Clinical and Pathophysiologic Insights of Free triiodothyronine/Free thyroxine Ratio in Patients with Heart Failure with Preserved Ejection Fraction: Data from the NETDiamond Cohort

Background: Thyroid dysfunction is common in patients with heart failure (HF). Impaired conversion of free T4 (FT4) into free T3 (FT3) is thought to occur in these patients, decreasing the availability of FT3 and contributing to HF progression. In HF with preserved ejection fraction (HFpEF), it is not known whether changes in conversion of thyroid hormones (THs) are associated with clinical status and outcomes. Objectives: The objective of this study was to evaluate the association of FT3/FT4 ratio and TH with clinical, analytical, and echocardiographic parameters, as well as their prognostic impact in individuals with stable HFpEF. Methods: We evaluated 74 HFpEF participants of the NETDiamond cohort without known thyroid disease. We performed regression modeling to study the associations of TH and FT3/FT4 ratio with clinical, anthropometric, analytical, and echocardiographic parameters, and survival analysis to evaluate associations with the composite of diuretic intensification, urgent HF visit, HF hospitalization, or cardiovascular death over a median follow-up of 2.8 years. Results: The mean age was 73.7 years and 62% were men. The mean FT3/FT4 ratio was 2.63 (standard deviation: 0.43). Subjects with lower FT3/FT4 ratio were more likely to be obese and have atrial fibrillation. Lower FT3/FT4 ratio was associated with higher body fat (beta = -5.60 kg per FT3/FT4 unit, p = 0.034), higher pulmonary arterial systolic pressure (PASP) (beta = -10.26 mm Hg per FT3/FT4 unit, p = 0.002), and lower left ventricular ejection fraction (LVEF) (beta = 3.60% per FT3/FT4 unit, p = 0.008). Lower FT3/FT4 ratio was associated with higher risk for the composite HF outcome (HR = 2.50, 95% CI: 1.04-5.88, per 1-unit decrease in FT3/FT4, p = 0.041). Conclusions: In patients with HFpEF, lower FT3/FT4 ratio was associated with higher body fat, higher PASP, and lower LVEF. Lower FT3/FT4 predicted a higher risk of diuretic intensification, urgent HF visits, HF hospitalization, or cardiovascular death. These findings suggest that decreased FT4 to FT3 conversion might be a mechanism associated with HFpEF progression.