Radioiodine therapy of thyroid cancer was the first successful radionuclide therapy in the treatments of cancer, although its clinical use is empirical and not based on precise dosimetry. 124I is a positron-emitting radionuclide and positron emission tomography/computed tomography (PET/CT) with 124I currently provides the most accurate estimation of the absorbed (radiation) dose to thyroid cancer lesions. In the application, serial 124I PET/CT scans are performed to determine the time uptake curves and to delineate the volumes of the lesions. The 124I data are then used to project the absorbed dose per unit administered 131I activity. The results are part of the decision-making process to individually guide treatment plans, in particular by tailoring the therapeutic 131I activity in radioiodine therapy. The aim of this review is to provide an overview of 124I PET/CT lesion dosimetry of differentiated thyroid cancer including: 1) an historical overview; 2) the general properties of 124I and its activity measurement; 3) the main factors impairing PET image quantification; 4) an optimized lesion dosimetry protocol used in our group to make this manuscript self-contained; as well as 5) a summary of important clinical studies.