Clinical significance of patterns of incidental thyroid uptake at 18F-FDG PET/CT

Incidental uptake of 2-[18F]-fluoro-2-deoxy-d-glucose (18F-FDG) in the thyroid gland is not uncommonly encountered in day-to-day practice of oncological 18F-FDG positron-emission tomography/computed tomography (PET/CT). These are often felt to be “nuisance lesions” by referring clinicians and radiologists alike. However, recognition of the importance of different patterns of FDG uptake in the thyroid gland and knowledge of the possible underlying aetiologies are crucial in ensuring that patients are managed appropriately in the clinical context of their primary diagnosis, as the underlying pathological condition may be clinically important in a significant minority of such cases. This review describes the various patterns of 18F-FDG uptake within the thyroid and discusses the clinical significance and possible impact on patient management. Incidental low-grade homogeneous diffuse increased thyroid 18F-FDG uptake is usually seen in the patients with chronic thyroiditis, Grave's disease, and hypothyroidism. Thyroid function tests and antibody profiling are advised in these patients. Incidental focal 18F-FDG thyroid uptake should raise the possibility of underlying malignancy. Ultrasound with or without fine-needle aspiration cytology is usually recommended for the evaluation of these lesions. Heterogeneous uptake with prominent focal uptake in the thyroid should be further evaluated to exclude malignancy.