ENDOCRINOLOGY AND HORMONES: GENERAL CONCEPTS
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.
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)
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.
THE HYPOTHALAMIC-PITUITARY-ENDOCRINE AXIS
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.
Table 38–1Hormones that Integrate the Hypothalamic-Pituitary-Endocrine Axis ||Download (.pdf) Table 38–1 Hormones that Integrate the Hypothalamic-Pituitary-Endocrine Axis
|HYPOTHALAMIC-RELEASING HORMONE ||PITUITARY TROPHIC (SIGNAL) HORMONE ||TARGET HORMONE(S) |
|Growth hormone-releasing hormone (GHRH) ||Growth hormone (GH) ||IGF-1 |
|Somatostatin (SST)a ||Growth hormone || |
|Dopamine (DA)b ||Prolactin ||— |
|Corticotropin-releasing hormone (CRH) ||Corticotropin ||Cortisol/DHEA |
|Thyrotropin-releasing hormone (TRH) ||Thyroid-stimulating hormone (TSH) ||Thyroid hormone |
|Gonadotropin-releasing hormone (GnRH) || |
Follicle-stimulating hormone (FSH)
Luteinizing hormone (LH)
Organization of the anterior and posterior pituitary gland. Hypothalamic neurons in the supraoptic (SON) and paraventricular (PVN) nuclei synthesize arginine vasopressin (AVP) or oxytocin (OXY). Most of their axons project directly to the posterior pituitary, from which AVP and OXY are secreted into the systemic circulation to regulate their target tissues. Neurons that regulate the anterior lobe cluster in the mediobasal hypothalamus, including the PVN and the arcuate (ARC) nuclei. They secrete hypothalamic releasing hormones, which reach the anterior pituitary via the hypothalamic-adenohypophyseal portal system and stimulate distinct populations of pituitary cells. These cells, in turn, ...