FT3, FT4 and TSH are all indicators of the thyroid gland. Specifically, FT3 refers to serum free triiodothyronine, FT4 refers to serum free thyroxine, and TSH refers to thyroid stimulating hormone, and these three indicators reflect the function of the thyroid gland. If FT3 and FT4 are low and TSH is high, the body is considered to be in a state of hypothyroidism, a condition commonly seen in primary hypothyroidism, Hashimoto’s thyroiditis and late stage subacute thyroiditis. If this occurs, it is recommended to go to the hospital for a thyroid ultrasound and blood tests for TRAb, TGAb and TPO to further clarify the condition. If FT3 and FT4 are high and TSH is low, the body is considered to be in a hyperthyroid state, which is commonly seen in hyperthyroidism, often called hyperthyroidism, and pre-acute thyroiditis. If this happens, it is also important to visit a hospital for a thyroid ultrasound and a blood test for thyroid-related antibodies to further clarify the condition.