Objective <p>To investigate the relationship between non-suicidal self-injury (NSSI) and thyroid hormone levels in female adolescents with mental disorders and to provide a theoretical basis for psychiatrists to identify and intervene in NSSI.</p> Methods <p>In this study, we enrolled 289 female adolescents diagnosed with NSSI (NSSI group) according to the Diagnostic and Statistical Manual of Mental Disorders (5th edition), alongside a control group of 285 healthy female adolescents. The clinical data of the study subjects were collected and the levels of thyroid hormones were determined. The thyroid function of the subjects was evaluated according to the levels of FT3, FT4, T4, T3 and TSH, and the differences between the NSSI group and the control group were analyzed. The Ottawa Self-Injury Inventory was used to evaluate the self-injury status of the study subjects, and the HAMD-17 was used to assess the degree of clinical depression.</p> Results <p>In this study, the TSH level in the NSSI group was higher than that in the control group (<i>P</i> &lt; 0.01), and the T3, T4, and FT3 levels were lower than those in the control group (<i>P</i> &lt; 0.05). The HAMD-17 score in the NSSI group was higher than that in the control group (<i>P</i> &lt; 0.01), and the score on the self-injury questionnaire in the NSSI group was higher than that in the control group (<i>P</i> &lt; 0.01).</p> Conclusion <p>NSSI in female adolescents with depression or bipolar disorder may be associated with low thyroid hormone levels.</p>