TTN and hypothyroidism: Titin isoform switching from the N2BA to the N2B isoform (reduced N2BA:N2B ratio) increases Fpassive, as observed in diastolic dysfunction in heart failure with preserved ejection fraction (HFpEF), while the inverse reduces Fpassive, as shown in dilated cardiomyopathy (DCM) and hypothyroidism (HT) (Figure 3B).30