Objectives <p>To explore the relationship between the triglyceride-to-high-density lipoprotein cholesterol(TG/HDL-C) ratio and spinal deformity severity.</p> Methods <p>This retrospective study analyzed 159 preoperative spinal deformity patients from Guangzhou First People’s Hospital (2019–2025). Spinal deformity severity was assessed using Cobb angle measurements. Serum TG and HDL-C levels were measured to calculate the TG/HDL-C ratio. Multivariable logistic regression evaluated the association between the TG/HDL-C ratio and deformity severity. Subsequent restricted cubic spline (RCS) analysis with four knots examined potential nonlinear relationships. Receiver operating characteristic (ROC) curve analysis assessed the ratio’s predictive performance. Bootstrap validation with 1,000 resamples was used for internal validation. Comprehensive subgroup analyses with interaction tests evaluated the consistency and potential effect modification of observed associations. Three sensitivity analyses were performed to validate robustness.</p> Results <p>The significant inverse association was observed between TG/HDL-C ratio and spinal deformity severity. Multivariable regression confirmed a stable negative correlation after adjustment (OR = 0.44). Quartile analysis showed the highest ratio quartile had significantly lower deformity severity than the lowest quartile. RCS revealed a linear dose-response relationship, while ROC demonstrated moderate predictive value (AUC = 0.720). Excellent calibration was achieved (mean absolute error = 0.037). Subgroup analyses indicated consistent associations across populations, though no interaction terms survived multiple comparison correction. Sensitivity analyses confirmed that the association remained consistent across alternative modeling strategies, reinforcing the stability of the main findings.</p> Conclusions <p>Lower TG/HDL-C ratio correlates with severe spinal deformities, possibly reflecting metabolic dysfunction, inflammation, or bone loss.</p>