Low thyroid stimulating hormone in pregnant women suggests the possibility of transient thyrotoxicosis of pregnancy, subclinical or clinical hyperthyroidism (hyperthyroidism is short for hyperthyroidism). 1. Transient thyrotoxicosis of pregnancy: It is caused by human chorionic gonadotropin stimulation of thyroid hormone receptors in the thyroid gland, and usually resolves at 14 to 18 weeks. 2. Subclinical hyperthyroidism: serum free triiodothyronine and free thyroxine are within the normal range, but the serum thyrotropin level decreases, and there may be no obvious symptoms. 3. Clinical hyperthyroidism: a decrease in serum thyroid stimulating hormone level and a high serum thyroid hormone level may cause tachycardia, fatigue, excessive sweating, irritability, insomnia and other symptoms. Pregnant women with low thyroid stimulating hormone are advised to go to the hospital and ask the doctor to make a judgment and clarify the cause of the disease.