Thyroid hormone metabolism can be closely associated with cardiovascular disorders. We examined the relationship between low triiodothyronine (T3) levels and heart failure status, including B-type natriuretic peptide (BNP) levels, in 625 patients with cardiovascular disorders who underwent cardiac catheterization. A multiple regression analysis revealed that the left ventricular ejection fraction (LVEF), hemoglobin (Hb) levels, sex (male), free T3 (FT3) levels, and estimated glomerular filtration rate (eGFR) were significantly negatively associated with the log BNP value, while age was significantly positively associated with the log BNP value (P P P = 0.002), and the same relationship remained in the patient group with normal-range BNP values (β = −0.198, P = 0.008). We demonstrated a significant relationship between high BNP and low serum FT3 levels, and this relationship remained significant in patients with normal BNP levels. These results indicate that low T3 is associated with high plasma BNP levels rather than worsening of hemodynamics.