Clinical significance of diffusely increased ¹⁸F-FDG uptake in the thyroid gland

Our purpose was to determine the clinical significance of diffusely increased ¹⁸F-FDG uptake in the thyroid gland as an incidental finding on PET/CT. Methods: All patients who were found to have diffuse thyroid uptake on ¹⁸F-FDG PET/CT in our institution between November 2004 and June 2006 were investigated and compared with an age- and sex-matched control group. The ¹⁸F-FDG uptake in the thyroid was semiquantified using maximum standardized uptake value and correlated to the available serum thyroid-stimulating hormone (TSH) and thyroid peroxidase (TPO) antibody levels using regression analysis. Results: Of the 4,732 patients, 138 (2.9%) had diffuse thyroid uptake. Clinical information was available for 133 of the 138 patients. Sixty-three (47.4%) had a prior diagnosis of hypothyroidism or autoimmune thyroiditis, of whom 56 were receiving thyroxine therapy. In the control group, consisting of 133 patients with no thyroid uptake, there were 13 (9.8%) with a prior diagnosis of hypothyroidism, 11 of whom were receiving thyroxine therapy. In the study group, 38 (28.6%) of 133 patients did not undergo any further investigation for thyroid disease, whereas 32 (24.1%) of 133 patients were examined for thyroid disease after PET. Nineteen were found with autoimmune thyroiditis or hypothyroidism, and replacement therapy was initiated in 12. No significant correlation was found between maximum standardized uptake value and TSH (P = 0.09) orTPO antibody (P = 0.68) levels. Conclusion: The incidental finding of increased ¹⁸F-FDG uptake in the thyroid gland is associated with chronic lymphocytic (Hashimoto's) thyroiditis and does not seem to be affected by thyroid hormone therapy. SUV correlated neither with the degree of hypothyroidism nor with the titer of TPO antibodies.