Serum free thyroxine, which is thyroxine unbound to albumin, is not affected by substances in the plasma in laboratory tests and is more accurate than total thyroxine for understanding the functional status of the thyroid gland, and can be used to aid in the diagnosis of hyperthyroidism if it is increased. Increased serum free thyroxine is mainly used to diagnose hyperthyroidism, and if it is decreased it is mainly present in hypothyroidism, which can be seen in nephrotic syndrome. Serum free thyroxine can be used to assist in the diagnosis, and the concentration of the substance can be checked during the later treatment to identify the effect of the treatment, and the treatment can also be followed up and controlled. The normal value of serum free thyroxine is between 10.3 and 25.7 pmol/L (it may vary slightly from hospital to hospital depending on the test method) and is used as a reference value in laboratory tests to assist in the diagnosis of thyroid disorders.