Free thyroxine level is associated with both relapse rate and poor neurofunction in first-attack Neuromyelitis Optica Spectrum Disorder (NMOSD) patients

Background: To investigate whether the serum free thyroxine (FT4) level is a prognostic factor for the first-attack neuromyelitis optica spectrum disorders (NMOSD). Methods: This retrospective study enrolled 109 patients with first-attack NMOSD. The Expanded Disability Status Scale (EDSS) and the relapse rate were used to evaluate the outcomes. The logistic regression model was used to analyze the independent effects of FT4 on relapse and final EDSS. Kaplan-Meier analysis, scatter plot smoothing method, and two-phase piecewise linear regression model were used to investigate the relationship between the FT4 level and the relapse rate. Results: Multivariate analysis revealed that serum FT4 level might be a risk factor for both final EDSS (beta = 0.17; 95% confidence interval: 0.03-0.32) and the relapse rate (HR = 1.18; 95% confidence interval: 1.05-1.32). Furthermore, 1400 days after the onset, nearly 100% of patients in the high-FT4 group relapsed, while only 40% of the patients in the low-FT4 group relapsed. Finally, we found that the relationship between the FT4 level and the NMOSD relapse rate was nonlinear. The risk of NMOSD relapse increased with the FT4 level up to the inflection point of 12.01 pmol/L (HR = 1.45; 95% confidence interval: 1.06-1.98). When the FT4 level was > 12.01 pmol/L, there was no correlation between the FT4 level and the risk of NMOSD relapse (HR = 1.05; 95% confidence interval: 0.78-1.41). Conclusion: Serum FT4 level may be a prognostic indicator for the first-attack in patients with NMOSD. High FT4 levels are associated with poor neurofunctions and a high relapse rate in patients with the first-attack in patients with NMOSD.