High T3 and T4 may be the manifestation of hyperthyroidism, and the main causes of this manifestation include Graves’ disease, Hashimoto’s thyroiditis, subacute thyroiditis and pituitary tumor. 1. Graves’ disease: diffuse toxic goiter, clinically most hyperthyroidism is caused by Graves’ disease, is an autoimmune disease, TRAb abnormally high, leading to thyroid follicular cell damage, caused by the rise in blood levels of T3, T4 hormones. 2. Hashimoto’s thyroiditis: Hashimoto’s thyroiditis is a type of autoimmune thyroiditis. Abnormalities in thyroid autoantibodies, TPOAb and TgAb, are the main causative factors of Hashimoto’s thyroiditis. 3. Subacute thyroiditis: It is closely related to the pathogenic infection, which induces an autoimmune reaction and causes destruction of thyroid follicular cells, leading to an increase of hormones in the blood. 4. Pituitary tumor: Pituitary tumors secrete thyroid-stimulating hormones, which can promote the secretion of the thyroid gland, leading to excessive secretion of thyroid hormone levels, resulting in an increase in the corresponding indicators. High T3 and T4 levels require timely medical attention and standardized treatment under the guidance of clinicians.