Free T4 is commonly used to measure thyroid function and is normal when it is 18 pmol/L, which is within the normal reference value of 11.2~20.1 pmol/L. However, the reference value range may be different in different hospitals and with different reagents. This index can be used to assist in the diagnosis of various diseases causing hyperthyroidism and excessive release of thyroid hormones such as subacute thyroiditis. Free T4 is one of the items in the thyroid function test. When Graves’ disease, multinodular goiter, initial Hashimoto’s thyroiditis, pituitary disease and other diseases occur in the body, they can cause hyperthyroidism, resulting in an increase in free T4; if the thyroid gland is destroyed and causes excessive release of thyroid hormones, it will also cause an increase in free T4. When the result of free T4 is abnormal, you should go to the hospital in time, clarify the cause of the disease, and carry out active treatment under the guidance of the doctor.