Free thyroxine, FT4, is a hormone secreted by the thyroid gland and plays a very important role in maintaining body functions. It can be elevated in the following cases: i. Hyperfunction of the thyroid gland itself, such as diffuse toxic goiter or high-functioning adenoma. In this case, the thyroid gland secretes more FT4 and its level will be increased. Second, when the thyroid gland is damaged or injured, such as septic thyroiditis, subacute thyroiditis, damage to the thyroid gland or surrounding tissue during surgery, may cause destruction of the thyroid gland, causing the release of hormones from the follicles into the blood, and the corresponding FT4 level will increase. Some autoimmune thyroiditis, such as Hashimoto’s thyroiditis, can be characterized by thyrotoxicosis at a certain stage, when thyroid hormone levels are high. Fourth, some specific types of hyperthyroidism with unknown causes may have elevated FT4 alone.