Low triiodothyronine syndrome (LT3S), which also named as normal thyroidal syndrome or non-thyroidal illness syndrome, refers to a variety of stress state of thyroid hormone metabolism disorders. LT3S is characterized by decreased total serum T3 and free T3, increased serum reverse triiodothyronine and normal thyroxine and thyrotropin in laboratory and no symptoms of thyroid dysfunction in clinic. It was reported that, severe disease patients were often along with abnormal thyroid hormone levels, in which LT3S was the most common and closely related with the prognosis and long-term survival rate of patients. Since the important significance of LT3S in clinical work, the pathogenesis of LT3 and the relationship between LT3S and the related diseases are reviewed in the paper.