Theranostics, the integration of diagnostic imaging and targeted radionuclide therapy, has transformed oncological practice. While receptor-targeted tracers such as somatostatin receptor PET/computed tomography (CT) and prostate-specific membrane antigen PET/CT anchor most theranostic workflows, 18F-fluorodeoxyglucose (FDG) PET/CT retains an indispensable role across the spectrum of theranostic diseases. FDG PET images glucose metabolism and thereby captures biological information that receptor-targeted tracers cannot. This review synthesizes current evidence, guideline recommendations, and emerging applications, including total-body PET, AI-assisted quantification, radiomics, and novel fibroblast activation protein inhibitor targeted theranostics, to define the evolving position of FDG-PET/CT as a gatekeeper in the theranostic oncological workflow.