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ENDOCRINE SYSTEM
The endocrine system is all the organs of the body that are endocrine glands.
An endocrine gland secretes endocrine hormones.
Endocrine hormones are hormones that are secreted into the blood.
Hormones are substances that are secreted by one group of cells that affects the
physiology of another group of cells (organs). The endocrine system is controlled by the
pituitary gland and the hypothalamus.
Compared to most other organs in the body, endocrine organs are well vascularized.
The Endocrine System:
• A system of ductless glands
– Secrete messenger molecules called hormones
• Interacts closely with the nervous system
• Endocrinology – study of hormones and endocrine glands
Major Endocrine Glands
• Hypothalamus
• Pituitary Gland
• Thyroid Gland
• Parathyroid Glands
• Thymus Gland
• Adrenal Glands
• Pancreas
• Ovaries
• Testes
• Pineal Gland
Endocrine Organs:
• Scattered throughout the body
• Pure endocrine organs
– Pituitary, pineal, thyroid, parathyroid, and adrenal glands
• Organs containing endocrine cells
– Pancreas, thymus, gonads, and the hypothalamus
• Richly vascularized
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Control of Hormone Secretion
•The endocrine system is controlled by the pituitary gland and the hypothalamus.
•Always controlled by feedback loops
–Blood concentration declines below a minimum: more hormone is secreted
–Blood concentration exceeds maximum: Hormone production is halted
HYPOTHALAMUS
This is located at the base of the brain.
– Secretes releasing factors to release hormones
– Secretes inhibiting hormones to turn off secretion of hormones
PITUITARY GLAND
• The adenohypophysis portion of the pituitary gland actually develops from an
pouch that grows upward from the pharynx!
• One type of diabetes (insipidus) can be caused by trauma to the pituitary gland.
The other type of diabetes (mellitus) is caused by a problem with the pancreas.
• A tumor of the pituitary gland can lead to blindness because it is so close to the
optic nerve.
• Secretes nine major hormones
• Attached to the hypothalamus by the infundibulum (stalk)
The pituitary gland has two parts.
ADENOHYPOPHYSIS is the anterior pituitary.
NEUROHYPOPHYSIS is the posterior pituitary.
ADENOHYPOPHYSIS (Anterior Pituitary) Secretes hormones which affect other glands.
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Growth hormone (GH): Causes the body to grow
Prolactin (PRL)
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Stimulates lactation (milk production) in females
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Produces desire to cry
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Decreases in adolescent males, decreased desire to cry
Thyroid Stimulating Hormone (TSH)
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Causes the thyroid gland to release thyroid hormone
Adrenocorticotropic hormone (ACTH)
– Acts on adrenal cortex to stimulate the release of cortisol
– Helps people cope with stress
Melanocyte-stimulating hormone (MSH)
– Darkens skin pigmentation
– increases during pregnancy.
Follicle-stimulating hormone (FSH)
– Present in males and females, affects both
– Stimulates maturation of sex cells
Luteinizing hormone (LH)
– Induces ovulation in females
– Induces testosterone in males
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Study Tip to remember the hormones secreted by the anterior pituitary gland
“Melons grow and produce through late fall” stands for the hormones made in the
anterior pituitary.
• Melanocyte stimulating hormone (MSH)
• Growth Hormone (GH)
• Adrenal corticotropic Hormone (ACTH)
• Prolactin (PRL)
• Thyroid stimulating hormone (TSH)
• Luteinizing Hormone (LH)
• Follicle stimulating Hormone (FSH)
NEUROHYPOPHYSIS (Posterior Pituitary)
• Oxytocin
– Childbirth contractions
• Antidiuretic hormone (ADH)
– Signals kidneys to increase water reabsorption
HYPOTHALAMUS
The hypothalamus produces hormones which affect the pituitary, for example:
Thyroid Stimulating Hormone Releasing Hormone (TSH-RH).
This affects the adenohypophysis to secrete Thyroid Stimulating Hormone (TSH).
TSH affects the thyroid gland to secrete Thyroid hormone (TH)
There are many more hormones, all with the same pattern.
The Hypothalamus can also secrete inhibiting hormones:
TSH-Inhibiting Hormone prevents the adenohypophysis from secreting TSH.
The hypothalamus affects the adenohypophysis, and that’s about it.
