Introduction to Endocrinology

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FAQ and Introduction to Endocrinology:

1. What is Endocrinology?

Endocrinology is the study of the endocrine system (mostly hormones), related diseases and treatments.

2. What is the difference between an exocrine gland and an endocrine gland?

Exocrine glands secrete their products into ducts that carry the secretions into body cavities, into the lumen of an organ, or to the outer surface of the body.

Endocrine glands, on the other hand, secrete their products (hormones) into the interstitial fluid surrounding the secretory cells, rather than into ducts.

3. Hormones may be classified according to their site of action (in reference to their site of production), as circulating and local; locally acting hormones can be further sub classified. Differentiate between endocrine, paracrine and autocrine hormones.

• Endocrine hormone: A hormone secreted by an endocrine gland.

• Paracrine hormone: Hormones that act on local neighboring cells.

• Autocrine hormone: Hormones that act on the same cell that secreted them.

4. Describe the main chemical classifications of hormones.

Hormones can be divided into two main groups:

Lipid-soluble hormones:

• Steroid Hormones - derived from cholesterol.

• Thyriod Hormones - T3 and T4 synthesized by attaching iodine to the amino acid tyrosine.

• Nitric oxide - a gas hormone that also acts as a neurotransmitter.

Water-soluble hormones:

• Amine hormones - synthesized by decarboxylating and modifying certain amino acids.

• Peptide hormones or protein hormones - amino acid polymers.

• Eicosaniod hormones - hormones derived from arachidonic acid, a 20-carbon fatty acid.

5. How are hormones transported in plasma?

Water soluble hormones are transported freely in blood, whereas lipid hormones are transported in transport proteins. These transport proteins are synthesized in the liver.

6. What is the significance of free hormone in the plasma?

Free hormones in plasma diffuse out of capillaries, bind to receptors, and trigger responses.

7. Contrast the mechanism of action on the target cell of a lipid-soluble hormone and a water-soluble hormone.

Action of lipid hormones:

• Lipid hormones bind to receptors within target cells.

• A free lipid-soluble hormone diffuses from the blood, through the interstitial fluid, and through the lipid bilayer of the plasma membrane into a cell.

• The hormone binds to and activates receptors within the cell, which in turn alter the activity of specific gene expression by turning nuclear DNA on or off.

• Thus, new proteins are produced altering the cell's functionality.

Action of water-soluble hormones:

• A water soluble hormone diffuses from the blood through the interstitial fluid and then binds to its receptor at the exterior of the cell. The hormone-receptor complex activates a G-protein which in turn activates adenylate cyclase.

• Adenylate cyclase converts ATP into cyclic AMP which is the second messenger.

• Cyclic AMP activates one or more protein kinases; these kinases phosphorylate cellular proteins which alter local enzymes and cellular functionality.

• After some time, an enzyme called phosphodiesterase inactivates cAMP.

8. Which of the above mechanisms of action does the action of thyroid hormone most closely resemble?

Thyriod hormone is a lipid-soluble. Its mechanism would resemble mechanisms of lipid-soluble hormones.

9. What is the difference between hormones and prostaglandins?

A prostaglandin is a membrane-associated lipid; released in small quantities and acts as a local hormone. Prostaglandins are examples of eicosanoid hormones.

Additional Reading:

Basic Endocrinology

1. Introduction to Endocrinology
2. Hypothalamic-pituitary system
3. Adrenal Hormones
4. Antidiuretic Hormone (ADH) and ECF Regulation
5. Endocrine Pancreas
6. Growth Hormone
7. Adrenal Medulla
8. Hormonal Control of Calcium and Phosphate
9. Thyroid Hormones
10. Hormones of Male Reproduction
11. Hormones of Female Reproduction
12. Fluid Compartments of the Body
13. Notes on Hypothalamus Anterior Pituitary and Thyroid
14. Additional Notes on Female Reproduction
15. Hormonal Signaling Pathways
16. FAQ on Adrenal Hormones
17. FAQ on Male Reproduction
18. Synthesis and Deficiencies of Adrenal Hormones
19. Significance of Glycosylated Hemoblogin (HbA1c)
20. Significance of Measuring Albumin while with Calcium Levels
21. Stepwise Approach to Treatment of Ascites
22. How to differentiate between Diabetes Insipidus vs Psychogenic Polydipsia

Related Topics

1. Histology of the Endocrine System
2. Histology of the Male Reproductive System
3. Histology of the Female Reproductive System

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