It is not a big problem for a pregnant woman to have a somewhat low serum free T4 (free tetraiodothyronine). A somewhat low serum free T4 accompanied by a low T3 (free triiodothyronine) and an elevated TSH (thyroid-stimulating hormone) may be hypothyroidism, which may have a certain effect. The body’s need for thyroxine levels increases after pregnancy. Inadequate production by the body or insufficient intake of raw materials such as iodine affects the synthesis of thyroxine levels, resulting in thyroid insufficiency, as evidenced by low serum free T4. A low serum free T4 alone, while T3 and TSH are within the normal range, needs to be re-checked in 1-2 weeks for thyroid function and reassessment, which is usually not a problem. When the serum free T4 is a little low accompanied by low T3 and elevated TSH, it is necessary to consider the existence of hypothyroidism in the patient. Failure to treat it effectively and in time may cause the child’s mental retardation, short stature, growth retardation, and cretinism, which will have an impact on the child. After pregnancy check out with hypothyroidism, need to actively supplement thyroxine, regular review of thyroid function, to prevent oversupplementation of thyroxine, the disease is effectively controlled, the impact on the fetus is relatively small. Pregnant women’s serum free T4 is a little low, it is recommended to go to the hospital for examination and treatment.