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, eating too few calories, starving patients and newborns at birth. It is generally believed that low T3 syndrome is a protective adaptation mechanism of the body in critical illnesses, and that the decrease in serum T3 reduces the metabolism of liver, kidney, heart, skeletal muscle and other tissues that are mainly dependent on circulating T3, which helps to conserve energy and reduce consumption. However, some experts believe that low T3 syndrome is a damaging consequence of critical illness. Low T3 syndrome is the result of the effect of a serious disease on thyroid hormones. Measurement of T3, T4 and rT3 concentrations, especially the T3/rT3 ratio, is an important reference for the evolution of the disease, the observation of the outcome and the prognosis. The decrease of T3 and T4 when the disease is critical reflects the poor prognosis of the disease. However, in recent years, some scholars believe that replacement therapy with appropriate amounts of eugenol may be an ideal method, but further research is needed to determine whether this conclusion is correct.