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Introduction Fibrotic interstitial lung disease (f-ILD) has a high mortality, yet no cure. The causal role of short telomeres in f-ILD is well-established. 1 Shorter telomere length is associated with lower serum sex hormone concentrations. STARSHIP study demonstrated lower bioavailable oestrogen and testosterone in patients with f-ILD compared with age/sex-matched controls, and an association of lung function with sex hormone levels.2 We sought to understand the impact of low testosterone on survival outcomes in patients with f-ILD.Methods 102 patients with f-ILD attending a regional ILD clinic were recruited between March 2022-February 2023. Serum testosterone levels were measured at baseline. Date of death was recorded to June 2025.Kaplan-Meier and Cox proportional hazard models were used to compare male patients with f-ILD grouped by normal versus low free testosterone (<225pmol/L) and female patients grouped by free androgen index (FAI). Cox regressions were adjusted for age, forced vital capacity (FVC) and lung diffusion capacity (DLCO).Results Mean free testosterone concentration was lower in male patients (N=80; 211pmol/L [95% CI: 192–230]) than age/sex-matched controls (265pmol/L [95% CI: 243–287]). By June 2025, 31/50 (62%) male patients with low testosterone and 9/30 (30%) with normal testosterone had died. Kaplan-Meier analysis demonstrated a significant difference in survival between the two groups (log-rank test Chi 2=6.6, p=0.0092).In Cox proportional hazard modelling, low testosterone increased risk of death (HR=2.23, p=0.038) when accounting for expected variables (baseline age, FVC and DLCO). Survival was influenced by an interaction between DLCO and low testosterone that varied with time (p=0.027).The study was under-powered to analyse female patients (N=22); however, dichotomising by group mean FAI, 8/12 (67%) female patients below vs 3/10 (30%) above the mean had died, although did not reach statistical significance.Conclusions Low testosterone in males with f-ILD increases risk of mortality compared to male patients with normal testosterone, including when adjusting for age, FVC and DLCO. Similar findings are suggested in female patients; more data is required for confirmation. These findings may inform future research into new treatment options for f-ILD.References Duckworth, et al. Lancet Respiratory Medicine. 2020. https://doi.org/10.1016/S2213-2600(20)30364-7.Duckworth, et al. A3695-A3695. 10.1164/ajrccmconference.2024.209.1_MeetingAbstracts.A3695.Abstract S17 Figure 1Kaplan-Meier survival estimates for male patients with f-ILD compared by free testosterone from trail recruitment to census date June 2025