Abstract: Objective:To highlight the clinical characteristics of autoimmune thyroiditis that spontaneously developed from hyperthyroidism to hypothyroidism. Methods:The clinical data of two cases of autoimmune thyroiditis processing from hypothyroidism to hyperthyroidism was analyzed and literatures in Chinese and English were reviewed. Results:The two autoimmune thyroiditis patients had very similar medical history. They were first diagnosed with hypothyroidism and treated with levothyrocine as replacement therapy. However, two patients spontaneously developed to hyperthyroidism after using levothyrocine for 18 months and 9 months respectively. Their function of the thyroid returned to normal after giving anti-thyroid drugs. So far, there were only 52 cases reported in the documents, 17 were children and 35 were adults. These patients aged from 3 to 80 years old (mean age 32.8 years). The male-to-female ratio was about 1∶4. The primary disease was all autoimmune thyroiditis. Of all cases, 18 cases had not been detected with TRAb, TBSb and TSAb, and the other cases showed similar symptoms. During the phase of hypothyroidism, TPOAb, TGAb and TBAb increased significantly while TSAb was normal or decreased. In the phase of hyperthyroidism, TPOAb and TGAb maintained in the high level while TBAb decreased dramatically and TSAb increased steadily. In some cases only TRAb was tested, and TRAb was increased in all of them during the phase of hyperthyroidism. Conclusion:It is quite rarely seen that autoimmune thyroiditis progresses from hypothyroidism to hyperthyroidism, yet much attention should be paid to the clinical treatment. Though the mechanism of occurrence is unclear so far, it is probably closely related to TRAb. TBAb and TSAb testing at early stage might facilitate the early diagnosis, treatment afterwards and follow-up.