Although there have been many studies on the effects of the gut microbiota on host thyroid hormone synthesis and secretion as well as on the potential association between gut microbiota and HT, there have been no systematic studies on the gut microbiota trends during the gradual transition from healthy people to different stages of HT development or on the correlation between changes in the gut microbiota during this process and different environmental factors, such as age, gender, and hormones. This evidence concerns the gene TG and hematocrit.