scRNAseq of thyroid eye disease orbital fat demonstrates fibroblast thyroid hormone signalling and SPARC production

Introduction Thyroid eye disease (TED) is a debilitating extrathyroidal manifestation of Graves’ disease in which orbital inflammation, extracellular matrix overproduction, and adipose tissue expansion cause proptosis, disfigurement, and sight loss. There remains an unmet need for reliable serum biomarkers of active disease to enable earlier diagnosis and inform treatment decisions. We used single-cell RNA sequencing (scRNAseq) of orbital fat to characterise the pathological landscape of TED, then investigated candidate secreted mediators as circulating biomarkers across multiple independent clinical cohorts. Methods Orbital fat obtained from decompressions in four people with TED was profiled by scRNAseq. In three separate cohorts, serum levels of SPARC, fibronectin, IGF-1, and other serum proteins were measured by ELISA in healthy controls and participants with thyroid disease stratified by TED severity and validated in an independent cohort. Correlations with thyroid function tests (FT3, FT4, and TSH) and longitudinal changes over 48 weeks were also assessed. In additional orbital fat samples, SPARC protein distribution was characterised by immunofluorescence. Results scRNAseq identified orbital fibroblast subsets expressing thyroid hormone receptors (THRA, THRB), the T4/T3 transporter SLC16A2, DIO2, IGF1R, and extracellular matrix proteins including SPARC. Serum SPARC was significantly elevated in people with thyroid disease compared to healthy controls and was highest in those with moderate-to-sight-threatening TED across all three primary cohorts, with validation in an additional cohort. In moderate-to-sight-threatening TED, serum SPARC correlated positively with FT3 and negatively with TSH. SPARC and IGF-1 were positively correlated regardless of TED status. Longitudinal follow-up demonstrated that serum SPARC declined significantly by week 48 as clinical activity scores fell. Immunofluorescence confirmed heterogeneous SPARC expression by fibroblasts in both control and TED orbital fat. Discussion SPARC is highly expressed by orbital fat fibroblasts, its serum levels are elevated in active TED and correlate with both disease severity and thyroid hormone status. These findings suggest SPARC as a promising biomarker of active TED and provide evidence linking circulating T3 to orbital fibrotic processes, further supporting strict thyroid function control in TED management.