Objective To investigate the association between thyroidectomy and suicide attempt. Methods A nationwide population-based electronic medical records database of South Korea between January 1, 2009 and June 30, 2016 was used to investigate incidence rate ratios (IRRs) of suicide attempts and probable suicide attempts before and after thyroidectomy using a self-controlled case series design. Results In 2,986 patients who attempted suicide or probable suicide, the IRRs of suicidal behaviors during risk periods one year before and after thyroidectomy were investigated. Generally, after thyroidectomy, there was no increase in IRR compared to the non-risk period. When data were analyzed according to thyroidectomy type, after partial thyroidectomy, IRR increased up to 1.43 (95% CI: 1.03-1.98, p=0.032) in the days 91-181 period. In the subgroup with major depressive disorder (MDD), the IRR increased up to 1.74 (95% CI: 1.21-2.51, p=0.003) before thyroidectomy, and increased up to 1.67 (95% CI: 1.16-2.41, p=0.006) after thyroidectomy. Conclusion Although the general risk of suicide attempt was not increased after thyroidectomy, patients with MDD showed increased risk of suicide attempt before and after thyroidectomy. These results suggest that suicidality should be evaluated when depressive symptoms are present in patients who have undergone thyroidectomy.