Triiodothyronine (T₃) is a thyroid hormone secreted by the thyroid gland, which is a key indicator of thyroid function. A low level of this indicator and a normal level of the rest of the indicators may be a normal phenomenon; or it may be caused by disease. Whether treatment is needed cannot be generalized. 1. Normal phenomenon: during the application of glucocorticoids and iodine-containing drugs to treat the disease, the T₃ value may decline, in which case treatment is generally not required, and timely consultation with the doctor to adjust the program can be. 2. Diseases: Decreased T₃ values may also occur in mucous edema, cretinism, chronic thyroiditis, decreased binding capacity of thyroid-binding globulin, and low T₃ syndrome in non-thyroidal diseases, which need to be judged in combination with the indicators and the patient’s performance. Therefore, it is not possible to generalize whether treatment is needed for low triiodothyronine but normal others. It is recommended to consult a doctor in time, make comprehensive judgment under the guidance of the doctor, make a clear diagnosis, and target treatment.