Low triiodothyronine can be considered to be caused by primary hypothyroidism, Hashimoto’s thyroiditis, subclinical hypothyroidism, etc. The prognosis of active treatment is good and usually there will not be any serious consequences, but if the treatment is not standardized, it may induce serious complications such as mucus edema and coma, and even threaten the life. It is recommended to follow the doctor’s advice to actively treat. 1. Primary hypothyroidism: there may be low triiodothyronine, accompanied by elevated thyrotropic hormone, etc. After taking levothyroxine sodium tablets as prescribed by the doctor to control the level of thyroid hormone in a reasonable range, the prognosis is better, and there will be no serious consequences; however, non-standardized treatment may induce heart failure, mucous oedema coma and other serious complications, and even threaten the lives of the patients. 2. Subclinical hypothyroidism: If the patient is accompanied by symptoms of hypothyroidism, positive TPOAb, dyslipidemia or atherosclerotic disease, thyroid hormone treatment should be given; if the patient is not accompanied by the above conditions, regular monitoring of TSH changes should be carried out under the guidance of the doctor. It should be noted that some subclinical hypothyroidism may progress to clinical hypothyroidism. 3. Hashimoto’s thyroiditis: It can be accompanied by hypothyroidism, and the symptoms can be improved by taking levothyroxine sodium tablets, which usually does not cause serious consequences. It is advisable to go to the hospital and ask the doctor to make a judgment if you have a low triiodothyronine level, rather than making a blind judgment on your own.