Association between serum TSH levels and all-cause mortality in critically ill patients

Thyroid dysfunction is common in critical illness and may influence prognosis. However, the value of TSH in patients with severe diseases remains unclear. The aim of this study was to investigate the association between TSH and the clinical prognosis of critically ill patients. Methods: This retrospective study identified patients who were admitted to the ICU in the Medical Information Mart for Intensive Care (MIMIC-IV) database (version 2.2). A total of 6432 patients were divided into four groups based on TSH quartiles (Q1, <0.92 mIU/L; Q2, 0.92-1.07 mIU/L; Q3, 1.07-3.10 mIU/L; Q4, >3.10 mIU/L). The clinical outcomes were defined as all-cause 7-, 30-, and 90-year mortality after ICU admission. Restricted cubic splines (RCSs) for nonlinear associations were generated to visualize the relationship between TSH levels and clinical outcomes. The survival differences among the four groups were also analyzed using Kaplan‒Meier curves and log rank tests. Univariable and multivariable Cox proportional hazards regression were further used to assess the association between TSH levels and clinical outcomes. Results: After multivariate adjustment, a U-shaped relationship was observed between TSH levels and all-cause 7-, 30-, and 90- mortality among patients with severe disease (all P < 0.05 for nonlinearity). The plot showed a risk reduction in the low range of TSH, which reached the lowest risk at approximately 2.9 μIU/mL and then increased thereafter. Compared with patients with Q3 TSH levels, those with Q1, Q2, and Q4 TSH levels had a significantly higher risk of all-cause 30-day mortality (Q1: hazard ratio, 1.28; 95% CI, 1.06-1.54; Q2: hazard ratio, 1.22; 95% CI, 1.01-1.48; Q4: hazard ratio, 1.25; 95% CI, 1.04-1.50). For all-cause 90-day mortality, only the Q4 group had a significantly higher mortality risk than the Q3 group (hazard ratio, 1.24; 95% CI, 1.07-1.44). In subgroup analyses, we found that Q1 TSH levels were associated with higher mortality risk in men and older (≥65 years) patients, while Q4 TSH had a greater risk in men and younger (<65 years) patients. Conclusions: TSH was significantly associated with all-cause 7-, 30-, and 90-day mortality in critically ill patients after admission to the ICU. TSH may serve as a valuable biomarker for risk stratification in critically ill patients.