Objective   To evaluate the biochemical and structural changes of apatinib in patients with progressive radioactive iodine-refractory differentiated thyroid cancer (RAIR-DTC).

  Methods   The participants (n=10) were followed up since March 2016. Treatment effect was evaluated in using both biochemical thyroglobulin (Tg) and thyroglobulin antibody (Tg-Ab) and structural responses (target lesions, TL). Adverse events were also recorded over time.

  Results   The median follow-up was 7.9 months. The Tg level declined rapidly within 6 weeks after apatinib treatment, and the average decline ranged from 60% to 90%, indicating the immediate biochemical response of apatinib in progressive RAIR-DTC. The Tg level tended to stabilize thereafter. However, the Tg level rebounded by 4%–135% when withdrawal was performed for 3–14 days. The number of TLs decreased rapidly within 8 weeks, and the average decreased ranged from 40% to 60%, indicating the presence of rapid structural responses. Thereafter, the number of TLs continued to stabilize. TLs, in contrast to Tg, were not significantly affected by drug withdrawal. The rate of change in Tg (Tgvn) was positively correlated with the rate of change in TL (TLvn) TLvn=0.17×Tgvn+0.50 (r=0.56, P＜0.05). The apatinib dose was adjusted due to adverse events, which could be relieved after 3 to 14 days of withdrawal. Apatinib can effectively control the disease even at a reduced dose of 250 mg/d.

  Conclusion   Apatinib treatment showed a fast and sustainable biochemical and structural responses. Tg could be regarded as an objective indicator. Tgvn is positively correlated with TLvn, and the response of Tg is more sensitive than that of TLs.