Iodine-124 is a positron-emitting iodine isotope, enabling measurement of iodine uptake using positron emission tomography (PET). There is a number of situations where the use of (124)I-PET/computed tomography (CT) can improve the current clinical practice in the diagnosis and treatment of thyroid cancer. Firstly, (124)I-PET/CT can aid in the staging of patients, because of better detection of metastatic disease and measurement of metabolic tumour volume, and thus separate low-risk from high risk patients. Secondly, the much higher sensitivity and spatial resolution of PET compared to gamma scintigraphy can also improve detection of recurrent disease. Furthermore, (124)I-PET can be used for patient-specific radioiodine therapy radiation dosimetry. Simultaneous administration of the therapeutic dose of (131)I and a tracer dose of (124)I allows for accurate measurement of iodine uptake during therapy. The decay scheme of (124)I, with few positrons and many gamma rays emitted per decay, often simultaneously, poses a challenge to quantitative PET imaging. Improved correction methods and the use of last-generation PET/CT scanners with faster electronics and better energy resolution can overcome this.