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The Thyroid Gland — Structure — Function — TeachMePhysiology

The thyroid gland is an endocrine organ found in the neck, it is responsible for regulating the body's metabolic rate via hormones it produces. In this article, we will be looking at its anatomy, its cellular structure, its endocrine physiology and its clinical relevance.

Anatomy

The thyroid gland is a ductless alveolar gland found in the anterior neck, just below the laryngeal prominence (Adam’s apple). It is roughly butterfly-shaped, with two lobes wrapping around the trachea and connected in the middle by an isthmus. The thyroid gland is not usually palpable.

  • It is supplied by superior and inferior thyroid arteries, drained via superior, middle and inferior thyroid veins and has a rich lymphatic system.
  • You can read more about the anatomy of the Thyroid gland here.
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Fig 1 – Anterior view of the neck, showing the anatomical position of the thyroid gland

Cellular Structure

The function of the Thyroid gland is to produce and store thyroid hormones. Thyroid epithelia form follicles filled with colloid – a protein-rich reservoir of the materials needed for thyroid hormone production. These follicles range in size from 0.02-0.3mm and the epithelium may be simple cuboidal or simple columnar.

In the spaces between the follicles, parafollicular cells can be found. These cells secrete calcitonin, which is involved in the regulation of calcium metabolism in the body.

Function

The thyroid gland is one of the main regulators of metabolism. T3 and T4 typically act via nuclear receptors in target tissues and initiate a variety of metabolic pathways. High levels of them typically cause these processes to occur faster or more frequently. Metabolic processes increased by thyroid hormones include:

  • Basal Metabolic Rate
  • Gluconeogenesis
  • Glycogenolysis
  • Protein synthesis
  • Lipogenesis
  • Thermogenesis

This is achieved in a number of ways, such as increasing the size and number of mitochondria within cells, increasing Na-K pump activity and increasing the presence of β-adrenergic receptors in tissues such as cardiac muscle.

Thyroid Hormone Synthesis

There are six steps in the synthesis of thyroid hormone, and you can remember them using the mnemonic ATE ICE:

  • Active transport of Iodide into the follicular cell via the Sodium-Iodide Symporter (NIS). This is actually secondary active transport, and the sodium gradient driving it is maintained by a Sodium-Potassium ATPase.
  • Thyroglobulin (Tg), a large protein rich in Tyrosine, is formed in follicular ribosomes and placed into secretory vesicles.
  • Exocytosis of Thyroglobulin into the follicle lumen, where it is stored as colloid. Thyroglobulin is the scaffold upon which thyroid hormone is synthesised.
  • Iodination of the Thyroglobulin. Iodide is made reactive by the enzyme thyroid peroxidase. Iodide binds to the benzene ring on Tyrosine residues of Thyroglobulin, forming monoiodotyrosine (MIT) then diiodotyrosine (DIT).
  • Coupling of MIT and DIT gives the Triiodothyronine (T3) hormone and coupling of DIT and DIT gives the Tetraiodothyronine (T4) hormone, also known as Thyroxine.
  • Endocytosis of iodinated thyroglobulin back into the follicular cell. Thyroglobulin undergoes proteolysis in lysosomes to cleave the iodinated tyrosine residues from the larger protein. Free T3 or T4 is then released, and the Thyroglobulin scaffold is recycled.

T3 and T4 are the active thyroid hormones. They are fat soluble and mostly carried by plasma proteins – Thyronine Binding Globulin and Albumin.

While T3 is the more potent form, it also has a shorter half-life due to its lower affinity for the binding proteins. Less than 1% of T3 and T4 is unbound free hormone.

At the peripheries, T4 is deiodinated to the more active T3.

T3 and T4 are deactivated by removing iodine. This happens in the liver and kidney. As T4 has a longer half-life, it is used in the treatment of hypothyroidism over T3 as its plasma concentrations are easier to manage.

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Fig 2 — Overview of the synthesis of thyroid hormones

Thyroid Hormone Release

Thyroid hormones are released as part of the hypothalamic-pituitary-thyroid axis. The Hypothalamus detects a low plasma concentration of thyroid hormone and releases Thyrotropin-Releasing Hormone (TRH) into the hypophyseal portal system.