Some Hypothalamus Hormones
• Growth Hormone Releasing Hormone (GH-RH)
• Prolactin Releasing Hormone (PRL-RH)
• Thyroid Stimulating Hormone Releasing Hormone (TSH-RH)
• Adrenocorticotropic Hormone Releasing Hormone (ACTH-RH)
• Melanocyte Stimulating Hormone Releasing Hormone (MSH-RH)
• Follicle Stimulating Hormone Releasing Hormone (FSH-RH)
• Luteinizing Hormone Releasing Hormone (LH-RH)
More Hypothalamus Hormones
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Growth Hormone Inhibiting Hormone (GH-IH)
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Prolactin Inhibiting Hormone (PRL-IH)
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Thyroid Stimulating Hormone Inhibiting Hormone (TSH-IH)
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Adrenocorticotropic Hormone Inhibiting Hormone (ACTH-IH)
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Melanocyte Stimulating Hormone Inhibiting Hormone (MSH-IH)
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Follicle Stimulating Hormone Inhibiting Hormone (FSH-IH)
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Luteinizing Hormone Inhibiting Hormone (LH-IH)
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Pituitary Disorders
•Diabetes insipidus
–Not enough ADH (anti-diuretic hormone; a diuretic takes out excess fluid from the
body)
–Because they lack ADH, the person urinates frequently (polyuria), so they are
thirsty and drink a lot of water (polydipsia). Their blood glucose is normal.
•Gigantism and Acromegaly
–Hypersecretion of GH in children
–Giantism is overall growth
–Acromegaly is enlarged hands and feet
–GH needs for thyroid hormone (TH) to be present. GH stimulates all cells to
increase protein synthesis, fat utilization, and use up sugar.
–Gigantism is the result of excess GH during pre-puberty and acromegaly is the
result of excess GH after growth plates closed.
–The genetic determination of a person’s height has multiple genes involved, so
parents might be tall and have smaller children. There are no rules to predict it. A
child may also be small due to a defect in the placenta, blocking nutrients during
development.
•Pituitary dwarfism
–Hyposecretion of GH
–Normal proportions of head and limbs; overall size is small
THYROID GLAND
Located in the anterior neck, inferior to thyroid cartilage
• Largest pure endocrine gland
• Produces two hormones
• Thyroid hormone (TH)
– Acts on most cells of the body
– Increases metabolic rate
– Iodine is needed to make TH
• Thyroid gland also produces Calcitonin
– Lowers blood calcium levels in children
– Slows osteoclasts to allow for bone deposition
(Vitamin D is synthesized and secreted by the dermis)
THYROID HORMONE
Thyroid Gland
• The functional unit of the thyroid gland is the thyroid follicle. The cells
making up the perimeter of the follicle are called follicular cells. They make and
secrete the light purple liquid within the follicle, called colloid. Colloid is water,
filled with a lot of protein called thyroglobulin, which is made by the follicular
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cells. Since thyroglobulin is a protein, there is a gene that codes for it. TSH is
what stimulates the follicular cells to make thyroglobulin. TSH also increases
the size of the follicular cells to accommodate all this protein.
Thyroglobin
• When thyroglobulin is made, it is exocytosed from the follicular cell and stored
outside of the cell, in the follicle. As it moves across the cell membrane, a
peroxidase enzyme attaches iodine to the tyrosine (amino acid) portion of the
thyroglobulin. This process is iodination.
• To make thyroid hormone, you need iodine in your body. Iodized salt has enough
to meet this need.
Thyroid Hormone Effects
• All cells respond to thyroid hormone, increasing their metabolic rate (heart speeds up,
beats with greater force, more nutrients are used, etc).
• Too much thyroid hormone is hyperthyroidism; these people are thin and active.
• When levels of TH are too low, it is called hypothyroidism; these people are
overweight, move slowly, have no energy.
• TH also stimulates neurons; the person feels more alert, observing their
environment with more interest. With not enough TH, they lose interest, become
sluggish.
• When there is too much TH, they get muscles tremors and increased blood
glucose levels (hyperglycemia).
• When there is not enough TH, they get low blood glucose levels (hypoglycemia).
Mechanism of release of thyroid hormone
• Thyroid hormone is partly made of iodine. Iodine is essential for the formation of
thyroxin. If a person doesn’t eat enough iodine, they can’t make thyroid hormone.
• The hypothalamus responds by putting out more TSH-RH.
• The pituitary will respond by releasing TSH.
• But the thyroid can’t respond by releasing TH if it does not have the iodine to make
the hormone, so it the size of the follicle grows  gland grows  GOITER.
Role of Hypothalamus
• The hypothalamus is like the boss of a company; the pituitary gland is like the boss’
manager, and the thyroid gland is the worker.
• The boss tells the manager to tell the worker to write more papers.
• The manager tells the worker to write more papers.
• The worker writes more papers. The papers then go out to every cell in the body.
