Chapter 32

Endocrinology involves the study of glands that secrete hormones into the circulation for effects at distant target sites. At present, more than 100 hormones are known to be released into the circulation, with more than 200 types of receptors on target cells in the human body. Many of these cells contain literally thousands of receptors on their surfaces. Because these hormone receptors are so ubiquitous throughout the body, the presence or absence of a single hormone can have multiple effects on one or more organ systems, including the cardiovascular system. This chapter considers most of the common and some uncommon endocrinopathies that can affect the heart, addressing specifically how they can be recognized and treated to best restore cardiovascular health.

Thyroid hormone has profound effects on the cardiovascular system, regulating vascular tone and contractility and the metabolic demands of the body. Thyroid disease often presents solely with cardiovascular manifestations, necessitating a thorough search for this potentially reversible cause of heart disease.

Thyroid hormone regulates oxidative and metabolic processes throughout the body by directing cellular protein synthesis at the nuclear level. Both overproduction or underproduction of thyroid hormone can disrupt normal metabolic function. Under the control of pituitary release of thyroid-stimulating hormone (TSH), the thyroid gland secretes tetraiodothyronine (T4) and triiodothyronine (T3), mostly bound to plasma proteins. The free, or unbound, fraction of hormone negatively feeds back at the level of the hypothalamus and pituitary to suppress further release of thyroid-releasing hormone (TRH) and TSH. One step necessary in the production of thyroid hormone is the trapping of iodine by the gland.

### Hyperthyroidism

#### Essentials of Diagnosis

• Suppressed TSH below the lower normal limits.
• High free T4, total T4, and free thyroxine index, or high free T3 or total T3 radioimmunoassay in T3 toxicosis.
• High 24-hour radioactive iodine uptake in Graves disease or toxic multinodular goiter; low uptake in thyroiditis or exogenous cause.
• Symmetric goiter (often with bruit) and exophthalmos in Graves disease.

#### General Considerations

In hyperthyroidism, increased levels of thyroid hormone result in a hyperdynamic cardiovascular system. The enhanced diastolic and systolic performance is due to the effect of T3 on the regulation of specific cardiac genes. These genes promote the expression of structural proteins of the contractile apparatus of the cardiac myocyte. In addition, thyroid hormone increases calcium-activated adenosine triphosphatase (ATPase), which increases intracellular calcium concentration and inotropism.

In general, the hyperdynamic activity of the hyperthyroid cardiovascular system is similar to that of other conditions in which the sympathetic nervous system is activated with the release of catecholamines. There is enhanced cardiac output, increased stroke volume, enhanced left ventricular contractility, decreased systemic vascular resistance, tachycardia with a wide pulse pressure, a hyperdynamic precordium, increased myocardial oxygen consumption, and increased coronary flow. Although systolic ...

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