TRH binds to receptors found on thyrotrophic cells of the anterior pituitary gland, causing them to release Thyroid Stimulating Hormone (TSH) into the systemic circulation. TSH binds to TSH receptors on the basolateral membrane of thyroid follicular cells and induces the synthesis and release of thyroid hormone.

  1. [start-clinical]

Clinical Relevance — Goitre

A Goitre is the medical term for an enlarged thyroid gland. The organ swells up to a palpable, and often visible, size within the neck. This may be due to an over or under active thyroid, iodine deficiency and in rare cases thyroid cancer.

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Fig 3 — A thyroid goitre

Clinical Relevance — Hyperthyroidism

Hyperthyroidism is the medical term for an overactive thyroid gland. One common cause of Hyperthyroidism is Grave’s Disease – an autoimmune condition where antibodies are produced that stimulate the TSH receptors on follicular cells. It affects roughly 1% of the population and is 10 times more common in women than in men.

Patients may present with heat intolerance, weight loss, tachycardia, nervousness, increased sweating, exophthalmos and increased bowel movements. Hyperthyroidism can be treated with Carbimazole which inhibits iodine binding to thyroglobulin.

Clinical Relevance — Hypothyroidism

Hypothyroidism is an underactive thyroid gland. One common cause of Hypothyroidism is Hashimoto’s Disease – an autoimmune condition where thyroid follicles are destroyed or antibodies are produced that block the TSH receptor on follicle cells.

Endocrine Glands — Definition, Examples, Function

Endocrine glands are tissues or organs that excrete chemical substances (hormones) directly into the blood. Common endocrine glands are the hypothalamus, pineal, and adrenal glands. Endocrine glands secrete hormones directly into the bloodstream or into the intercellular space, allowing the hormones to reach their target.

Endocrine system glands are spaced throughout the entire body. They release a wide number of hormones which control the metabolism and function of other cells. Exocrine glands, by comparison, secrete substances inside and outside of the body using ducts. These two methods of transport mark the difference between exocrine and endocrine glands.

While in the bloodstream, the hormones are able to travel through the body’s circulatory system to reach distant targets.

Hormones, in turn, will carry out varied functions in the body depending on the receptors they bind and the quantity of the hormone that is present.

These changes will reflect the balance of secretion and excretion of hormones in the body. Their duration will depend on the hormone’s inherent half-life and activity levels.

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As you can see in the image above, men and women share all of the same endocrine glands, besides the reproductive organs. While reproductive organs have the primary function of creating and releasing gametes, they also release a number of hormones which affect the body in different ways.

The pineal gland is a small gland located within the brain that serves as a great example of endocrine glands in general. The pineal gland is activated by neurons connected to your eyes. When these nerves are activated by light, the pineal gland is repressed. When nighttime comes, and the light reaching your eyes decreases, the pineal gland becomes activated.

The pineal gland secretes melatonin, a hormone which activates our sleep cycle. By releasing this hormone when it gets dark, the pineal gland is helping your body coordinate for sleeping. This includes changing your respiratory rate, brain patterns, and even digestive patterns.

The thyroid gland is found in your throat, just below the jaw. This gland secretes a number of hormones that act on your metabolism. Thyroid hormones can increase the rate of your cellular metabolism, or decrease it.

These activities are in part directed by another endocrine gland, the pituitary, which signals to the thyroid which hormones to release. In turn, your metabolism is regulated.

In fact, a nonfunctioning thyroid gland often leads to drastic weight gain or loss, depending on the malfunction.

The endocrine system derives its power from coordinating the interactions that take place between the hormones that are released by this network of glands. Endocrine glands themselves will inherently be able to make, secrete, and store hormones for future use.

Division of endocrine glands (EG). Paraganglia. — презентация

1 Prepared by: Alimova A., Ashim K., Aubakirova G. 335-group, GM faculty

2 Endocrine apparatus- combines anatomical and topographical disparate bodies

3 1. They are anatomically and topographically separate. 2. Dont excretory ducts. 3. Produce hormones.

