Ft3, or FT3, is free triiodothyronine in serum, referred to as free T3, and belongs to one of the thyroid hormones. low FT3 is generally evaluated in combination with thyroid stimulating hormone, TSH, and serum free T4, FT4 levels, etc. low FT3 may be associated with hypothyroidism, low T3 syndrome, and chronic lymphocytic thyroiditis. 1. thyroid function Hypothyroidism: It is manifested by increased TSH in the serum, but total serum thyroxine (TT4) will appear to be reduced along with FT4, and the increased TSH and reduced levels of TT4 and FT4 correlate with the extent of the condition. Serum total triiodothyronine (TT3) and FT3 are normal in the early stages and decreased in the late stages, as FT3 is mainly derived from the conversion of FT4 in peripheral tissues and is therefore only used as a reference indicator. The goal of treatment is to restore serum TSH and thyroid hormone levels to within the normal range, and levothyroxine sodium tablets are generally given for treatment, often requiring lifelong administration; 2. Low T3 syndrome: also known as non-thyroidal morbid syndrome, refers to syndromes in which non-thyroidal disease causes FT3 to decrease, such as severe diabetic disease, surgical trauma, etc. The main manifestations are reduced levels of TT3 and FT3, and the degree of FT3 reduction is generally related to the severity of the disease, mainly for the treatment of the original disease, such as the application of insulin to control blood sugar as prescribed by the doctor, or the postoperative diet of eggs, milk and other high-quality protein to promote recovery; 3, chronic lymphocytic thyroiditis: also known as Hashimoto’s thyroiditis, the pathogenesis is unclear, and is currently considered to belong to a polygenic The pathogenesis is not clear, but is considered to be polygenic. If only FT3 is low and the rest of the indicators are normal, no special treatment is needed. If there are no obvious clinical symptoms and normal thyroid function tests, patients can usually be observed on a follow-up basis; 4. Other causes: elderly people may have a decrease in FT3 and FT4 after using drugs such as phenytoin sodium, dopamine and glucocorticoids. Such people should change or stop the medication under the guidance of the doctor, after which such indicators can gradually rise.