Hypothyroidism is categorized into secondary hypothyroidism and primary hypothyroidism, in which secondary hypothyroidism affects milk, while primary hypothyroidism usually does not affect milk, and may affect milk in those with long-term primary hypothyroidism. Secondary hypothyroidism is hypothyroidism caused by hypothalamic pituitary lesions, mainly seen in patients with Schihan’s syndrome or pituitary tumors. In addition to a decrease in thyroid hormones, patients with this condition also have a decrease in prolactin, sex hormones, and adrenal hormones, which affects milk because of the decrease in prolactin. Primary hypothyroidism, mainly seen in Hashimoto’s thyroiditis, after radioactive iodine 131 treatment, after thyroid cancer surgery. In this case, if you take levothyroxine sodium tablets regularly for replacement therapy, it usually does not have much effect on milk and you can breastfeed normally. The pituitary gland is often enlarged in long-term primary hypothyroidism, which may be accompanied by increased prolactin and overflow, thus affecting milk. For patients with hypothyroidism, whether primary or secondary, it is recommended to consult a doctor for timely treatment.