Hypothalamus, Anterior Pituitary, and Thyroid

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Notes on Hypothalamus

• Hypothalamus is the master gland in the endocrine system.

• It is the link between the nervous and endocrine system.

• Input to the hypothalamus comes from the limbic system>cerebral cortex>thalamus>reticular activating system.

• Pain, stress, and emotions cause changes in hypothalamic activity.

• The hypothalamus controls the autonomic nervous system, and regulates body temperature, thirst, hunger, sexual behavior and defense mechanisms such as fear and rage.

• Cells in the hypothalamus synthesize at least 9 hormones.

• Hormones in the hypothalamus are synthesized by the hypothalamic neuronsecretory cells and carried by axons and released at axon terminals. The hormones then diffuse into capillaries of the primary plexus of the hypophyseal portal system and are carried by the hypophyseal portal veins to the secondary plexus of the hypophyseal portal system for distribution to target cells in the anterior pituitary gland.

• The pituitary gland secretes 7 hormones.

• Hormones are controlled by feed-back systems.

• Hypothalamic hormones are: somatostatin, thyrotropin-releasing hormone (TRH), MSH, gonadotropin-releasing hormone (GnRH), CRH, and GHRH.

Notes on Anterior Pituitary Gland

• This is a small gland located at the base of the brain.

• It enlarges during pregnancy.

• It is attached to the hypothalamus.

• The anterior lobe constitutes 80% of the gland.

• The anterior pituitary has a portal vascular system that is necessary for transporting hypothalamic hormones from hypothalamus to pituitary.

• There are 5 types of cells in the anterior pituitary.

Notes on Thyroid Gland

• This is a butterfly-shaped gland located inferior to the larynx (voice box).

• It produces hormones that are necessary for normal growth and maturity.

• It is also responsible for maintenance of normal oxidative metabolism of tissues.

• Thyroid hormones include T3, T4 and calcitonin.

• Low blood levels of T3 and T4 stimulate the secretion of TRH by the hypothalamus; TRH stimulates the secretion of TSH by the anterior pituitary gland.

• TSH stimulates thyroid follicles to produce T3 and T4.

Endocrine Hormones FAQ

1. Describe the mechanisms that control hormone secretion.

Hormone secretion is regulated by signals from the nervous system, chemical changes in the blood, and other hormones. Most mechanisms work via feedback systems like negative and positive feedback.

2. What roles does the autonomic nervous system play in controlling hormone secretion?

The autonomic nervous system is controlled by the hypothalamus. The autonomic nervous system and endocrine work hand-in-hand to maintain homeostasis in the body. The ANS regulates body temperature, thirst, hunger, sexual behavior and defense mechanisms.

3. Describe the connection of the hypothalamus with the pituitary gland.

The hypothalamus controls the pituitary gland. They are connected through the hypophyseal portal system.

4. What is the definition of a portal system?

A portal system is an entrance, a connection between two or more tissues/organs that use it to exchange nutrients. In a portal system, blood flows from one capillary network into a portal vein, then into a second capillary network without passing through the heart.

5. What is the function of the hypophyseal portal veins?

The hypophyseal portal veins connect the hypothalamus and pituitary gland. Blood flows from capillaries in the hypothamalus into portal veins that carry blood to capillaries of anterior pituitary.

6. Give reasons for why there may be variations in plasma hormone concentration.

There is variation in plasma hormone concentration due to the fact that homeostasis is continuously being disturbed in the body. Thus, hormonal systems come into play on a continuous basis using negative and positive loops.

7. In addition to control of synthesis and release of hormones there are two other mechanisms that may limit the hormone's availability. Briefly describe them.

The two mechanisms that control hormone concentration are negative and positive feedback systems.

8. What are tropic hormones? Give examples of the tropic hormones.

Tropic hormone is a hormone that targets another endocrine gland. Examples would be FSH and LH that regulate functions of gonads.

9. Which endocrine glands are under direct regulation by hormones of the anterior pituitary?

Ovaries, testes, thyroid gland, and cortex of adrenal gland.

10. Which endocrine glands are not under direct regulation by hormones of the anterior pituitary?

Adrenal medulla, pancreatic islets, pineal gland, parathyroid, and thymus.

11. In simple terms, how are endocrine disorders best described?

Disorders of the endocrine system often involve either hyposecretion or hypersecretion.

12. In endocrinology, disorders may be classified into primary, secondary, tertiary & quaternary. What are the criteria for these classifications? Provide an example of each to help sort out the classes.

Primary:

They originate within the gland that produces the hormone.

Secondary:

The hormone producing gland is normal, but it is not properly being controlled by the hypothalamic-pituitary system.

Tertiary:

Involves the dysfunction of the hypothalamus and its releasing hormones.

Quaternary:

This disorder lies in the inability of target tissue to respond to 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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