4

5 1. distance action 2. specific action 3. small doses 4. very high activity 5. action only to alive cells

6

7

8 Glands secrete 4 groups : 1. Exocrine having ducts in the cavity : — Large gland of the oral cavity ; — Small gland of the mouth and gastrointestinal tract ; — Liver.

9 2. Endocrine, ductless and releasing his secret directly into the blood and lymph systems : — Pituitary ; — Epiphysis ; — Thyroid gland ; — Parathyroid glands ; — Adrenal glands.

10

11 3. Mixed in which both are present exocrine and endocrine parts: — Pancreas; — Gonads.

12 4. Apocrine or also called paracrine — it is glands, which are located in organs or tissues (heart, stomach, kidney, liver, oral cavity, lungs, etc.).

13 1. Central endocrine organs: the hypothalamus, pituitary, pineal. 2. Peripheral organs of the endocrine system: thyroid gland, parathyroid glands, adrenal glands. 3.

Bodies unite endocrine and not endocrine functions: the gonads (testes, ovary), placenta, pancreas. 4.

Single hormone producing cells: neuroendocrine cells (nervous origin), single hormone producing cells (not nervous origin).

14 (depending on their origin) I.Glands endodermal origin: 1. Branchial group — derived epithelium of the pharynx and gill pockets embryo : — Thyroid gland ; — Parathyroid glands ; — Thymus. 2. Glands of the intestinal tube : — Pancreatic islets.

15 II. Glands of mesodermal origin (this group is selected only in recent years ) : — Adrenal cortex — interrenal system ; — Gonads.

16 III. Glands of ectodermal origin : 1. Neurogenic group — derivatives diencephalon : — Neurohypophysis ( posterior lobe of the pituitary gland) ; — Epiphysis. 2.

Derivatives of Rathke's pouch epithelium — the epithelium of the oral bay : — Adenohypophysis. 3.

Group sympathetic nervous system — derivatives of sympathetic nervous system: — Adrenal medulla ; — Paraganglia ( chromaffin body).

17 GLAND ORIGIN FROM DIFFERENT PRIMORDIA ENDOCRINE GLANDS 1. Ectodermal-branchial 1. Thyroid 2. Parathyroid 3. Thymus 2. Ectodermal medium-intestinal 1. Endocrine part of the pancreas 3. Mesodermal- interrenal 1. Adrenal cortex 4.

Mesodermal-mesenchymal 1. Endocrine elements of the gonads (testis, ovary) 5. Ectodermal-neurogenic 1. Neurohypophysis 2. Pineal body 3. Chromaffin body (paraganglia) 4. Adrenal medulla 6. Ectodermal-mouthparts 1.

Adenohypophysis

18 From the ectoderm develop: — Pituitary; — Epiphysis; — Adrenal medulla; — Chromaffin bodies. Develop from the endoderm: — Thyroid gland; — Parathyroid glands; — Thymus; — Insular apparatus of the pancreas. Develops from the mesoderm: — Adrenal cortex; — Endocrine part of the gonads.

19 Additional endocrine glands develop from embryonic sources sympathetic nervous system. They are also known as chromaffin bodies.

These include: 1) intercarotid chromaffin body (somnolent glomerulus) at the beginning of the external and internal carotid arteries 2) lumbo-aortic chromaffin body located the front surface of the abdominal aorta 3) unstable under cardiac chromaffin body between the pulmonary artery and aorta.

20

21 Cells of paraganglia secreted catecholamines. Involution anatomically separate paraganglia begins in years and ends at the end of puberty.

Lumbo-aortic paraganglia well expressed in neonates and infants. This small thin strips on either side of the aorta at the level of the beginning of the inferior mesenteric artery.

The newborn sizes up paraganglia (8-15) * (2-3) mm in infants — about 3 cm

22

23

Endocrinology. Sections  Anatomy and Physiology  Endocrine Disorders and Emergencies. — ppt download

1 Endocrinology

2 Sections  Anatomy and Physiology  Endocrine Disorders and Emergencies

3 Anatomy & Physiology  Endocrine Glands  Have systemic effects.  Act on specific target tissues in specific ways.  May have single or multiple targets.  Disorders  Disorders result from over- or underproduction of hormone(s).

