Low free T4 is because the body itself thyroid epithelial follicular cell secretion of a variety of thyroxine content is reduced, but also may be iodine-deficient areas of the human body iodine deficiency in the manufacture of T4 iodine is insufficient, resulting in the human body by the thyroid hormone effect of the various target organs and tissues functioning abnormally, indicating that the human body appeared in the symptoms of hypothyroidism. It may also be due to hyperthyroidism patients take too much medication, beyond the body’s own adaptive range, resulting in the body’s thyroid hormone content is too small and the free T4 low, in addition to the thyroid system diseases, other organs in the body can also lead to disease. Diseases of the liver system in the human body can lead to the lowering of the immune system in the human body, the content of various kinds of immune cells continues to decrease, and various kinds of inflammatory cell reactions occur frequently in the human body, resulting in the thyroid gland being affected, and low T4 occurs secondary to the disease.