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INTRODUCTION

Endocrinology is the discipline that focuses on hormones, their interaction with specific receptors and related diseases. The endocrine system is involved in multiple functions such as growth, metabolic homeostasis, production and storage of energy, blood pressure regulation and reproduction. Hormones are defined as activated chemical signals secreted into the bloodstream that act on distant receptors in different tissues, regulated by precise feedback mechanisms.

In relation to the heart and the cardiovascular system, hormones produced in different glands (ie, cortisol and adrenal, triiodothyronine, and thyroid gland) or other organs (ie, renin and kidney or adiponectin and adipose tissue) will elicit indirect and direct biological effects depending on the localization of the receptors on target tissues. Furthermore, it is now clear that the heart is not just a target organ for different type of hormones but also is an “endocrine organ,” by secreting natriuretic peptides that play a central role in fluid and electrolyte homeostasis. Since there are hundreds of different hormones and because receptors are so ubiquitous throughout the body, the effects of endocrine dysregulation on the cardiovascular system is frequently observed in clinical practice; many endocrine diseases are diagnosed due to cardiovascular manifestations. This chapter will update most of the endocrinopathies that can affect the heart, including the interplay of the endocrine system and the cardiovascular system, with special emphasis on evidence-based information from recent years.

THYROID & THE HEART

The thyroid gland is regulated by pituitary thyroid-stimulating hormone (TSH) and secretes mainly the pro-hormone thyroxine (T4) that is deiodated in peripheral tissues to active triiodothyronine (T3). Thyroid hormones are mostly bound to plasma proteins. Free T3 is the predominant inhibitor of the hypothalamus and pituitary to suppress further release of thyroid-releasing hormone (TRH) and TSH.

Thyroid diseases are quite common, affecting approximately up to 15% of the adult female population and a smaller percentage of males. This sex-specific prevalence likely results mainly from autoimmune diseases, however, with advancing age the incidence of thyroid disease in men reaches that of women. Thyroid hormone modulates oxidative and metabolic processes throughout the body by regulating, at the nuclear level, cellular protein synthesis. Thyroid hormones also regulate growth, central nervous system development, heart rate, gut motility, and thermoregulation. Nuclear thyroid receptors are widely distributed including the heart and cardiovascular system. Nongenomic actions of thyroid hormones have also been recognized based on rapid tissue responses outside of the nucleus, through regulation of ion transporter activity. Both overproduction and underproduction of thyroid hormone can disrupt normal metabolism and cardiovascular function.

The cardiovascular signs and symptoms of thyroid dysfunction are some of the most characteristic and clinically relevant signs and symptoms observed in clinical practice. Both hyperthyroidism or hypothyroidism induce changes in cardiac contractility, myocardial oxygen consumption, cardiac output, blood pressure, lipid parameters, systemic vascular resistance and thrombogenicity. For example, it is well known that hyperthyroidism can induce atrial fibrillation, ...

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