LBBAP and Thyroid Function: Does Left Bundle Branch Area Pacing Protect Against the Low T3 Syndrome Caused by Pacing-Induced Cardiomyopathy?

The Clinical Question

Left bundle branch area pacing (LBBAP) has emerged as a physiological alternative to conventional right ventricular pacing (RVP), primarily valued for its ability to maintain synchronous left ventricular activation and reduce the risk of pacing-induced cardiomyopathy (PICM). But could the benefits extend beyond hemodynamics? Specifically, does the prevention of PICM through LBBAP also protect against the thyroid hormone derangements — particularly the low T3 syndrome — that accompany heart failure progression?

This question connects two well-established bodies of evidence that, surprisingly, have not been directly studied together in the pacing literature.

The Heart Failure–Thyroid Axis: Established Evidence

The relationship between heart failure severity and thyroid hormone metabolism is one of the most consistent findings in cardiovascular endocrinology. Cross-sectional studies demonstrate that approximately 20–30% of patients with overt congestive heart failure have low circulating T3 levels, and the magnitude of this decrease is directly proportional to NYHA functional class severity. In advanced heart failure (NYHA III–IV), the prevalence may be even higher.

This pattern, known as the low T3 syndrome or non-thyroidal illness syndrome (NTIS), is characterized by decreased serum triiodothyronine (T3) with normal or near-normal thyrotropin (TSH) and free thyroxine (fT4). It results from two concurrent mechanisms: impaired hepatic conversion of T4 to the biologically active T3 by 5′-monodeiodinase, and — critically — the upregulation of type 3 deiodinase (D3) within cardiomyocytes themselves.

Type 3 Deiodinase: The Local Cardiac Thyroid Inactivator

Type 3 deiodinase is the primary enzyme responsible for inactivating thyroid hormones at the tissue level. Under normal physiological conditions, the adult heart has negligible deiodinase activity and depends entirely on circulating plasma T3. However, during cardiomyopathy — whether dilated, hypertrophic, or ischemic — D3 is dramatically induced within cardiomyocytes, sometimes up to 15-fold above baseline.

This D3 induction creates a state of local cardiac hypothyroidism, even when systemic thyroid function tests appear normal. The consequences are significant: T3 regulates the expression of key cardiac genes controlling myosin heavy chain isoforms, SERCA2a calcium handling, beta-adrenergic receptor density, and Na⁺/K⁺-ATPase activity. A local deficit in T3 contributes to impaired contractility, diastolic dysfunction, and adverse remodeling — phenotypic features that mirror the hypothyroid heart.

The PICM Pathway: How RVP Connects to Thyroid Derangement

Right ventricular pacing introduces iatrogenic left bundle branch block-type dyssynchrony. While not all patients develop clinically significant cardiomyopathy, observational registry data show a cumulative heart failure incidence of approximately 10.6% at 2 years in paced patients, with the excess risk concentrated in the early months post-implantation. Longer-term data suggest incidence rates approaching 21% at 5 years.

The pathophysiological cascade is well characterized: RVP-induced dyssynchrony leads to asymmetric septal activation, altered regional wall stress, impaired diastolic filling, progressive left ventricular dilation, mitral regurgitation, and ultimately systolic dysfunction. Wider paced QRS durations — particularly exceeding 150 ms — are strongly associated with increased PICM risk.

As heart failure progresses, the D3 induction pathway activates. Peripheral T4-to-T3 conversion is impaired, circulating T3 falls, and the local cardiac T3 deficit worsens. This creates a potential vicious cycle: PICM drives heart failure, which drives low T3, which impairs myocardial recovery, which worsens heart failure. Notably, hypothyroidism itself can increase pacing capture thresholds, adding a further layer of clinical risk including potential loss of capture.

How LBBAP May Disrupt the PICM–Thyroid Cascade

Left bundle branch area pacing maintains physiological left ventricular activation by stimulating the left bundle branch or the left side of the interventricular septum, preserving the native His-Purkinje conduction network. This approach avoids the iatrogenic dyssynchrony that is the initiating event in PICM.

The clinical evidence for LBBAP's superiority in preserving cardiac function is substantial. Studies consistently show that LBBAP significantly improves or preserves LVEF compared to RV pacing, lowers BNP levels, improves 6-minute walk distance, and reduces left atrial dimensions. In patients upgraded from failed conventional CRT or coronary venous leads, LBBAP produces QRS narrowing from approximately 170 ms to 139 ms and LVEF improvement from 29% to 40%.

By preventing the development of PICM, LBBAP theoretically interrupts the entire downstream cascade that leads to D3 upregulation and the low T3 state. If the heart does not develop failure, the stimulus for D3 induction is absent, and thyroid hormone metabolism is preserved.

Supporting Evidence: CRT and Thyroid Recovery

While no study has directly examined thyroid function in LBBAP recipients, indirect evidence from cardiac resynchronization therapy (CRT) provides preliminary support. A small study documented that CRT recipients showed improvements in free T3 levels and the free T3/free T4 ratio — intriguingly, this improvement appeared to occur regardless of the degree of reverse cardiac remodeling, suggesting that the restoration of ventricular synchrony itself may directly influence thyroid hormone metabolism through mechanisms beyond simple hemodynamic improvement.

Additionally, among CRT recipients, those with pre-existing hypothyroidism had significantly higher all-cause mortality and heart failure hospitalizations compared to euthyroid patients, suggesting that thyroid status is not merely an epiphenomenon but may actively modulate the response to pacing therapy.

Clinical Implications and Knowledge Gaps

The hypothesis that LBBAP protects against pacing-related thyroid derangement carries several practical implications. First, it adds another mechanistic argument to the growing case for conduction system pacing as a primary strategy, especially in patients with high anticipated pacing burden. Second, it suggests that routine thyroid panel monitoring — including free T3 alongside the standard TSH — may be warranted in patients receiving chronic RVP, particularly those showing early signs of PICM.

Third, in patients who develop low T3 syndrome while on chronic RVP, the standard clinical response may focus on thyroid supplementation while missing the pacing-related etiology. Upgrading to LBBAP could address the root cause by preventing further cardiomyopathic progression and allowing natural recovery of thyroid hormone homeostasis.

The critical knowledge gap is the absence of prospective studies directly measuring serial thyroid panels (TSH, free T4, free T3, reverse T3) in matched LBBAP versus RVP cohorts. Such a study, particularly if it also measured myocardial D3 activity or gene expression markers in device-extracted tissue, would definitively establish whether the physiological benefits of conduction system pacing extend to the thyroid axis.

Conclusion

The mechanistic chain linking right ventricular pacing to thyroid hormone derangement — through PICM, heart failure progression, D3 induction, and the low T3 syndrome — is well supported by existing literature. While no direct evidence yet confirms that LBBAP prevents these thyroid consequences, the physiological rationale is compelling. By maintaining synchronous ventricular activation and preventing cardiomyopathic progression, LBBAP addresses the upstream trigger of a cascade that ultimately extends beyond hemodynamics into endocrine homeostasis. Prospective studies comparing thyroid function across pacing modalities represent an important and clinically relevant frontier in conduction system pacing research.