Some of the papers land on the desk of the boss. When his desk is covered with
papers, he tells the manager to stop the orders for more papers. If not enough papers
are on his desk, he tells the manager to keep sending out the order for more papers.
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This is what happens in the body:
• Hypothalamus (the boss) makes TSH-RH (thyroid stimulating hormone releasing
hormone)
• Pituitary (the manager) makes TSH (thyroid stimulating hormone)
• Thyroid gland (the worker) makes TH (thyroid hormone)
• The hypothalamus releases its hormone (TSH-RH) to the pituitary, telling the
pituitary to release its hormone (TSH), which tells the thyroid gland to release thyroid
hormone (TH).
• When thyroid hormone is released, it will circulate throughout the body, causing an
increase in metabolism in all of those cells. Some of the TH will bind to receptors in
the hypothalamus, and then the hypothalamus knows there is enough TH, and it will
stop releasing TSH-RH. Until the receptors in the hypothalamus are bound with the
resulting thyroid hormone, the hypothalamus is not satisfied that there is enough
thyroid hormone present.
What if the hypothalamus released its signal and the thyroid released too much
hormone?
• The hypothalamus will stop secreting its releasing hormone. This is a negative
feedback signal.
• When very few TH receptors are bound on the hypothalamus, it will keep releasing
its hormone. When its thyroid receptors are saturated, will stop.
Positive and Negative Feedback
• The presence of thyroid hormone is what will stop the hypothalamus from wanting
more.
• This is negative feedback, which is what most hormones have.
What if a gland disobeys the negative feedback?
• Example: Thyroid gland is impaired by a tumor.
• A thyroid tumor might cause it to over-secrete or under-secrete TH.
• Under-secreting thyroid tumor: what happens to the other hormone levels? Start
with the problem area (in this case, the thyroid is the place with the tumor), and then
evaluate the other glands. (Start with the problem area, the thyroid gland)
– TH will be low (hypothyroidism)
– TSH-RH will be high, since only a few hypothalamus receptors are bound
– TSH levels will be high.
• Over-secreting thyroid tumor:
– TH will be high (hyperthyroidism)
– TSH-RH will be low
– TSH levels will be low. This combination tells you the source of the problem
is the thyroid.
Another Example: Pituitary tumor
• Under-secreting pituitary tumor (Start with the problem area, the pituitary)
– TSH is low
– TH is low (hypothyroidism)
– TSH-RH is high
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Over-secreting pituitary tumor
– TSH is high
– TH is high (hyperthyroidism)
– TSH-RH is low
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NOTE: If the problem is the TSH, we don’t bother injecting TSH, we just give the
hormone that is lacking: Thyroid hormone.
Another Example: Hypothalamic Tumor
• Under-secreting hypothalamic tumor (Start with the problem area, the
hypothalamus)
– TSH-RH is low
– TSH is low
– TH is low (hypothyroidism)
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Over-secreting hypothalamic tumor
– TSH-RH is high
– TSH is high
– TH is high (hyperthyroidism)
NOTE: The adrenal cortex has the same cycle as thyroid hormone; it needs ACTH-RH
(adrenocorticotropic releasing hormone), ACTH, CH (cortisol hormone).
Problems with Thyroid
•Goiter
–Too little iodine in the diet
•Hyperthyroidism (Graves’ Disease)
–Caused by autoimmune disorder
–Leads to nervousness, weight loss, sweating, and rapid heart rate
•Hypothyroidism
–Decreases metabolism, causes obesity
GOITER
• This is usually caused by too little iodine in diet.
• That’s why salt is iodized.
• Iodine is only found in seafood, so if salt wasn’t iodized, a lot of people wouldn’t get
enough iodine, and there would be a lot of goiters.
• There are more problems with the thyroid gland than any other organ.
HYPERTHYROIDISM
(Most commonly caused by Graves Disease)
• Signs include thinness, eyes that stick out like a bug (exophthalmoses).
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HYPOTHYROIDISM
This can be caused by
• Hashimoto’s thyroiditis
• Iodine deficiency
• Tumor
• Defective enzyme in thyroid.
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Hashimoto’s Thyroiditis - adult hypothyroidism
– Antibodies attack and destroy thyroid tissue
– Low metabolic rate and weight gain are common symptoms
– Myxedema: non-pitting edema associated with hypothyroidism
Cretinism – hypothyroidism in children
– Short, disproportionate body, thick tongue and mental retardation
PARATHYROID GLANDS
These are four glands, embedded in the thyroid gland.
