The Endocrine System

The Natural Thyroid Diet

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Chapter 2 considered the general issue of chemical communication between cells. This section focuses on the classic endocrine glands and their hormones. A hormone is a chemical substance that is released into the blood in small amounts and that, after delivery by the circulation, elicits a typical physiologic response in other cells. An endocrine gland is a group of cells that produce and secrete a hormone. Endocrine glands are also called ductless glands to distinguish them from exocrine glands, which deliver their products through ducts to the outside of the body or to the lumen of the gastrointestinal tract. Endocrine glands typically are highly vascular, and their capillaries are usually fenestrated. Both of these features presumably facilitate two-way trafficking of signals to the gland and hormones to the blood. Except for these vascular features, endocrine glands have no special distinguishing characteristics. They are usually, but not always, bilateral structures, and they may arise during development from any of the primitive germ layers. Although the discussion in this text is limited largely to the classic endocrine glands listed in Table 1, it is abundantly clear that many other organs, including brain, kidney, heart, and even adipose tissue, also behave as endocrine glands. Unfortunately, endocrine physiology can sometimes involve a bewildering array of facts, not all of which can be derived from basic principles. The outline of goals and objectives shown in Fig. 1 should be useful for organizing and digesting this necessarily large volume of material.

Table 1 Classical Endocrine Glands

Pituitary gland

Thyroid gland

Parathyroid glands

Islets of Langerhans

Adrenal glands

Gonads (ovaries and testes)

Placenta

SYNTHESIS, SECRETION, AND METABOLISM OF HORMONES

The classic hormones fall into three categories: (1) derivatives of the amino acid tyrosine; (2) steroids, which are derivatives of cholesterol; and (3) peptides and proteins, which comprise the most abundant and diverse class of hormones. Table 2 lists some examples of each category. Endocrine glands synthesize their secretory products from simple precursors such as amino acids and acetate or transform complex precursors taken up from the blood. General steps in protein biosynthesis, storage, and secretion common to all protein and peptide hormones are reviewed in Chapter 2. These hormones are stored, sometimes in large amounts, in membrane-bound secretory vesicles, and can be released

Goals and Objectives

A. The student should be familiar with

1. Essential features of feedback regulation

2. Essentials of competitive binding assays

B. For each hormone, the student should know:

1. Its cell of origin

2. Its chemical nature, including a. Distinctive features of its chemical composition b. Biosynthesis c. Whether it circulates free or bound to plasma proteins d. How it is degraded and removed from the body

3. Its principal physiological actions a. At the whole body level b. At the tissue level c. At the cellular level d. At the molecular level e. Consequences of inadequate or excess secretion

4. What signals or perturbations in the internal or external environment evoke or suppress its secretion a. How those signals are transmitted b. How that secretion is controlled c. What factors modulate the secretory response d. How rapidly the hormone acts e. How long it acts f. What factors modulate its action

FIGURE 1 Goals and objectives in endocrine physiology.

37. Introduction to Endocrine Physiology Table 2 Chemical Nature of the Classical Hormones

Tyrosine derivatives

Steroids

Peptides (<20 amino acids)

Proteins (>20 amino acids)

Epinephrine Norepinephrine Dopamine Triiodothyronine

Thyroxine

Testosterone Estradiol Progesterone Cortisol

Aldosterone Vitamin D

Oxytocin

Vasopressin

Angiotensin

Melanocyte- stimulating hormone Somatostatin Thyrotropin-releasing hormone Gonadotropin-releasing hormone

Insulin Glucagon

Adrenocorticotropic hormone Thyroid-stimulating hormone Follicle-stimulating hormone Luteinizing hormone Growth hormone Prolactin

Growth hormone-releasing hormone Parathyroid hormone Calcitonin

Chorionic gonadotropin Corticotropin-releasing hormone Placental lactogen rapidly. In general, the same stimuli that provoke secretion of peptide hormones also activate transcription of the genes that encode them so that new synthesis replenishes the storage pools. In contrast, steroid hormones are not stored in significant amounts and must be synthesized de novo. Secretion of these hormones therefore increases slowly. Relevant details of synthesis and storage of the amino acid and steroid hormones are presented in the discussion of each gland.

Protein and peptide hormones are synthesized on ribosomes as larger molecules (prohormones and prepro-hormones) than the final secretory product and undergo a variety of postsynthetic steps of transformation into the final secretory product. Postsynthetic processing to the final biologically active form is not limited to peptide hormones or to the time prior to secretion. Postsecretory transformations to more active forms may occur in liver, kidney, fat, or blood, as well as in the target tissues themselves. For example, thyroxine, the major secretory product of the thyroid gland, is converted extra-thyroidally to triiodothyronine, which is the biologically active form of the hormone. Testosterone, the male hormone, is converted to a more potent form, dihydro-testosterone, within some target tissues and is converted to the female hormone, estrogen, in other tissues. Peripheral transformations add another level of complexity that must be considered when evaluating causes of endocrine disease.

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