The document summarizes key aspects of the endocrine system and hormone signaling. It describes how hormones are chemical messengers that circulate through the bloodstream and activate target cells. The endocrine system works alongside the nervous system to regulate slower but longer-acting responses, such as reproduction and metabolism. Examples are provided of hormone types, the mechanisms of hormone signaling, feedback loops that regulate hormone levels, and specific endocrine glands like the pituitary, thyroid, and parathyroid glands. Diseases associated with hormonal imbalances like diabetes and hyperthyroidism are also mentioned.

1. Hormones & Endocrine System
M. Faisal Shahid and Humera Waheed, Ph.D.
PCMD, ICCBS.
University of Karachi

2. Long-Distance Regulators
• Bio-chemical signals “Secreated” in
circulatory system to modulate regulatory
messages in the organ system.
• Activate specifically on equipped target cells
i.e. which have receptor.

3. • Two systems coordination:
– a) Endocrine (Ductless) system
– b) Nervous system
• Endocrine system secretes hormones that
coordinate slower but longer-acting
responses
– E.g. Reproduction & Development, Energy
metabolism, growth, and behavior
• Nervous system conveys high-speed
electrical signals along specialized cells
called neurons.

4. Classification of Secreted Signaling Molecules
– Hormones: Long Range, Slow acting, Long Lasting
(E.g. Insulin)
– Local regulators: Small Range, Swift acting, Long to
Medium Lasting. (E.g. TNFα)
– Neurotransmitters: Single Cell Ranged, Highly
Focused and Fast Acting (e.g. Acetylcholine)
– Neurohormones: Long Range, Highly Specific (ATCH,
FSH)
– Pheromones: Shed in environment, v. long range, for
mating processes. (E.g. Chilogttone)

5. The Basics
• Endocrine signals travel via the bloodstream
• Endocrine glands are ductless and secrete
hormones directly into surrounding fluid

6. Signaling Pattern
Followed by
Hormones
Blood
vessel Response
Response
Response
Response
(a) Endocrine signaling
(b) Paracrine signaling
(c) Autocrine signaling
(d) Synaptic signaling
Neuron
Neurosecretory
cell
Figure 1: Signaling Classification
Blood
vessel
Synapse
Response

7. Blood
vessel Response
Response
Response
(a) Endocrine signaling
(b) Paracrine signaling
(c) Autocrine signaling
Figure 2: Signaling Classification (Generalized)

8. Neurotransmitters
• Neurons (nerve cells) contact target cells at
synapses (Infinitesimally short distances).
• Neurotransmitters play a role in sensation,
memory, cognition, and movement

9. Response
(d) Synaptic signaling
Neuron
Neurosecretory
cell
Blood vessel
Synapse
Response
Figure 3: Signaling Classification (Continued)

10. • Neurohormones: Hormones that originate
from neurons of brain and diffuse through the
bloodstream

11. Pheromones
• Pheromones are chemical signals that are
released from the body and used to
communicate with other individuals in the
species
• Pheromones mark trails to food sources, warn
of predators, and attract potential mates

12. Chemical Classes of Hormones
• Molecular function of hormones in vertebrates:
– Polypeptides (proteins and peptides)
– Amines derived from amino acids
– Steroid hormones

13. • Lipid-soluble: Cross cell membranes ACTS
INSIDE CELLS.
• Water-soluble hormones: Act extracellularly

14. Figure 4: Hormones according to solubilty
Water-soluble Lipid-soluble
Steroid:
Cortisol
Polypeptide:
Insulin
Amine:
Epinephrine Amine:
Thyroxine

15. • The Endocrine Response: 3 Phases
– Reception: Input
– Signal transduction: Processing
– Response: Output

16. NUCLEUS
Signal
receptor
(a) (b)
TARGET
CELL
Signal receptor
Transport
protein
Water-
soluble
hormone
Fat-soluble
hormone
Figure 5 (a)

17. Figure 5 (b)
Signal
receptor
TARGET
CELL
Signal receptor
Transport
protein
Water-
soluble
hormone
Fat-soluble
hormone
Gene
regulation
Cytoplasmic
response
Gene
regulation
Cytoplasmic
response
OR
(a) NUCLEUS (b)

18. Signaling
• Binding of a hormone onreceptor initiates a
signal transduction.
• It leads to responses in the cytoplasm for
either:
– Enzyme activation
– Change in gene expression

