Low serum total thyroid hormone may be caused by low binding protein in blood or hypothyroidism. 1. Low serum total thyroid hormone with normal TSH, FT3 and FT4 may be caused by low binding protein in the blood, and it is recommended to have regular checkups without special treatment. 2. Low serum total thyroid hormone, low FT3 and FT4, and elevated TSH are indicative of hypothyroidism and should be treated with thyroid hormone replacement therapy under the guidance of a doctor. Therefore, if the serum total thyroid hormone is low, you should go to the hospital in time, improve the examination, clarify the cause of the disease, and carry out the treatment under the guidance of the doctor.