The main reason for iodine deficiency during pregnancy is that due to various endocrine and metabolic changes, the body’s demand for thyroid hormones increases, and the demand for iodine as a raw material for the synthesis of thyroid hormones increases. After the third trimester, the fetal thyroid gland gradually becomes capable of synthesizing hormones and the mother’s ability to transfer iodine via the placenta increases, making it more prone to iodine deficiency. Since 1995, China has been implementing universal salt iodization, but large domestic studies still show that 50% and 57% of pregnant women with normal thyroid function and subthyroidism have different degrees of iodine deficiency, respectively. The fetus forms its own thyroid gland around 3 months of gestation and gradually establishes independent pituitary-thyroid axis function. The placenta is less permeable to maternal T4 synthesis, and most of the thyroid hormones for neurological development are synthesized by the fetus itself, while the required iodine is provided by the mother. The U-shaped curve between iodine and thyroid function suggests that iodine deficiency and excess iodine are not only the main causes of subclinical hypothyroidism (hypothyroidism), but also the main cause of low FT4. The reason is that in iodine deficiency, pregnant women can only increase TSH secretion to increase thyroid hormone secretion, and when they cannot compensate, they may develop low FT4emia, while in iodine excess, the secretion of thyroid hormone will be suppressed, thus making TSH compensatingly increased. According to the latest WHO recommendations, at least 250μg of iodine should be taken during pregnancy (the total daily iodine intake is not recommended to exceed 500ug), and the latest guidelines for the diagnosis and treatment of thyroid disease during pregnancy in China also suggest that in addition to the normal diet, an additional 150μg of iodine should be taken daily to ensure the above-mentioned iodine intake (150ug of iodine can be provided by many commercially available vitamins for pregnant women). However, dietary supplementation is not recommended because of the tendency to oversupply iodine in seafood such as kelp and shrimp, and because excessive iodine intake can also cause TSH to rise. It is recommended that women who are able to do so should have a urine iodine test to help determine iodine supplementation.