The stage of thyroid cancer includes four stages: 1. Stage I: differentiated thyroid cancer before 45 years old without other metastases, tumor less than 2 centimeters; 2. Stage II: differentiated thyroid cancer before 45 years old with brain metastases and lung metastases, tumor more than 4 centimeters; 3. Stage III: tumor more than 4 centimeters with distant metastases; 4. Stage IV: differentiated thyroid cancer after 45 years old, tumor more than 4 centimeters with bone metastases, undifferentiated thyroid cancer.