T3 refers to triiodothyronine and T4 refers to thyroxine, and blood tests for both thyroid hormones need not be performed on an empty stomach. T3 and T4 are amino acid hormones, with most of the T4 existing in a protein-bound form, while 80% of the T3 is converted from T4 in peripheral tissues. The day before the test, try to eat light food and do not smoke or drink. After 10 p.m., it is best not to eat anything else. If symptoms such as dizziness and fatigue occur after the test, you should lie down immediately, and after the symptoms are relieved, you can proceed to other programs. Patients do not need to fast if they undergo blood tests for T3 and T4 alone. If other hormone levels or liver and kidney function tests are needed at the same time, the decision of whether or not to fast will be based on the doctor’s instructions.