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Endocrine System: Overview
• Endocrine system – the body’s second great
controlling system which influences metabolic
activities of cells by means of hormones
• Endocrine glands – pituitary, thyroid, parathyroid,
adrenal, pineal, and thymus glands
Basic Concepts of Endocrine System
• The pancreas and gonads produce both hormones and
exocrine products
Dr. C.H. Lai
• The hypothalamus has both neural functions and
releases hormones
28-01-10
Endocrinology
• The study of hormones
• Other tissues and organs that produce hormones –
adipose cells, pockets of cells in the walls of the small
intestine, stomach, kidneys, and heart
Hormones
Hormones may be defined as
• Chemical substances that are secreted by living cells
• Released in small quantities into the blood, and upon
delivery by the circulation to a specific site of action
• Act to regulate reactions that elicit a typical response
Hormones
Hormones
• This is endocrine action
• Other modes of action include paracrine (acting on
adjacent cells) and neurocrine (secreted at nerve endings)
Chemical Natures of Hormones
Mechanisms of Hormone Actions
•
Proteins and peptides (e.g. insulin, posterior pituitary
hormones, hypothalamic releasing hormones, and GI
hormones)
•
Amines (e.g. adrenaline, thyroxine)
• Alter plasma membrane permeability
•
Steroids (e.g. adrenal corticosteroids, androgens,
estrogen)
• Activate or deactivate enzyme systems
Hormones produce one or more of the following cellular
changes:
• Stimulate protein synthesis
• Induce secretory activity
• Stimulate mitosis
Location of the Major Endocrine Glands
• The major endocrine glands
include:
• Pineal gland, hypothalamus,
and pituitary
Pituitary Gland
Pituitary (hypophysis) – attached to the hypothalamus by
infundibulum
• Anterior (adenohypophysis)
• Posterior (neurohypophysis)
• Thyroid, parathyroid, and
thymus
• Adrenal glands and
pancreas
• Gonads – male testes and
female ovaries
The Anterior Pituitary Hormones
• Many of the hormones secreted are trophic hormones
that control the secretion of other endocrine glands.
• Excess trophic hormone can cause hypertrophy of target
gland.
• Insufficient amounts can cause atrophy of target gland
The Anterior Pituitary Hormones
• The six hormones of the anterior pituitary gland :
• Growth hormone (GH)
• Thyroid-stimulating Hormone (TSH)
• Adrenocorticotropic Hormone (ACTH)
• Luteinizing Hormone (LH)
• Follicle-stimulating Hormone (FSH)
• Prolactin (PRL)
• Regulate the activity of
other endocrine glands
The Anterior Pituitary Hormones
• GH – promotes movement of amino acid into cells.
• TSH – stimulates thyroid to secrete T4.
• ACTH – stimulates adrenal cortex to secrete
glucocorticoids.
• FSH – stimulates growth of follicles or production
of sperm.
• LH – stimulates ovulation and growth of corpus
luteum or secretion of testosterone.
The Anterior Pituitary Hormones
Growth hormone (GH)
• Stimulates bone growth
• Increases protein synthesis
• Stimulates gluconeogenesis, decreases utilization and
uptake of glucose
• Increase lipolysis
(breakdown of fat)
GH
• PRL – stimulates milk production & …
Endocrine Disturbances of Growth and Development
Dwarfism
Giantism
Enlarged hand of the same patient in
comparison with the hand of an adult
male with a height of 6'1".
Twelve-year-old boy with pituitary gigantism
measuring 6'5" with his mother. Note the
coarse facial features and prominent jaw.
Endocrine Disturbances of Growth and Development
Acromegaly
(adult)
The Anterior Pituitary Hormones
Thyroid-stimulating Hormone (TSH)
• TSH stimulates the synthesis and secretion of thyroid
hormones from the thyroid
The Anterior Pituitary Hormones
Adrenocorticotropic Hormone (ACTH)
• ACTH stimulates the synthesis and secretion of cortisol
from the adrenal cortex
The Anterior Pituitary Hormones
The Anterior Pituitary Hormones
Gonadotrophins
Gonadotrophins: FSH
Luteinizing Hormone (LH)
Follicle-stimulating Hormone (FSH)
In female:
• Promotes the development of the follicles in the ovary
In male:
• Stimulates the formation and maturation of sperms in the
testis
The Anterior Pituitary Hormones
Gonadotrophins: LH
In female:
• Involved in the maturation of follicles in the ovary
• Stimulates the secretion of estrogens
The Anterior Pituitary Hormones
Prolactin (PRL)
• It stimulates and maintains lactation after estrogen and
progesterone priming and reduces the responsiveness of
the gonads to gonadotrophins
• Stimulates the secretion of progesterone from the corpus
luteum
In male:
• Stimulates the secretion of testosterone from the Leydig
cells of the testis
The Anterior Pituitary Hormones
Hypothalamus / Pituitary Gland
The hypothalamic releasing hormones regulate the
synthesis and release of anterior pituitary hormones.
