Isotope thyroid function testing for many years, found that there are many patients with severe disease, low T3, or even T4 reduced, while TSH normal phenomenon, for such laboratory results, we are review after sending the report, but often the results are still the same after review, after reviewing the literature know that this is —– low T3 syndrome. The information will be collated and shared as follows: In various non-thyroidal systemic diseases, from time to time there will be lower blood T3, lower or normal T4, TSH is normal, generally called normal thyroid function pathological syndrome, and will be low T3, T4, TSH normal state called low T3 syndrome. Low T3 syndrome is seen in chronic liver disease, acute or chronic infections, kidney disease, heart and lung disease, tumors, post-surgery and trauma; it is also seen in some wasting diseases such as uncontrolled diabetes, high-dose glucocorticoid therapy, too few calories in the diet, starving patients and newborns at birth. It is generally believed that low T3 syndrome is a protective adaptation mechanism of the organism in critical illness, and that reduced serum T3 reduces the metabolism of liver, kidney, heart, skeletal muscle and other tissues that depend mainly on circulating T3, which facilitates energy conservation and reduces consumption. However, some experts believe that low T3 syndrome is an injurious outcome of critical illness. Low T3 syndrome is the result of the effect of severe disease on thyroid hormones. The measurement of T3, T4 and rT3 concentrations, especially the ratio of T3/rT3, is an important reference for the evolution of the disease, the observation of the efficacy and the prognosis. When the disease is critical, the decrease of T3 and T4 will occur, reflecting the poor prognosis of the disease. However, some scholars believe that replacement therapy with an appropriate amount of eugenol may be an ideal method, but further research is needed to determine whether this conclusion is correct.