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ENDOCRINOLOGY AND HORMONES: GENERAL CONCEPTS
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Endocrinology analyzes the biosynthesis of hormones, their sites of production, and the sites and mechanisms of their action and interaction. The major functions of hormones include the regulation of energy storage, production, and utilization; the adaptation to new environments or conditions of stress; the facilitation of growth and development; and the maturation and function of the reproductive system. Although hormones were originally defined as products of ductless glands, we now appreciate that many organs not classically considered as "endocrine" (e.g., the heart, kidneys, GI tract, adipocytes, and brain) synthesize and secrete hormones that play key physiological roles. In addition, the field of endocrinology has expanded to include the actions of growth factors acting by means of autocrine and paracrine mechanisms, the influence of neurons—particularly those in the hypothalamus—that regulate endocrine function, and the reciprocal interactions of cytokines and other components of the immune system with the endocrine system.
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Conceptually, hormones may be divided into 2 classes:
Hormones that act predominantly via nuclear receptors to modulate transcription in target cells (e.g., steroid hormones, thyroid hormone, and vitamin D)
Hormones that typically act via membrane receptors to exert rapid effects on signal transduction pathways (e.g., peptide and amino acid hormones)
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The receptors for both classes of hormones provide tractable targets for a diverse group of compounds that are among the most widely used drugs in clinical medicine.
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THE HYPOTHALAMIC-PITUITARY-ENDOCRINE AXIS
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Many of the classic endocrine hormones (e.g., cortisol, thyroid hormone, sex steroids, growth hormone) are regulated by complex reciprocal interactions among the hypothalamus, anterior pituitary, and endocrine glands (Table 38–1). The basic organization of the hypothalamic-pituitary-endocrine axis is summarized in Figure 38–1.
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