-or multi-nodular goiter (n=26) and use of iodine-contrast medium within 2 months (n=32). Thyroid function tests were checked within 14 days before the thyroid SPECT/CT. Twenty minutes after injection of Tc-99m pertechnetate (185 MBq), planar scan (1 minute) and quantitative SPECT/CT (1 minute) were sequentially performed. T/B ratio was calculated by putting rectangular region-of-interest over the thyroid and left upper chest as background on the planar scan, whereas %uptake of the thyroid was obtained by manually segmenting thyroid on the quantitative SPECT/CT. Graves’ disease was defined as hyperthyroidism with elevated TSH-receptor antibody, while destructive thyroiditis was thyrotoxicosis without TSH-receptor antibody elevation. Univariate and multivariate logistic regression analyses were conducted for the diagnostic accuracy estimation.

Results: Of the newly diagnosed 298 cases of thyrotoxicosis (female:male=206:92, mean age = 46.15 [range : 19-89]), 142 (=47.7%) were Graves’ disease, 139 (=46.6%) were destructive thyroiditis, and the remaining 17 (=5.7%) were indeterminate. %uptake was significantly greater in Graves’ disease (mean±std; 5.63±5.64%) than in destructive thyroiditis (0.21±0.20%) (p<0.0001 by Mann-Whitney test). T/B ratio was also significantly higher in Graves’ disease (13.47±10.06) than in destructive thyroiditis (2.25±0.62) (p<0.0001 by Mann-Whitney test). For the diagnosis of Graves’ disease, %uptake (p<0.0001) and T/B ratio (p<0.0001) were significant variables by univariate analyses, but only %uptake remained significant by multivariate analysis (p<0.0001, odds ratio (OR) 9.2941 with 95% confidence interval (CI) 5.2386-16.4891). For the diagnosis of destructive thyroiditis, %uptake (p<0.0001) and T/B ratio (p<0.0001) were significant in univariate analyses, but again only %uptake was a significant variable by multivariate analysis (p<0.0001, OR 0.0021 with 95% CI 0.0003-0.0128). With excluding the indeterminate 17 cases, the results were almost the same.

Conclusions: Absolute %uptake from quantitative SPECT/CT is more accurate than relative uptake ratio from traditional planar scan regarding the diagnosis of Graves’ disease and destructive thyroiditis.