19. • The hormone epinephrine has multiple effects
in mediating the body’s response to short-term
stress
• Epinephrine binds to receptors on the plasma
membrane of liver cells
• This triggers the release of messenger
molecules that activate enzymes and result in
the release of glucose into the bloodstream

20. Figure 6: Epinephrine Cascade
cAMP Second
messenger
Adenylyl
cyclase
G protein-coupled
receptor
ATP
GTP
G protein
Epinephrine

21. Figure 6 (b): Epinephrine Cascade
cAMP Second
messenger
Adenylyl
cyclase
G protein-coupled
receptor
ATP
GTP
G protein
Epinephrine
Inhibition of
glycogen synthesis
Promotion of
glycogen breakdown
Protein
kinase A

22. Lipid-Soluble Hormones
• Usually cause gene expression alteration
• Steroids, thyroid hormones, and the hormonal
form of vitamin D enter target cells and bind to
protein receptors in the cytoplasm or nucleus
• Protein-receptor complexes then act as
transcription factors in the nucleus, regulating
transcription of specific genes

23. Figure 7. Steroid Hormones
Signaling
Hormone
(estradiol)
Hormone-receptor
complex
Plasma
membrane
Estradiol
(estrogen)
receptor

24. Figure 7 (b): Steroid Hormone Signaling
Hormone
(estradiol)
Hormone-receptor
complex
Plasma
membrane
Estradiol
(estrogen)
receptor
DNA
Vitellogenin
mRNA
for vitellogenin

25. Multiple Effects of Hormones
• The same hormone may have different effects
on target cells that have
– Different receptors for the hormone
– Different signal transduction pathways
– Different proteins for carrying out the response
• A hormone can also have different effects in
different species

26. Figure 8. Differential Effects of Same Hormone: Epinephrine
Glycogen
deposits
β receptor
Vessel
dilates.
Epinephrine
(a) Liver cell
Epinephrine
β receptor
Glycogen
breaks down
and glucose
is released.
(b) Skeletal muscle
blood vessel
Same receptors but different
intracellular proteins (not shown)

27. Glycogen
deposits
β receptor
Vessel
dilates.
Epinephrine
(a) Liver cell
Epinephrine
β receptor
Glycogen
breaks down
and glucose
is released.
(b) Skeletal muscle
blood vessel
Same receptors but different
intracellular proteins (not shown)
Epinephrine
β receptor
Different receptors
Epinephrine
α receptor
Vessel
constricts.
(c) Intestinal blood
vessel
Figure 8 (b). Differential Effects of Same Hormone: Epinephrine

28. Localized Signaling: Paracrine
• In paracrine signaling, non-hormonal chemical signals
called local regulators elicit responses in nearby target
cells
• Types of local regulators:
– Cytokines and growth factors (Interferon etc.)
– Nitric oxide (NO)
– Prostaglandins (The PAIN ALARMS)

29. Major endocrine glands:
Adrenal
glands
Hypothalamus
Pineal gland
Pituitary gland
Thyroid gland
Parathyroid glands
Pancreas
Kidney
Ovaries
Testes
Organs containing
endocrine cells:
Thymus
Heart
Liver
Stomach
Kidney
Small
intestine
Figure 9. Major
Hormone
Secretion Sites

30. Figure 10. The Feed-back Loops
Pathway Example
Stimulus Low pH in
duodenum
S cells of duodenum
secrete secretin ( )
Endocrine
cell
Blood
vessel
PancreasTarget
cells
Response Bicarbonate release
Negativefeedback
–

31. • A negative feedback loop inhibits a response
by reducing the initial stimulus
• Negative feedback regulates many hormonal
pathways involved in homeostasis

32. Insulin and Glucagon: Control of Blood Glucose
• Insulin and glucagon are antagonistic
hormones that help maintain glucose
homeostasis
• The pancreas has clusters of endocrine cells
called islets of Langerhans with alpha cells
that produce glucagon and beta cells that
produce insulin

33. Figure 11. The Insulin Loop
Homeostasis:
Blood glucose level
(about 90 mg/100 mL)
Insulin
Beta cells of
pancreas
release insulin
into the blood.
STIMULUS:
Blood glucose level
rises.

