The thyroid gland is an important endocrine gland in the body, and the main hormone secreted by the thyroid gland is thyroid hormone. The body regulates the secretion of thyroid hormones through thyrotropin, or TSH, which is secreted by the pituitary gland. Therefore, any factor that affects the hypothalamic-pituitary-thyroid axis can cause insufficient thyroid hormone secretion. The main causes are as follows: 1. Hypothalamic and pituitary lesions. Secondary hypothyroidism occurs when lesions occur in the hypothalamus or pituitary gland, such as pituitary tumors, resulting in a decrease in TSH secretion by the adenohypophysis, which leads to a decrease in T3 and T4 secretion by the thyroid gland secondary to hypothyroidism. The cause of secondary hypothyroidism lies in the hypothalamus or pituitary gland, while the thyroid gland itself is normal. 2. Thyroid hypothyroidism, also known as primary hypothyroidism, is mostly seen in lesions of the thyroid gland itself, such as autoimmune thyroiditis. After surgery or iodine-131 treatment for hyperthyroidism, the destruction of the thyroid gland can lead to insufficient secretion of thyroid hormones, resulting in primary hypothyroidism, mainly manifested as elevated TSH and reduced T3 and T4.