Objective To investigate the levels of free triiodothyronine（ FT3）， free thyroxine（ FT4） and thyroid stimulating hormone（ TSH） in patients with depression of different severity. Methods The 105 first episode depression inpatients in the First Specialized Hospital of Harbin from September 2017 to April 2018 and the employees who participated in the physical examination in the hospital in October 2017 were selected as the research subjects. The depression patients were divided into 3 sub-groups according to the Hamilton Depression Scale（ HAMD-24） score， namely mild depression group（ n=33）， moderate depression group（ n=44） and severe depression group（ n=28）. The levels of FT3， FT4 and TSH were tested by Autolumo A2000. Results There was no significant difference in TSH among the four groups（ F=0.687，P=0.561）. The FT3 level of the severe depression group was higher than that of the healthy group， the mild depression group and the moderate depression group（ P ＜ 0.01）. There was no statistical significance in the difference of FT3 among the health group， mild depression group and moderate depression group（ P＞0.05）. The level of FT4 of the 3 depression groups were all higher than that of the health group（ P＜0.05）， however there was no statistical significance in the difference among the 3 depression groups（ P＞0.05）. Conclusions The relative level of FT4 in serum may indicate the existence of depressive symptoms， while the relative level of FT3 in serum of depressive patients may indicate the severity of the disease.