As color doppler ultrasound listed as the thyroid to regular physical examination project,the positive detection rate of benign and malignant thyroid diseases has increased year by year.The population of pa-tients after thyroid surgery is getting larger and larger,however,the research on how to replace the absent thyroid with exogenous thyroid hormone is not perfect.As the switch of thyroid hormone activation and deactivation,deio-dinase plays an important role in maintaining the internal environment of human body.The expression of these en-zymes will change with the different needs of each organ in the individual.In the past,many authors have made a detailed description of deiodinase in basic medicine,but the principle of action in clinical aspects is relatively lack-ing.In this paper,the mechanism of deiodinase was reviewed through literature reading,to explore the basic princi-ple of deiodinase in exogenous thyroid hormone supplementation after thyroid surgery.