The normal value of free thyroxine is 11.5-22.7 pmol/L (chemiluminescent method), and the reference range may vary from hospital to hospital. Measurement of free thyroxine can be used to understand thyroid function. Serum free thyroxine (FT4) is a sensitive indicator of thyroid function in vitro test. It can reflect thyroid function more accurately even if plasma thyroid-binding protein binding and concentration are altered, and it has a special diagnostic value for pregnant women and oral contraceptive patients’ thyroid function. FT4 is small in the blood, and the normal value range is 11.5-22.7 pmol/L. The reference value range may be different in different hospitals. Patients are advised to consult their local hospitals for details.