Hypothalamus / Pituitary Gland
Hypothalamus / Pituitary Gland
• Anterior pituitary control is regulated by negative
feedback by two methods:
• The target gland hormone can act on the
hypothalamus and inhibit secretion of releasing
hormones.
• The target gland hormone can act on the anterior
pituitary and inhibit response to the releasing
hormone.
GHIH
PIH, dopamine
Hypothalamus / Pituitary Gland
Posterior Pituitary Hormones
Posterior Pituitary Hormones
• Oxytoxin and vasopressin are synthesized in the
hypothalamus and transported down the axons to the
posterior pituitary via axons of the hypothalamohypophyseal tract
Posterior Pituitary Hormones
Antidiuretic hormone (ADH, vasopressin).
• Antidiuretic action, increases water reabsorption in the
distal tubules and collecting ducts in the kidney
• Pressor action, increases blood pressure by
vasoconstriction
Posterior Pituitary Hormones
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Oxytocin
• Effect on milk ejection
- Causes contraction of the myoepithelium of the breast
• Effect on the uterus
Thyroid & Parathyroid Glands
- Causes contraction in parturition
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Thyroid Glands
Thyroid Glands
• Consists of 2 lateral lobes joined by a thin isthmus, on
either side of the trachea
Thyroid Glands
• Composed of follicles with colloid (thyroglobulin) in
the lumen
Thyroid Hormone (Thyroxine)
Physiological Effects
Colloid
(1) ↑ Basal metabolic rate and growth
(2) Carbohydrate metabolism
• ↑ Peripheral utilization of glucose (glycolysis)
• ↑ Glycogen breakdown
• ↑ Gluconeogenesis (amino acids, glycerol or
lactate to glucose/glycogen)
• ↑ GI glucose absorption
Thyroid Hormone (Thyroxine)
(3) Heart contraction
Thyroid Hormone (Thyroxine)
(5) Temperature regulation
(4) Central nervous system
• ↑ Heat production (↑ Cellular activities)
• Essential for the normal development within the
critical period (first year of life).
•
Important for response to cold
• Hyperthyroidism → hyper-excitability, irritability,
restlessness
• Hypothyroidism → listlessness, lack of energy,
decrease sensory capacity
Diseases of the Thyroid
Diseases of the Thyroid
• Hypothyroid – too little hormone secreted
• Goiter – caused by lack of iodine
TSH is a trophic hormone, it stimulates not only
T3/T4 secretion but also protein synthesis in
follicular cells. Therefore, ↑ exposure in thyroid ↑
size
Diseases of the Thyroid
Hyperthyroid – too much hormone secreted.
• Graves’ disease –
Diseases of the Thyroid
Hyperthyroid –
Graves’ disease
• Autoantibodies that mimic TSH overstimulate
thyroid.
• Produces high BMR, irritability, weight loss,
exophthalmos.
Thyroid Glands
Calcitonin –
• Secreted by the parafollicular cells (C-cells) of the thyroid
gland
• Blocks osteoclast activity
• Decrease the formation of 1,25 (OH)2 Vit. D (in kidney)
Swelling within the orbital
cavities and enlargement of
the perorbital muscles behind
the eyes
Thyroid Glands : Calcitonin
Calcitonin –
• Acts as an antagonist to parathyroid hormone (PTH)
Parathyroid Glands
Parathyroid Glands
Parathyroid glands – embedded in posterior surface of thyroid
• Secretes parathyroid hormone (PTH).
