When thyroid function is reduced, the first manifestation is the increase of thyroid stimulating hormone tsh, followed by the decrease of T4. Therefore, in patients with hypothyroidism, tsh increases and T4 decreases, including free T4 and total T4. Of course, this refers to primary hypothyroidism and secondary hypothyroidism, that is, hypothyroidism caused by diseases secondary to hypothalamus or pituitary gland. For example, if the pituitary thyroid function is decreased, the patient's tsh will not increase, but will decrease. That is to say, secondary hypothyroidism, for example, pituitary hypothyroidism, tsh, T3 and T4 also decline, especially T4.