4 Hypothalmus  Located deep within the cerebrum.  Some cells relay messages from the autonomic nervous system to the central nervous system.  Other cells respond as gland cells to release hormones.

  • 5 Posterior Pituitary  Diabetes Insipidus  Oxytocin and Pregnancy
  • 6 Anterior Pituitary
  • 7 Thyroid Gland  Hyperthyroidism & Hypothyroidism
  • 8 Parathyroid Gland
  • 9 Thymus Gland

10 Pancreas  Combination Organ  Exocrine tissues called acini secrete digestive enzymes into the small intestine.  Endocrine tissues secrete hormones.  Glycogenolysis.  Gluconeogenesis.

11 Pancreas

12 Adrenal Gland  Adrenal Medulla  Inner segment of adrenal gland.  Closely tied to autonomic nervous system.  Adrenal Cortex  Outer layers of endocrine tissue, which secrete steroidal hormones.

13 Adrenal Gland

14 Gonads  Female  Ovaries  Male  Testes

15 Pineal Gland  Located in the roof of the thalamus.  Related to the body’s “biological clock.”  Implicated in Seasonal Affective Disorder.

  1. 16  Placenta  Releases hCG throughout gestation  Digestive Tract  Gastrin and secretin  Heart  ANH  Kidneys  Renin Other Organs with Endocrine Activity
  2. 17  Disorders of the Pancreas  Disorders of the Thyroid Gland  Disorders of the Adrenal Glands Endocrine Disorders and Emergencies
  3. 18 Disorders of the Pancreas  Diabetes Mellitus  Glucose Metabolism  Metabolism Anabolism & catabolism

19 Disorders of the Pancreas  Insulin is required for glucose metabolism Presence of enough insulin to meet cellular needs. Ability to bind in a manner to stimulate the cells adequately.

 When unable to obtain energy from glucose, the body begins to use fatty stores. Ketones and ketosis.

 Regulation of Blood Glucose  Hypoglycemia and hyperglycemia  Role of pancreas, liver, and kidneys  Osmotic diuresis and glycosuria

20 Diabetes Mellitus  Type I Diabetes Mellitus  Also called juvenile or insulin-dependent diabetes mellitus (IDDM).  Characterized by low production of insulin. Closely related to heredity.

 Results in pronounced hyperglycemia. Symptoms of untreated Type I DM include polydipsia, polyuria, polyphagia, weight loss, and weakness.

Untreated or noncompliant patients may progress to ketosis and diabetic ketoacidosis.

21 Diabetes Mellitus  Type II Diabetes Mellitus  Also called adult-onset or non-insulin-dependent diabetes mellitus (NIDDM).  Results from decreased binding of insulin to cells. Related to heredity and obesity.

Accounts for 90% of all diagnosed diabetes patients. Less risk of fat-based metabolism.  Results in less-pronounced hyperglycemia. Hyperglycemic hyperosmolar nonketotic acidosis.

Managed with dietary changes and oral drugs to stimulate insulin production and increase receptor effectiveness.

  • 22 Diabetic Emergencies
  • 23
  • 24 Blood Glucose Determination Choose a vein, and prep the site.
  • 25 Blood Glucose Determination Perform the venipuncture.

26 Blood Glucose Determination Place a drop of blood on the reagent strip. Activate the timer.

  1. 27 Blood Glucose Determination Wait until the timer sounds.
  2. 28 Blood Glucose Determination Wipe the reagent strip.
  3. 29 Blood Glucose Determination Place the reagent strip in the glucometer.
  4. 30 Blood Glucose Determination Read the blood glucose level.
  5. 31 Blood Glucose Determination Administer 50% dextrose intravenously, if the blood glucose level is less than 80 mg.
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32 Diabetic Emergencies  Diabetic Ketoacidosis  Pathophysiology  Results from the body’s change to fat metabolism.  Continuous buildup of ketones produces significant acidosis.  Signs and Symptoms  Extended period of onset (12–24 hours).  Sweet, fruity breath odor.  Potassium-related cardiac dysrhythmias.  Kussmaul’s respiration.  Decline in mental status and coma.

