Thyroid hormone and adrenergic signaling in the heart.

Thyroid hormone action has profound consequences for the heart, ranging from atrial fibrillation to hemodynamic collapse. It has long been known that the cardiovascular signs and symptoms seen in thyrotoxicosis resemble those seen in states of catecholamine excess. However, measured concentrations of serum catecholamines in patients with thyrotoxicosis are typically normal or even low, suggesting an increase in the adrenergic responsiveness of the thyrotoxic heart. In spite of several decades of work, the question of whether thyroid hormone increases cardiac adrenergic responsiveness is still controversial. In this brief review, we consider the reasons underlying this controversy, focusing on the complexity of the adrenergic signaling cascade.