Nuclear imaging scans are especially useful in the diagnostic evaluation of thyroid disorders such as thyroid nodules and thyroiditis. The rationale lies in distinguishing hyperfunctioning thyroid tissue (eg. “hot” thyroid nodules and Graves’ disease show accentuated focal and diffuse radionuclide uptake respectively) from “cold” thyroid nodules and thyroiditis associated with attenuated radionuclide uptake. The diagnosis of thyroiditis is often suspected based on negligible uptake of either technetium-99m (99mTc) or radioiodine (123I or 131I) on thyroid scintigraphy. Here, we wish to highlight a case that showed a high uptake in the thyroid gland on 18F-fluorodeoxyglucose (18F-FDG) positron emission tomography (PET) scan which turned out to be due to autoimmune thyroiditis. A 25-year-old asymptomatic woman without any history of medical illnesses had volunteered for a brown fat research study. 18F-FDG PET imaging revealed abnormally increased tracer uptake over the anterior neck. The images appeared strangely analogous to typical 99mTc or radioiodine uptake scans among patients with hyperthyroidism. Further investigative workup confirmed autoimmune thyroiditis. In conclusion, while many clinicians are aware that reduced thyroid radionuclide uptake usually signifies thyroiditis, the inclusion of autoimmune thyroiditis into the list of differential diagnosis based on accentuated thyroid uptake of 18F-FDG tracer remains counterintuitive for most of them. As the incidence and prevalence of autoimmune thyroiditis is high coupled with an increasing use of 18F-FDG-PET scanning for various indications, physicians should recognize that diffusely increased 18F-FDG uptake by the thyroid can result from autoimmune thyroiditis which needs follow-up and treatment to avert the morbidity of hypothyroidism.