Tailored medicine is a customary concept in current medical practice, including nuclear medicine, which uses radiotracers that bind to specific molecular targets according to a patient's clinical situation or pathology. Once molecular images confirm high tumoral uptake of a specific radiotracer, a therapeutic option becomes available by replacing the radioactive isotope for another radioactive molecule, with higher ionizing capacity. This allows molecular guided radiotherapy, also known as theranostics (therapy and diagnostic), to be used in different pathologies where nuclear medicine plays an important role. The aim of this review will be to explore the following conditions: thyroid diseases and radioiodine; neuroendocrine tumors and radiolabeled somatostatin analogues; and prostatic cancer and radiolabeled prostatic specific membrane antigen (PSMA).