34. Figure 12. Insulin Loop
Homeostasis:
Blood glucose level
(about 90 mg/100 mL)
Insulin
Beta cells of
pancreas
release insulin
into the blood.
STIMULUS:
Blood glucose level
rises.
Liver takes
up glucose
and stores it
as glycogen.
Blood glucose
level declines.
Body cells
take up more
glucose.

35. Figure 13. The Insulin-Glucagon
Loop
Homeostasis:
Blood glucose level
(about 90 mg/100 mL)
Glucagon
STIMULUS:
Blood glucose level
falls.
Alpha cells of pancreas
release glucagon.

36. Figure 14: The Insulin Glucagon Loop
Homeostasis:
Blood glucose level
(about 90 mg/100 mL)
Glucagon
STIMULUS:
Blood glucose level
falls.
Alpha cells of pancreas
release glucagon.
Liver breaks
down glycogen
and releases
glucose.
Blood glucose
level rises.

37. Target Tissues for Insulin and Glucagon
• Insulin reduces blood glucose levels by
– Promoting the cellular uptake of glucose
– Slowing glycogen breakdown in the liver
– Promoting fat storage

38. • Glucagon increases blood glucose levels by
– Stimulating conversion of glycogen to glucose
in the liver
– Stimulating breakdown of fat and protein into
glucose

39. Diabetes Mellitus
• Diabetes mellitus is perhaps the best-known
endocrine disorder
• It is caused by a deficiency of insulin or a
decreased response to insulin in target tissues
• It is marked by elevated blood glucose levels

40. • Type I diabetes mellitus (insulin-dependent) is
an autoimmune disorder in which the immune
system destroys pancreatic beta cells
• Type II diabetes mellitus (non-insulin-
dependent) involves insulin deficiency or
reduced response of target cells due to change
in insulin receptors

41. Coordination of Endocrine and Nervous Systems
in Vertebrates

42. • The hypothalamus receives information from
the nervous system and initiates responses
through the endocrine system
• Attached to the hypothalamus is the pituitary
gland composed of the posterior pituitary and
anterior pituitary

43. • The posterior pituitary stores and secretes
hormones that are made in the hypothalamus
• The anterior pituitary makes and releases
hormones under regulation of the
hypothalamus

44. Spinal cord
Posterior
pituitary
Cerebellum
Pineal
gland
Anterior
pituitary
Hypothalamus
Pituitary
gland
Hypothalamus
Thalamus
Cerebrum
Figure 15: The Pituitary Gland

45. Figure 16. The Hormonal Index

46. Figure 16 (b). The Hormonal Index

47. Figure 16 (c) . The Hormonal Index

48. Figure 16 (d). The Hormonal Index

49. Figure 16(e). The Hormonal Index

50. Posterior Pituitary Hormones
• The two hormones released from the posterior
pituitary act directly on nonendocrine tissues

51. Figure 17. The Posterior Pituitary
Hormones
Posterior
pituitary
Anterior
pituitary
Neurosecretory
cells of the
hypothalamus
Hypothalamus
Axon
HORMONE OxytocinADH
Kidney tubulesTARGET Mammary glands,
uterine muscles

52. • Oxytocin induces uterine contractions and the
release of milk (positive feedback)
• Antidiuretic hormone (ADH) enhances water
reabsorption in the kidneys

53. Anterior Pituitary Hormones
• Hormone production in the anterior pituitary is
controlled by releasing and inhibiting hormones
from the hypothalamus
• For example, the production of thyrotropin
releasing hormone (TRH) in the hypothalamus
stimulates secretion of the thyroid stimulating
hormone (TSH) from the anterior pituitary

54. Figure 18. Anterior Pituitary
Hormones
Hypothalamic
releasing and
inhibiting
hormones
Neurosecretory cells
of the hypothalamus
HORMONE
TARGET
Posterior pituitary
Portal vessels
Endocrine cells of
the anterior pituitary
Pituitary hormones
Tropic effects only:
FSH
LH
TSH
ACTH
Nontropic effects only:
Prolactin
MSH
Nontropic and tropic effects:
GH
Testes or
ovaries
Thyroid
FSH and LH TSH
Adrenal
cortex
Mammary
glands
ACTH Prolactin MSH GH
Melanocytes Liver, bones,
other tissues

55. Hormone Cascade Pathways
• A primary Hormone Targetting another
Hormone Secreting Gland is a TROPIC
HORMONE.
• Hormone cascade pathways are usually
regulated by negative feedback