• Increases serum calcium levels by:
− Stimulating osteoclasts to resorb bone
− Reabsorption of Ca++ by kidneys
− Blocks reabsorption of phosphate by
kidneys
− Promotes production of 1, 25-dihydroxyvitamin D3
PTH (parathormone）
PTH
• Causes bone resorption through rapid formation of new
osteoclasts and transient depression of osteoblasts
•Osteoclasts are for bone resorption and osteoblasts are for bone formation)
PTH
Resorption of bone by osteoclasts
PTH
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• Increases kidney calcium
reabsorption and
decreases phosphate
reabsorption
Adrenal Gland
Dr. C.H. Lai
• Increases the formation
of the active form of
vitamin D to increase GI
calcium and phosphate
absorption
Adrenal Gland
• The adrenals (suprarenals, in human) lie above the
kidneys
Adrenal Gland
Adrenal Cortex
• The outer cortex is made up of 3 zones: zona glomerulosa
(secretes aldosterone), z. fasciculata and z. reticularis
(secrete cortisol & androgens)
Adrenal Cortex
Aldosterone
Functions
Adrenal Cortex: Aldosterone
Functions
• Increases the reabsorption of sodium in the distal tubules
and hence excretion of potassium and hydrogen ions
• Increase in sodium reabsorption results in increase in
water reabsorption, hence increase in blood volume
Cortisol
Functions
• Carbohydrate
- ↑ gluconeogenesis (from amino acids)
- ↓ glucose utilization & uptake
- too much leads to ↑ blood glucose
• Protein - ↓ protein synthesis (except liver), ↑ protein
breakdown.
• Fat (triglycerides) - ↑ lipolysis (permissive).
Cortisol
Functions
Cortisol
Metabolic effects of glucocorticoids
• Response to stress – source of energy, synthesis of new
proteins, maintenance of blood pressure
• Anti-inflammatory action
Cortisol
Regulation of Secretion
• CRH stimulates ACTH synthesis/ secretion, and ACTH
stimulates cortisol synthesis/secretion
• Negative feed-back – by free circulating cortisol
• Stress stimulates
• Diurnal rhythm
– high in the morning,
low in the evening.
Adrenal Cortex: Hyper Secretion
e.g. Cushing's Syndrome
Adrenaline & Noradrenaline
Adrenal Gland
Adrenal medulla
Hormones
Actions
• Adrenaline & noradrenaline (also from sympathetic
nerve endings)
(1) Heart contraction, vasoconstriction (alpha),
vasodilation (beta 2)
Receptors
(2) Metabolism - glycogenolysis (liver/muscle), lipolysis
• Adrenaline- equally on alpha & beta receptors
(3) Contraction of smooth muscles
• Noradrenaline- mainly on alpha, also on beta 1 (heart)
Regulation of Secretion
(1) Sympathetic nervous system
(2) Stress
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Pancreatic Hormones
Insulin / Glucagon
Pancreatic Hormones &
Glucose Metabolism
Dr. C.H. Lai
• Both are peptide hormones secreted by the pancreas;
insulin by beta cells and glucagon by alpha cells
Pancreatic Hormones
Pancreatic Hormones
Insulin
(1) Physiological effects
•
Increases the uptake of glucose by muscle, adipose
tissue (transporters), and liver, increases glycolysis
- Excess is converted to glycogen or fat
Pancreatic Hormones
Glucagon
(1) Physiological effects
•
Increases blood glucose levels by increasing
glycogenolysis and gluconeogensis
•
Increases lipolysis
•
Decreases gluconeogenesis (in liver)
•
Decreases blood glucose level
•
Increases lipogenesis in the liver and decreases
lipolysis in the adipose tissue
•
Increases protein synthesis
Regulation of Insulin and Glucagon Secretion
Diseases of the Pancreas
Diseases of the Pancreas
Diabetes mellitus
• Two types: insulin dependent or non-insulin dependent;
due to lack of insulin secretion or response to insulin,
leading to hyperglycaemia (increase in blood glucose)
• Hyperglycemia – high plasma [glucose]
• IDDM (type I) – β cells do not secrete insulin. May
be an autoimmune disorder. Hyperglycemia and
glycosuria occur. Requires insulin injections.
• NIDDM (type II) – β cells decrease insulin output
and/or cells become resistant to insulin (e.g.
downregulate receptors). Hyperglycemia but not
glycosuria (except severe cases). Treated with oral
pancreatic stimulators and/or drugs that decrease
insulin resistance
Diabetes Mellitus (DM)
Diseases of the Pancreas
• Hypoglycemia - low plasma [glucose]
• If plasma [glucose] drops too low, coma develops.