33 Diabetic Emergencies  Assessment and Management  Focused History & Physical Exam Obtain SAMPLE and OPQRST histories. Look for medical identification.  Management Maintain airway and support breathing as indicated.

Determine blood glucose level and obtain blood sample. If blood glucose unknown, administer 25g 50% dextrose. Establish IV and administer normal saline per local protocol. Monitor cardiac rhythm and vital signs.

Expedite transport.

34 Diabetic Emergencies  Hyperglycemic Hyperosmolar Nonketotic (HHNK) Coma  Pathophysiology  Found in Type II diabetics.  Results in blood glucose levels up to 1000mg/dL.  Insulin activity prevents buildup of ketones.  Sustained hyperglycemia results in marked dehydration. Often related to dialysis, infection, and medications.  Very high mortality rate.

35 Diabetic Emergencies  Signs & Symptoms  Gradual onset over days.  Increased urination and thirst, orthostatic hypotension, and altered mental status.  Assessment & Management  Difficult to distinguish from diabetic ketoacidosis in the prehospital setting.  Treatment is identical to diabetic ketoacidosis.

36 Diabetic Emergencies  Hypoglycemia  Pathophysiology  True medical emergency resulting from low blood glucose levels; rarely seen outside diabetics.  By the time signs and symptoms develop, most of the body’s stores have been used.  Diabetics with kidney failure are predisposed to hypoglycemia.

37 Diabetic Emergencies  Signs & Symptoms  Altered mental status with rapid onset Frequently involves combativeness.  Diaphoresis and tachycardia  Hypoglycemic seizure and coma  Assessment and Management  Focused History & Physical Exam Obtain SAMPLE and OPQRST histories. Look for medical identification.

38 Diabetic Emergencies  Management Maintain airway and support breathing as indicated. Determine blood glucose level and obtain blood sample. Establish IV access. If blood glucose

Endocrinology | Encyclopedia.com

views updated May 14 2018

  • History of endocrinology
  • Basic endocrine principles
  • The pituitary
  • The pineal
  • The thyroid
  • The parathyroids
  • The thymus
  • The pancreas
  • The adrenals
  • The ovaries
  • The testes
  • Endocrine disorders
  • Resources

The endocrine system is the human body’s group of specialized organs and tissues that produce, store, and secrete chemical hormones. This network of nine glands and over 100 hormones that maintain and regulate numerous events throughout the body. The glands of the endocrine system include the pituitary, thyroid, parathyroids, thymus, pancreas, pineal, adrenals, and ovaries or testes: in addition, the hypothalamus, in the brain, regulates the release of pituitary hormones. Each of these glands secrete hormones (chemical messengers) into the blood stream. Once hormones enter the blood, they travel throughout the body and are detected by receptors that recognize

specific hormones. These receptors exist on target cells and organs. Once a target site is bound by a particular hormone, a cascade of cellular events follows that culminates in the physiological response to a particular hormone.

The endocrine system differs from the exocrine system in that exocrine glands contain ducts that direct their hormones to specific sites; whereas endocrine hormones travel through blood until they reach their destination.

The endocrine is also similar to the nervous system, because both systems regulate body events and communicate through chemical messengers with target cells.

However, the nervous system transmits neurotransmitters (also chemical messengers) between neighboring neurons via nerve extension, and neurotransmitters do not generally enter the circulation.

Yet, some overlap between hormones and neurotransmitters exists, which gives rise to chemical signals called neurohormones that function as part of the neuroendocrine system. The endocrine system oversees many critical life processes involving metabolism, growth, reproduction, immunity, and homeostasis. The branch of medicine that studies endocrine glands and the hormones that they secrete is called endocrinology.

History of endocrinology

Although some ancient cultures noted biological observations grounded in endocrine function, modern understanding of endocrine glands and how they secrete hormones has evolved only in the last 300 years.

Ancient Egyptian and Chinese civilizations castrated (removed the testicles of) a servile class of men called eunuchs.