56. Tropic Hormones
• A tropic hormone regulates the function of
endocrine cells or glands
• The four strictly tropic hormones are
– Thyroid-stimulating hormone (TSH)
– Follicle-stimulating hormone (FSH)
– Luteinizing hormone (LH)
– Adrenocorticotropic hormone (ACTH)

57. Nontropic Hormones
• Nontropic hormones target nonendocrine
tissues
• Nontropic hormones produced by the anterior
pituitary are
– Prolactin (PRL)
– Melanocyte-stimulating hormone (MSH)

58. • Prolactin stimulates lactation in mammals but
has diverse effects in different vertebrates
• MSH influences skin pigmentation in some
vertebrates and fat metabolism in mammals

59. Growth Hormone
• Growth hormone (GH) is secreted by the
anterior pituitary gland and has tropic and
nontropic actions
• It promotes growth directly and has diverse
metabolic effects
• It stimulates production of growth factors
• An excess of GH can cause gigantism, while a
lack of GH can cause dwarfism

60. Thyroid Hormone: Control of Metabolism and
Development
• The thyroid gland consists of two lobes on the
ventral surface of the trachea
• It produces two iodine-containing hormones:
triiodothyronine (T3) and thyroxine (T4)

61. • Thyroid hormones stimulate metabolism and
influence development and maturation
• Hyperthyroidism, excessive secretion of thyroid
hormones, causes high body temperature,
weight loss, irritability, and high blood pressure
• Graves’ disease is a form of hyperthyroidism in
humans
• Hypothyroidism, low secretion of thyroid
hormones, causes weight gain, lethargy, and
intolerance to cold

62. Normal
iodine
uptake
High level
iodine
uptake
Figure 19: Iodine 131 Uptake Micrograph

63. Parathyroid Hormone and Vitamin D: Control of
Blood Calcium
• Two antagonistic hormones regulate the
homeostasis of calcium (Ca2+
) in the blood of
mammals
– Parathyroid hormone (PTH) is released by
the parathyroid glands
– Calcitonin is released by the thyroid gland

64. Figure 20. Parathyroid Loop
PTH
Parathyroid gland
(behind thyroid)
STIMULUS:
Falling blood
Ca2+
level
Homeostasis:
Blood Ca2+
level
(about 10 mg/100 mL)

65. Figure 20 (b). The Parathyroid Loop
PTH
Parathyroid gland
(behind thyroid)
STIMULUS:
Falling blood
Ca2+
level
Homeostasis:
Blood Ca2+
level
(about 10 mg/100 mL)
Blood Ca2+
level rises.
Stimulates Ca2+
uptake in kidneys
Stimulates
Ca2+
release
from bones
Increases
Ca2+
uptake
in intestines
Active
vitamin D

66. • PTH increases the level of blood Ca2+
– It releases Ca2+
from bone and stimulates
reabsorption of Ca2+
in the kidneys
– It also has an indirect effect, stimulating the
kidneys to activate vitamin D, which promotes
intestinal uptake of Ca2+
from food
• Calcitonin decreases the level of blood Ca2+
– It stimulates Ca2+
deposition in bones and
secretion by kidneys
Control of Blood Calcium

67. Adrenal Hormones: Response to Stress
• The adrenal glands are adjacent to the kidneys
• Each adrenal gland actually consists of two
glands: the adrenal medulla (inner portion) and
adrenal cortex (outer portion)

68. Catecholamines from the Adrenal Medulla
• The adrenal medulla secretes epinephrine
(adrenaline) and norepinephrine
(noradrenaline)
• These hormones are members of a class of
compounds called catecholamines
• They are secreted in response to stress-
activated impulses from the nervous system
• They mediate various fight-or-flight responses

69. • Epinephrine and norepinephrine
– Trigger the release of glucose and fatty acids
into the blood
– Increase oxygen delivery to body cells
– Direct blood toward heart, brain, and skeletal
muscles, and away from skin, digestive
system, and kidneys
• The release of epinephrine and norepinephrine
occurs in response to nerve signals from the
hypothalamus

