Fluorodeoxyglucose-avid thyroid incidentaloma

Positron emission tomography–computed tomography (PET-CT) has become a standard investigation oncology workup.18F-fluorodeoxyglucose (18F-FDG) is the main radiopharmaceuticals for most of the studies. Since malignant cells overexpress glucose transporter receptors, they internalize FDG, and FDG undergoes the first rate-limiting step of glycolysis to form FDG-6-phosphate. In a whole-body PET-CT study, a lot of incidental findings are noticed and often bear significant clinical relevance. A normal thyroid gland does not show any demonstrable FDG uptake. However, sometimes, a normal-looking thyroid gland shows up increased FDG uptake in a diffuse manner in both lobes without any obvious focal FDG-avid lesion. This is attributed to autoimmune lymphocytic thyroid diseases and hypothyroidism. A hyperthyroid gland may appear diffusely enlarged with intense FDG uptake throughout the gland suggesting the hyperfunctioning of its follicular cells. However, any focal FDG uptake in either lobe of the thyroid needs an evaluation, particularly if there is any hypodense nodule on corresponding CT study. It is observed that FDG-avid thyroid nodules can harbor malignancy in almost 33% of cases. Hence, any FDG-avid incidentaloma needs an image-guided fine-needle aspiration cytology to rule out occult malignancy.