It was noted that eunuchs were less aggressive than other men, but the link of this behavior to testosterone was not made until recently.

Light was shed on endocrine function during the seventeenth and eighteenth centuries by a few significant advances. Seventeenth century English scientist, Thomas Wharton (1614–1673) noted the distinction between ductile and ductless glands.

In the 1690s, Dutch scientist Fredrik Ruysch (1638–1731) first stated that the thyroid secreted important substances into the blood stream.

A few decades later, Theophile Bordeu (1722–1776) claimed that “emanations” were given off by some body parts that influenced functions of other body parts.

One of the greatest early experiments performed in endocrinology was published by A. A. Berthold (1803–1861) in 1849. Berthold took six young male chickens, and castrated four of them. The other two chickens were left to develop normally; thus, they were used comparatively as control samples.

Two of the castrated chickens were left to become chicken eunuchs. However, what Berthold did with the other two castrated chickens is what really changed endocrinology. He transplanted the testes back into these two chickens at a distant site from where they were originally.

The two castrated chickens never matured into roosters with adult combs or feathers. However, the chickens that received transplanted testes did mature into normal adult roosters.

This experiment revealed that hormones that could access the blood stream from any site would function correctly in the body and that hormones did, in fact, travel freely in the circulation.

The same year Berthold published his findings, Thomas Addison (1793–1860), a British scientist, reported one of the first well documented endocrine diseases which was later named Addison’s disease (AD).

AD patients all had a gray complexion with sickly skin; they also had weak hearts and insufficient blood levels of hemoglobin necessary for oxygen transport throughout the body. On autopsy, each of the patients Addison studied was found to have diseased adrenal glands.

This disease can be controlled today if it is detected early. President John F. Kennedy (1917–1963) suffered from AD.

Basic endocrine principles

Most endocrine hormones are maintained at specific concentrations in the plasma, the non-cellular, liquid portion of the blood.

Receptors at set locations monitor plasma hormonal levels and inform the gland responsible for producing that hormone if levels are too high or too low for a particular time of day, month, or other life period. When excess hormone is present, a negative feedback loop is initiated such that further hormone production is inhibited.

Most hormones have this type of regulatory control. However, a few hormones operate on a positive feedback cycle such that high levels of the particular hormone will activate release of another hormone.

With this type of feedback loop, the end result is usually that the second hormone released will eventually decrease the initial hormone’s secretion. An example of positive feedback regulation occurs in the female menstrual cycle, where high levels of estrogen stimulate release of the pituitary hormone, luteinizing hormone (LH).

All hormones are influenced by numerous factors. The hypothalamus can release inhibitory or stimulatory hormones that determine pituitary function. In addition, every physiological component that enters the circulation can affect some endocrine function. Overall, this system uses multiple bits of chemical information to hormonally maintain a biochemically balanced organism.

Endocrine hormones do not fall into any one chemical class, but most are either a protein (polypep-tides, peptides, and glycoproteins are also included in this category) or a steroid.

Protein hormones bind cell-surface receptors and activate intracellular events that carry out the hormone’s response. Steroid hormones, on the other hand, usually travel directly into the cell and bind a receptor in the cell’s cytoplasm or nucleus.

From there, steroid hormones (bound to their receptors) interact directly with genes in the DNA (deoxy-ribonucleic acid) to elicit a hormonal response.

The pituitary

The pituitary gland has long been called the master gland, because it secretes multiple hormones that, in turn, trigger the release of other hormones from other endocrine sites.

The pituitary is roughly situated behind the nose and is anatomically separated into two distinct lobes, the anterior pituitary (AP) and the posterior pituitary (PP).

The entire pituitary hangs by a thin piece of tissue, called the pituitary stalk, beneath the hypothalamus in the brain. The AP and PP are sometimes called the adenohypophysis and neurohypophysis, respectively.

The PP secretes two hormones, oxytocin and anti-diuretic hormone (ADH), under direction from the hypothalamus. Direct innervation of the PP occurs from regions of the hypothalamus called the supraoptic and paraventricular nuclei.

Although the PP secretes its hormones into the bloodstream through blood vessels that supply it, it is regulated in a neuroendocrine fashion.

