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Background Vitamin D is essential to healthy immune system function, and deficiency has been implicated in the pathogenesis of autoimmune disease. 1 Its role in autoimmune hepatitis (AIH) is unclear, but recent studies have linked severe deficiency to increased risk of disease flare, cirrhosis and mortality.2 3 Methods We analysed data from the prospectively recruited UKAIH cohort, for whom vitamin D, ALT, IgG and cirrhosis status were available at baseline (n=106), 12 months (n=75), or both (n=74).Biochemical remission was defined as both ALT and IgG below the upper limit of normal (ULN). For categorical analysis, vitamin D was sufficient (>20 ng/ml), insufficient (10–20 ng/ml) or deficient (<10 ng/ml), as per local laboratory values.Results 81/106 patients were female (76.4%); median age at diagnosis was 60.5 (18–83) years. All but 1 patient had abnormal ALT and/or IgG at time of diagnosis and 15 (14.1%) were cirrhotic. At 12 months from diagnosis (n=75), 49 (65.3%) were in complete biochemical remission.At diagnosis, 72 patients (67.9%) had inadequate vitamin D levels: 26 (24.5%) were vitamin D deficient, 46 (43.4%) insufficient, and 14 (32.1%) sufficient. At 12 months, 23 patients (30.7%) had inadequate vitamin D levels; 2 patients (2.7%) vitamin D deficient, 21 (28%) insufficient, and 52 (69.3%) sufficient.Table 1 summarises the results assessing associations between vitamin D status, ALT/IgG level, cirrhosis and biochemical remission status at 12 months. Median vitamin D at baseline was significantly lower in cirrhotic than non-cirrhotic patients (p=0.036). Vitamin D level was not associated with disease activity or remission status at 12-months of follow-up. Over 12 months, 1 patient newly developed cirrhosis, and 1 patient died.Abstract P37 Table 1Summary of associations between vitamin D level and markers of disease activity and severity at baseline and 12 months ALT IgG Cirrhosis at presentation (Y/N) Remission at 12 months (Y/N) % change in ALT %change in IgG Baseline 12 months Baseline 12 months Baseline Vitamin D (continuous) p=0.221a p=0.224a p=0.036b p=0.879b Vitamin D (categorical) p=0.379c p=0.236c p=0.239d p=0.384d 12 months Vitamin D (continuous) p=0.676a 0=0.442a p=0.876b Vitamin D (categorical) p=0.277c p=0.090c p=0.144d Comparative % change in vitamin D (baseline to 12 months) p=0.821b p=0.579a p=0.503a Legend: a=Spearman’s rank, b= Mann-Whitney U, c=Kruskal-Wallis, d=Chi-squareConclusion To our knowledge, this is the 1st study to report prospective, longitudinal vitamin D measurement in incident patients with AIH. Our data showed lower vitamin D levels in cirrhotic patients at AIH diagnosis. However, vitamin D deficiency is well-recognised in cirrhosis of all aetiologies. We did not identify associations between vitamin D status/absolute values with either disease activity or biochemical remission status. It would be valuable to study a larger patient cohort with longer follow-up to hard outcome measures to further examine if there is any potential for vitamin D as a disease modifier.References Athanassiou L, Kostoglou-Athanassiou I, Koutsilieris M, et al. Vitamin D and autoimmune rheumatic diseases. Biomolecules. 2023;13:709 [Internet]. 2023 [cited 2025 Jun 5];13(4):709.Kilani Y, Alsakarneh S, Madi MY, et al. Autoimmune hepatitis and vitamin D deficiency: a nationwide perspective. Aliment Pharmacol Ther [Internet]. 2025 [cited 2025 Mar 4];61(4):682–692.Ebadi M, Bhanji RA, Mazurak VC, et al. Severe vitamin D deficiency is a prognostic biomarker in autoimmune hepatitis. Aliment Pharmacol Ther [Internet]. 2019 [cited 2025 Apr 23];49(2):173–182.