Background: Prognostic markers reflecting nutritional vulnerability in idiopathic pulmonary fibrosis (IPF) remain poorly defined. Methods: In this prospective cohort study, 63 stable outpatients with IPF were followed for 3 years. Sarcopenia was defined according to the 2019 Asian Working Group for Sarcopenia criteria. Serum transthyretin levels were measured concurrently. Cox proportional hazards regression, binary logistic regression, and Kaplan–Meier survival analyses were performed. Results: During follow-up, 18 patients (29%) died and 21 (33%) experienced respiratory-related hospitalization. Serum transthyretin was an independent predictor of both 3-year mortality and respiratory-related hospitalization, even after adjusting for the Gender–Age–Physiology index. Conversely, sarcopenia and low appendicular skeletal muscle mass index (ASMI) were not independently associated with either outcome. Kaplan–Meier analysis demonstrated significant differences in both mortality and hospitalization according to serum transthyretin levels. Low ASMI evaluated using sex-specific cutoffs was associated with higher mortality in the unadjusted analysis, but not with hospitalization; sarcopenia was not significantly associated with either endpoint. Conclusions: Serum transthyretin may serve as a practical biomarker of nutritional vulnerability, providing complementary prognostic information beyond muscle mass-based assessment in IPF.

Background: Idiopathic pulmonary fibrosis (IPF) is a progressive lung disease associated with high mortality. Serum transthyretin and sarcopenia may influence patient outcomes; however, their long-term prognostic significance has not been fully elucidated. This study investigated whether serum transthyretin levels and sarcopenia predict mortality and respiratory-related hospitalization within 3 years in patients with IPF. Methods: This prospective pilot study enrolled 63 stable outpatients with IPF. Sarcopenia was defined according to the 2019 Asian Working Group for Sarcopenia criteria. Serum transthyretin levels were measured concurrently with the assessment of sarcopenia. Cox proportional hazards regression, binary logistic regression, and Kaplan–Meier survival analyses were performed. Results: The cohort had a mean age of 73.7 years and a mean percent predicted forced vital capacity of 82.3%. Twenty-four patients (38%) were diagnosed with sarcopenia. During the 3-year follow-up period, 21 patients (33%) experienced respiratory-related hospitalization, and 18 patients (29%) died. Serum transthyretin was identified as an independent predictor of both mortality and respiratory-related hospitalization within 3 years, whereas sarcopenia was not. Kaplan–Meier survival analyses stratified by serum transthyretin levels (cutoff value: 22.6 mg/dL) demonstrated significant differences in mortality (P = 0.018, log-rank test) and respiratory-related hospitalization (P = 0.008). Conclusion: Serum transthyretin independently predicts mortality and respiratory-related hospitalization within 3 years in patients with IPF, irrespective of sarcopenia status.