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The AP, on the other hand, receives hormonal signals from the blood supply within the pituitary stalk.

AP cells are categorized according to the hormones that they secrete. The hormone-producing cells of the AP include: somatotrophs, corticotrophs, thyrotrophs, lactotrophs, and gonadotrophs.

Somatotrophs secrete growth hormone; corticotrophs secrete adrenocortico-tropic hormone (ACTH); thyrotrophs secrete thyroid stimulating hormone (TSH); lactotrophs secrete prolactin; and gonadotrophs secrete LH and follicle stimulatory hormone (FSH). Each of these hormones sequentially signals a response at a target site.

While ACTH, TSH, LH, and FSH primarily stimulate other major endocrine glands, growth hormone and prolactin primarily coordinate an endocrine response directly on bones and mammary tissue, respectively.

The pineal

The pineal gland is a small cone-shaped gland believed to function as a body clock. The pineal is located deep in the brain just below the rear-most portion of the corpus callosum (a thick stretch of nerves that connects the two sides of the brain).

The pineal gland, also called the pineal body, has mystified scientists for centuries. Seventeenth century French mathematician and philosopher Rene´ Descartes (1596–1650) speculated that the pineal was the seat of the soul.

However, its real function is somewhat less grandiose than that described by Descartes.

The pineal secretes the hormone melatonin, which fluctuates on a daily basis with levels highest at night.

Although its role is not well understood, some scientists believe that melatonin helps to regulate other diurnal events, because melatonin fluctuates in a 24-hour period.

Exactly what controls melatonin levels is not well understood either; however, visual registration of light may regulate the cycle.

The thyroid

The thyroid is a butterfly-shaped gland that wraps around the back of the esophagus. The two lobes of the thyroid are connected by a band of tissue called the isthmus.

An external covering of connective tissue separates each lobe into another 20 to 40 follicles. Between the follicles are numerous blood and lymph vessels in another connective tissue called stroma.

The epithelial cells around the edge of the follicles produce the major thyroid hormones.

The major hormones produced by the thyroid are triiodothyronine (T3), thyroxine(T4), and calcitonin. T3 and T4 are iodine-rich molecules that fuel metabolism. The thyroid hormones play several important roles in growth, metabolism, and development. The thyroid of pregnant women often become enlarged in late pregnancy to accommodate metabolic requirements of both the woman and the fetus.

Thyroid hormones accelerate metabolism in several ways. They promote normal growth of bones and increase growth hormone output. They increase the rate of lipid synthesis and mobilization.

They increase cardiac output by increasing rate and strength of heart contractions. They can increase respiration, the number of red blood cells in the circulation, and the amount of oxygen carried in the blood.

In addition, they promote normal nervous system development including nerve branching.

The parathyroids

Endocrine Glands

Endocrine glands are glands that have no duct and release their secretions directly into the intercellular fluid or into the blood.

Differentiate among the types of endocrine glands (pituitary [posterior pituitary, anterior pituitary], thyroid, parathyroid, adrenal, and pancreas) in the endocrine system

Key Takeaways

Key Points

  • The main endocrine glands are the pituitary (anterior and posterior lobes), thyroid, parathyroid, adrenal (cortex and medulla), pancreas, and gonads.
  • The parathyroid glands are four masses of tissue, two embedded posteriorly in each lateral mass of the thyroid gland.
  • The pancreas lies along the lower curvature of the stomach, close to where it meets the first region of the small intestine, the duodenum.

Key Terms

  • adrenal gland: This gland is responsible for releasing hormones in response to stress through the synthesis of corticosteroids, such as cortisol and catecholamines (epinephrine (adrenaline) and norepinephrine), as well as the production of androgens.
  • thyroid: One of the largest endocrine glands, it is responsible for the secretion of thyroxine which controls how quickly the body uses energy, makes proteins, and is sensitive to other hormones.

Endocrine glands are ductless and release their secretions directly into the intercellular fluid or into the blood. A collection of endocrine glands makes up the endocrine system: the pituitary (anterior and posterior lobes), thyroid, parathyroid, adrenal (cortex and medulla), pancreas and gonads.

