J-shaped association of free triiodothyronine to free thyroxine ratio with 5-year mortality among patients with chronic kidney disease: a prospective cohort study

Purpose To investigate the association of free triiodothyronine to free thyroxine (FT3/FT4) ratio with 5-year mortality in patients with chronic kidney disease (CKD) using data from the National Health and Nutrition Examination Survey (NHANES) through 2001–2002 and 2007–2012. Methods Two thousand four hundred twenty-four patients with CKD stage 1–5 were included. Patients’ baseline characteristics were collected. Cox proportional hazards models were used to investigate the association of FT3/FT4 ratio with 5-year all-cause and cardiovascular mortality. Subgroup analysis was performed. Results Within 5 years of follow-up duration, 425 (17.53%) deaths were recorded, and 154 patients died from cardiovascular disease. A J-shaped association between FT3/FT4 ratio and 5-year mortality was observed. After fully adjustment, the elevated FT3/FT4 ratio was significantly associated with a decreased 5-year all-cause mortality risk (HR: 0.79, 95% CI 0.63–0.99) among patients with CKD stage 1–5 when FT3/FT4 ratio < 4.71, and is significantly associated with an increased 5-year all-cause mortality risk in CKD patients with advanced CKD stages or low UACR when FT3/FT4 ratio ≥ 4.71 (HR: 2.74, 95% CI 1.20–6.29; HR: 3.09, 95% CI 1.12–8.57, respectively). The elevated FT3/FT4 ratio also showed a J-shaped association with the 5-year cardiovascular mortality which disappears after fully adjustment. Conclusion The FT3/FT4 ratio is closely associated with 5-year mortality risk among patients with CKD, indicating a potential role of FT3/FT4 ratio as a biomarker for mortality prediction in CKD patients.