Low T3 syndrome in heart failure: prevalence and its relationship with disease severity and ejection fraction

Low T3 syndrome is a common hormonal alteration observed in patients with heart failure and has been associated with disease severity and adverse clinical outcomes. The prevalence of low T3 syndrome and its relationship with heart failure severity and ejection fraction phenotypes remain clinically relevant issues. This study aimed to evaluate the prevalence of low T3 syndrome in patients with heart failure and to investigate its association with disease severity and left ventricular ejection fraction.

In this prospective observational study, 132 patients with chronic heart failure (81 males, 51 females) who were evaluated at a tertiary care hospital between June 2021 and November 2021 were consecutively enrolled. Patients with known thyroid disease, use of thyroid hormone therapy, positive anti-thyroid antibodies, severe anemia, malignancy, active inflammatory disease, or medications affecting thyroid function were excluded. Low T3 syndrome was defined as a serum total triiodothyronine level below 2.6 ng/L in the presence of normal thyroid-stimulating hormone concentrations. Patients were classified according to left ventricular ejection fraction as reduced (< 50%) or preserved (≥ 50%). Clinical characteristics, laboratory parameters, inflammatory markers, and hospitalization indicators were analyzed in relation to serum T3 levels and ejection fraction phenotypes.

Low T3 syndrome was detected in 60 patients (45.5%). Reduced ejection fraction was present in 82 patients (62.1%), while 50 patients (37.9%) had preserved ejection fraction. The prevalence of low T3 syndrome was significantly higher in patients with reduced ejection fraction compared to those with preserved ejection fraction. However, this association appeared less pronounced than its relationship with overall disease severity. Patients with low T3 syndrome demonstrated features of increased disease severity, including a non-significant trend toward longer hospital stays and worse clinical profiles. Lower T3 levels were associated with indicators of more advanced heart failure, whereas the relationship with ejection fraction phenotype was less pronounced.

Low T3 syndrome is highly prevalent in patients with heart failure and is associated with increased disease severity. Its relationship with ejection fraction phenotype appears secondary to its association with clinical severity. Assessment of T3 levels may provide additional insight into the clinical status of patients with heart failure beyond conventional systolic function evaluation.