Endocrine glands in the human head and neck: The endocrine system is the system of glands, each of which secretes different types of hormones directly into the bloodstream (some of which are transported along nerve tracts) to regulate the body.

Pituitary Gland

The hypothalamus makes up the lower region of the diencephalons and lies just above the brain stem. The pituitary gland (hypophysis) is found in the inferior part of the brain, attached to the bottom of the hypothalamus by a slender stalk called the infundibulum.

The pituitary gland consists of two major regions, the anterior pituitary gland (adenohypophysis) and the posterior pituitary gland (neurohypophysis). The hypothalamus also controls the glandular secretion of the pituitary gland.

Posterior Pituitary Gland

Communication between the hypothalamus and the posterior pituitary occurs through neurosecretory cells that span the short distance between them. Hormones produced by the cell bodies of the neurosecretory cells are packaged in vesicles and transported through the axon and stored in the axon terminals that lie in the posterior pituitary.

When the neurosecretory cells are stimulated, the action potential generated triggers the release of the stored hormones from the axon terminals to a capillary network within the posterior pituitary. Two hormones—oxytocin and antidiuretic hormone (ADH)—are produced and released this way.

Anterior Pituitary Gland

The anterior pituitary is involved in sending hormones that control all other hormones of the body. Its lobe is derived from oral ectoderm and is composed of glandular epithelium.

Communication between the hypothalamus and the anterior pituitary occurs through hormones (releasing hormones and inhibiting hormones) that are produced by the hypothalamus and delivered to the anterior pituitary via a portal network of capillaries. The releasing and inhibiting hormones are produced by specialized neurons of the hypothalamus called neurosecretory cells.

These hormones are released into a capillary network that supplies the anterior pituitary. The hormones then diffuse from this secondary plexus into the anterior pituitary, where they initiate the production of specific hormones by the anterior pituitary.

Thyroid Gland

This is one of the largest endocrine glands in the body. It is positioned on the neck just below the larynx and has two lobes, one on either side of the trachea.

It is involved in the production of the hormones T3 (triiodothyronine) and T4 (thyroxine). These hormones increase the metabolic activity of the body‘s cells.

The thyroid also produces and releases the hormone calcitonin (thyrocalcitonin) that contributes to the regulation of blood calcium levels. Thyrocalcitonin or calcitonin decreases the concentration of calcium in the blood, where most of it is stored in the bones.

Parathyroid

There are four parathyroid glands, all located on the thyroid gland. One of its most important functions is to regulate the body’s calcium and phosphorus levels.

All four glands also secrete parathyroid hormone, or PTH, which causes calcium to be released from the bones back into the extracellular fluid. PTH is released directly into the bloodstream and travels to its target cells, which are often quite far away and found in bone, kidneys, and the gastrointestinal system.

Calcitonin, a hormone produced by the thyroid gland that also regulates ECF calcium levels, serves to counteract the calcium-producing effects of PTH.

Adrenal Glands

Adrenal glands: These are responsible for releasing hormones in response to stress through the synthesis of corticosteroids such as cortisol, and catecholamines such as epinephrine (adrenaline) and norepinephrine.

The adrenal glands are a pair of ductless glands located above the kidneys. Through hormonal secretions, they regulate many essential bodily functions including biochemical balances that influence athletic training and general stress response.

The glucocorticoids include corticosterone, cortisone, and hydrocortisone or cortisol. These hormones serve to stimulate the conversion of amino acids into carbohydrates, a process known as gluconeogenesis, and the formation of glycogen by the liver. They also stimulate the formation of reserve glycogen in the tissues, such as in the muscles.

Pancreas

The pancreas is a very important organ in the digestive and the circulatory systems that helps to maintain our blood sugar levels. It is considered to be part of the gastrointestinal system since it produces digestive enzymes. These are released into the small intestine to aid in reducing food particles to basic elements that can be absorbed by the intestine and used by the body.

It has another, very different function in that it forms insulin, glucagon, and other hormones that are sent into the bloodstream to regulate blood sugar levels and other activities throughout the body.

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