The normal range of free thyroxine (FT4) is generally considered to be 10.3-25.7 pmol/L. However, the reference index varies from hospital to hospital due to the influence of test methods and reagents. Meanwhile, FT4 is affected by many factors, so it should be analyzed specifically. FT4 is the free type of thyroxine in serum, serum FT4 enters the tissue cells to play its physiological role.FT4 is not affected by plasma thyroxine-binding globulin, and can accurately reflect the function of the thyroid gland. It is one of the sensitive indicators for detecting thyroid function in vitro. FT4 level is an important reference indicator for determining abnormal thyroid function, and other reference indicators include thyrotropin (TSH), free triiodothyronine (FT3) and so on. FT4 measurement methods include radioimmunoassay, chemiluminescence, electrochemiluminescence, etc. The reagents and units used in each method are different, and their normal ranges are also different. FT4 can be affected by a variety of factors. For example, FT4 is often elevated in patients undergoing heparin therapy, while FT4 generally continues to decline during pregnancy. FT4 is also affected by a variety of medications, such as propranolol and amiodarone. Therefore, if a patient obtains an FT4 measurement report, he or she should consult a hospital in a timely manner to interpret the results under the guidance of a specialist and to guide treatment.