Hypodense foci in the thyroid are described by ultrasound. The presence of hypodense foci is usually a sign of a thyroid nodule. Thyroid nodules are very common and are generally benign lesions, but the nature of the nodule needs to be generally determined based on the description of the ultrasound. Generally, benign nodules show clear borders, intact envelope, uniform echogenicity, regular morphology, no punctate calcification foci, aspect ratio less than one, and no abundant blood flow. If several indicators are not consistent with benign nodules, close observation or thyroid aspiration is required to determine the nature of the nodule. Generally benign nodules can be observed, while malignant nodules or nodules that are too large will require surgery. In addition, subacute thyroiditis may also present in isolated cases as localized hypodense foci, usually with fever and transient hyperthyroidism, and sometimes a thyroid aspiration is required to identify them.