Diffuse thyroid lesions are described by ultrasound or CT or MRI as diffuse lesions in the thyroid gland that are uniform and consistent changes. This can occur in many diseases, such as diffuse goiter with hyperthyroidism, as well as acute thyroiditis, subacute thyroiditis, and Hashimoto’s thyroiditis, all of which may have diffuse thyroid lesions. Diffuse thyroid lesions are described as lesions in the thyroid gland, but are not specific and are based on the patient’s clinical symptoms, signs, and relevant tests. If diffuse goiter with hyperthyroidism occurs, in addition to diffuse lesions, the patient will have hypermetabolic syndrome, such as panic attacks, sweating, fear of heat, and weight loss. Also thyroid function tests with elevated levels of thyroid hormones and decreased levels of thyroid stimulating hormones and some antibodies to the thyroid gland are correspondingly elevated, all of which support the diagnosis of hyperthyroidism. However, diffuse thyroid lesions can also be seen in patients with hypothyroidism, which presents with symptoms opposite to those of hyperthyroidism. Among other inflammatory conditions of the thyroid, such as subacute thyroiditis, a group of inflammatory conditions caused by a viral infection, patients can also develop diffuse lesions in the thyroid. Subacute thyroiditis due to viral infection is self-limiting and usually does not require special treatment and can heal on its own after about 3 months as long as symptomatic treatment is given. Therefore, when diffuse thyroid lesions occur, it is important to determine which type of diffuse thyroid lesion has occurred based on the patient’s clinical symptoms, signs, and laboratory tests, mainly ultrasound, thyroid hormone measurements, and antibody measurements. In addition, although most diffuse thyroid lesions are benign, regular follow-up is required for early detection of malignant nodules.