Objective: To investigate the direct radiation damage to the parathyroid glands and impact on the function of parathyroid glands due to 131I therapy after surgery for differentiated thyroid carcinoma, as well as the timing for postoperative 131I therapy. Methods: The clinical data of 281 patients with differentiated thyroid carcinoma undergoing the first postoperative 131I thyroid remnant ablation from January 2013 to February 2015 were retrospectively analyzed. According to parathyroid hormone level before 131I treatment, 238 patients had normal function of parathyroid glands and 43 patients had mild hypoparathyroidism. The serum levels of calcium and parathyroid hormone of the patients on postoperative day (POD) 1 and 6, as well as before, 1 week and 3 months after 131I therapy were determined and analyzed. Results: All patients had no manifestations of hypocalcemia before 131I therapy. In patients with postoperative normal function of parathyroid glands, the overall difference in serum calcium level was statistically significant among different time points (F=6.912, P<0.05), and the serum calcium showed the lowest level on POD 1 (P<0.05), while it showed no statistical difference among the other 4 time points (P>0.05); the overall difference in serum parathyroid hormone level was statistically significant among different time points (F=16.808, P<0.05), and the parathyroid hormone reached the lowest level on POD 1, then increased before 131I therapy, and reduced again 1 week after 131I therapy, and then, increased again 3 months after 131I therapy. In patients with postoperative mild hypoparathyroidism, hypocalcemia occurred with different degrees on average POD 7.5 d after 131I therapy; the overall difference in serum calcium level was statistically significant among different time points (F=37.710, P<0.05), and the serum calcium showed the lowest level on POD 1 and 1 week after 131I therapy; the overall difference in serum parathyroid hormone level was statistically significant among different time points (F=29.082, P<0.05), the serum parathyroid hormone showed the lowest level on POD 1, then increased before 131I therapy, and reduced again one week after 131I therapy, which approached the level on POD1 (P>0.05), and rose again 3 months after 131I therapy, which averagely reached the normal range. Conclusion: 131I thyroid remnant ablation exerts direct radiation damage to parathyroid glands, may cause hypoparathyroidism and aggravate the existing hypoparathyroidism. For patients with hypoparathyroidism after surgery for differentiated thyroid carcinoma, 131I thyroid remnant ablation is recommended to be delayed until the parathyroid hormone has completely recovered to normal level.