Thyroid hormone is a hormone secreted and synthesized by the follicular cells of the thyroid gland. When the function of the thyroid follicular cells decreases, a deficiency of thyroid hormone occurs, which is reflected in a decrease in the level of free thyroid hormone when a thyroid function test is performed. The specific causes of thyroid hormone deficiency are as follows: First, Hashimoto’s thyroiditis, a disease that causes damage to the thyroid cells due to high antibody levels. Second, thyroid surgery, many thyroid nodules or cysts, or adenomas, after the removal of most of the thyroid gland, can also lead to a lack of thyroid hormones. Third, after iodine-131 treatment for hyperthyroidism, most of the thyroid cells are destroyed, which can also lead to thyroid hormone deficiency. Fourth, abnormalities in pituitary gland function reduce the secretion of thyroid stimulating hormone and thus thyroid hormone secretion.