Objective To investigate the constituent changes of hospitalized thyroid cancer patients in Changhai Hospital(Shanghai, China) during nearly 20 years of universal salt iodization (USI), so as to understand the relationship between iodine intake and thyroid carcinoma. Methods We analyzed the patients with thyroid diseases in Changhai Hospital over the past 20 years before and after the adaptation of USI, and the annual constituent ratios of thyroid carcinoma were calculated. The patients with thyroid diseases were divided into four groups according to the years they were treated: 1993-1997 (group A), 1998-2002 (group B), 2003-2007 (group C), and 2008-2012 (group D). The constituent ratios of thyroid carcinoma were compared between different groups. Results After the adoption of universal salt iodization in 1994, the constituent ratio of thyroid carcinoma decreased from 11.36%(25/220) in 1993 to 7.59%(18/237), and then the ratio gradually increased again, reaching 20.76%(318/1 532) in 2012. The constituent ratios of thyroid carcinoma in group A, B, C and D were 8.01%(112/1 398), 9.61%(241/2 508), 11.65%(501/4 299) and 16.57%(1 101/6 645), respectively. The constituent ratios of thyroid cancer in A, B groups (initial salt iodization period) were not significantly different (P=0.095); while those in C, D groups (after long-term salt iodization) were increased significantly compared with those in A, B groups(P<0.001). Conclusion Short-term iodine supplementation may have a certain inhibitory effect against thyroid carcinoma. The detection rate of thyroid carcinoma is apparently increased 10 years after iodine supplementation; long-term iodine supplementation might be one of the reasons for the increase.