Hypodense foci are a manifestation of ultrasound or CT and MRI. On ultrasound, the density of a nodule in the liver or thyroid gland is lower than that of a normal liver or thyroid gland, so it is described as a hypointense foci. Hypodense foci are usually foreign bodies growing on a substantial organ, also known as a nodule. We cannot say that hypodense foci are benign or malignant, but we need to determine whether the border is clear, whether there is punctate calcification inside, and other tests. If the hypodense foci are suspected to be malignant, fine needle aspiration can be performed if necessary to obtain pathology for examination, and tissue biopsy can also be performed for pathological results. If the pathological result is benign, the patient can continue to be followed up. If the pathology is malignant, radical surgery is required.