70. Figure 21:The Adrenal Hormones Loop
Stress
Adrenal
gland
Nerve
cell
Nerve
signals
Releasing
hormone
Hypothalamus
Anterior pituitary
Blood vessel
ACTH
Adrenal cortex
Spinal cord
Adrenal medulla
Kidney
(a) Short-term stress response (b) Long-term stress response
Effects of epinephrine and norepinephrine:
2. Increased blood pressure
3. Increased breathing rate
4. Increased metabolic rate
1. Glycogen broken down to glucose; increased blood glucose
5. Change in blood flow patterns, leading to increased
alertness and decreased digestive, excretory, and
reproductive system activity
Effects of
mineralocorticoids:
Effects of
glucocorticoids:
1. Retention of sodium
ions and water by
kidneys
2. Increased blood
volume and blood
pressure
2. Possible suppression of
immune system
1. Proteins and fats broken down
and converted to glucose, leading
to increased blood glucose

71. Figure 21 (b): The Adrenal Hormones Loop
Stress
Adrenal
gland
Nerve
cell
Nerve
signals
Releasing
hormone
Hypothalamus
Anterior pituitary
Blood vessel
ACTH
Adrenal
cortex
Spinal cord
Adrenal
medulla
Kidney

72. Figure 21 (c) :The Adrenal Hormones Loop
(a) Short-term stress response
Effects of epinephrine and norepinephrine:
2. Increased blood pressure
3. Increased breathing rate
4. Increased metabolic rate
1. Glycogen broken down to glucose; increased blood glucose
5. Change in blood flow patterns, leading to increased
alertness and decreased digestive, excretory, and
reproductive system activity
Adrenal
gland
Adrenal medulla
Kidney

73. The Corticoids
• The adrenal cortex releases a family of steroids
called corticosteroids in response to stress
• These hormones are triggered by a hormone
cascade pathway via the hypothalamus and
anterior pituitary
• Humans produce two types of corticosteroids:
glucocorticoids and mineralocorticoids

74. Figure 22. Adrenal Hormones
(b) Long-term stress response
Effects of
mineralocorticoids:
Effects of
glucocorticoids:
1. Retention of sodium
ions and water by
kidneys
2. Increased blood
volume and blood
pressure
2. Possible suppression of
immune system
1. Proteins and fats broken down
and converted to glucose, leading
to increased blood glucose
Adrenal
gland
Kidney
Adrenal cortex

75. • Glucocorticoids, such as cortisol, influence
glucose metabolism and the immune system
• Mineralocorticoids, such as aldosterone,
affect salt and water balance
• The adrenal cortex also produces small
amounts of steroid hormones that function as
sex hormones

76. Gonadal Gender Specific Hormones
• The gonads, testes and ovaries, produce most
of the sex hormones: androgens, estrogens,
and progestins
• All three sex hormones are found in both males
and females, but in different amounts

77. • Testes: androgens, mainly testosterone,
which stimulate development and maintenance
of the male reproductive system
• Testosterone causes an increase in muscle
and bone mass and is often taken as a
supplement to cause muscle growth, which
carries health risks

78. • Estrogens, most importantly estradiol, are
responsible for maintenance of the female
reproductive system and the development of
female secondary sex characteristics
• In mammals, progestins, which include
progesterone, are primarily involved in
preparing and maintaining the uterus
• Synthesis of the sex hormones is controlled by
FSH and LH from the anterior pituitary

79. Melatonin and Biorhythms
• The pineal gland, located in the brain,
secretes melatonin
• Light/dark cycles control release of melatonin
• Primary functions of melatonin appear to relate
to biological rhythms associated with
reproduction

80. Figure 23. Hormonal Process of Milk Production

81. Now, One can:
1. Distinguish between the following pairs of
terms: hormones and local regulators,
paracrine and autocrine signals
2. Describe the evidence that steroid hormones
have intracellular receptors, while water-
soluble hormones have cell-surface receptors
3. Explain how the antagonistic hormones insulin
and glucagon regulate carbohydrate
metabolism
4. Distinguish between type 1 and type 2
diabetes

82. 5. Explain how the hypothalamus and the
pituitary glands interact and how they
coordinate the endocrine system
6. Explain the role of tropic hormones in
coordinating endocrine signaling throughout
the body
7. List and describe the functions of hormones
released by the following: anterior and
posterior pituitary lobes, thyroid glands,
parathyroid glands, adrenal medulla, adrenal
cortex, gonads, pineal gland

83. Thank You

84. Questions?