The study is a multi-center prospective cohort study of active surveillance in papillary thyroid cancer with low risk. The purpose of the study is to observe natural course of low risk papillary thyroid cancer in Korean population, and comparison of prognosis between active surveillance group and conventional surgery group. Patient will be well informed about their choice of active surveillance or surgery.

Inclusion Criteria:

• who agreed to the agreement form
• who is older than 18 years old
• who was diagnosed by fine needle aspiration or needle biopsy of Bethesda category V or VI
• whose thyroid cancer is smaller than 1 cm length

Exclusion Criteria:

• who cannot routinely follow-up according to the study schedule
• who was diagnosed by fine needle aspiration or needle biopsy of Bethesda category I, II, III, or IV.
• who have hyperthyroidism which needs treatment

Patient aged older than 18 years with papillary thyroid microcarcinoma less than 1 cm in length, who did not meet the exclusion criteria from Seoul National University Hospital, National Cancer Center, Seoul National University Bundang Hospital, and Borame Medical Center.