• Parathyroid hormone (PTH)
• Increases blood concentration of Ca2+
There are three ways that the parathyroid gland raises blood calcium levels
1) Stimulates osteoclasts to move bone calcium into the bloodstream
2) Stimulates the kidneys to stop excreting calcium
3) Stimulates the intestines to absorb more calcium from diet
Activates vitamin D which increases calcium uptake by intestines
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The antagonist of parathyroid hormone is calcitonin, which is produced in the
thyroid gland, and stimulates osteoblasts to take calcium from the blood and deposit it
in bone.
If blood calcium levels are low, parathyroid hormone is released.
If blood calcium levels are high, parathyroid levels are low.
THYMUS GLAND
Hormones produced by this organ stimulate the production of T cells (a type of white blood
cell).
ADRENAL GLANDS (“On top of the kidney”; Latin)
Located on the superior surface of the kidneys
Two endocrine glands in one (different embryological origin)
1) ADRENAL MEDULLA - Secretes epinephrine.
2) ADRENAL CORTEX - bulk of the adrenal gland; secretes most of the steroids:
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ADRENAL MEDULLA –
• Secretes mostly epinephrine
– Active in “fight, flight, and fright” response
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ADRENAL CORTEX – bulk of the adrenal gland
• Secretes aldosterone (salt and water balance for blood pressure)
• Secretes androgens and estrogens (sex hormones)
• Secretes cortisol (anti-stress and anti-inflammation hormone)
ADRENAL MEDULLA - secretes epinephrine (adrenalin)
ADRENALIN (AKA epinephrine “above the kidney”; Greek) is the neurotransmitter for
the sympathetic nervous system (fight or flight reactions; increased heart rate, etc).
When you are spooked, the adrenal medulla releases epinephrine.
ADRENAL CORTEX - Secretes a variety of hormones- all are steroids (steroids are made
from cholesterol)
1. CORTISOL helps the body deal with stressful situations like fasting, anxiety,
trauma, and infection. It keeps the blood protein and glucose levels high enough to
support the brain’s activities and affects the metabolic rate. When the brain perceives
a stressful situation, the hypothalamus tells the pituitary to secrete ACTH, which
travels to the adrenal gland and signals it to release cortisol to most of the cells of the
body.
2. ALDOSTERONE increases blood volume during hemorrhage or drop in blood
pressure. It causes kidney to reabsorb more sodium; water follows with it, so the
blood volume increases.
3. SEX HORMONES for the opposite sex: Males produce estrogen here, and females
produce testosterone.
Cortisol: Release Mechanism
• ACTH-RH is released by the hypothalamus.
• ACTH is released by pituitary.
• Cortisol (also called corticotropic hormone or CT).
• Cortisol affects almost all cells in body.
The adrenal gland also can release androgens.
• When there is an intense need to make cortisol in response to stress, and if the
body cannot keep up with the demand for cortisol, androgens are secreted instead
of cortisol.
• Excess androgens do not affect males, but females might develop more masculine
features.
ALDOSTERONE
Aldosterone targets the cells of kidney, increases the amount of salt and water that is
reabsorbed.
• It elevates blood pressure.
• It signals the kidneys to reabsorb sodium, and water comes with it and that
increases blood volume.
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Sex (Male and Female) Hormones
• Male and Female sex hormones are present in both males and females; the
pituitary gland affects these hormones in both sexes.
• Male sex hormones (androgens, such as testosterone) are made in the testes of
males, and made in the adrenal gland of females.
• Female sex hormones are made in the ovary of females and in the adrenal gland
of males.
Adrenal Gland Disorders
•Cushing’s syndrome
–Hypersecretion of cortisol
–High blood sugar
–Round “moon” face and “buffalo hump”
•Addison’s disease
–Hyposecretion of cortisol
–Low blood sugar
–Low blood pressure
–Also get hyperpigmentation
CUSHING’S DISEASE
• Patient has excess cortisol, high blood pressure, high blood glucose.
ADDISON’S DISEASE
• Caused from not enough cortisol.
• Mainly see effects in the hands, fingers, and gums.
• Symptoms of Addison’s disease are decreased glucose levels, a drop in blood
pressure from water and salt imbalance, and darkening of the skin.
PINEAL GLAND
• Shaped like a pinecone
• Pinealocytes secrete melatonin
– A hormone that regulates circadian rhythms (sense of daytime and night; it
regulates sleep cycle)
• “Pineal sand” is radiopaque
– Mineral deposits within pineal gland.
– Used as a landmark to identify other brain structures in X-Rays
PANCREAS
This is an endocrine and an exocrine gland.
Exocrine glands secrete into a duct.
The pancreas has channels that end in exocrine glands, and interspersed among these are
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PANCREATIC ISLETS, which are endocrine.
– Exocrine cells – acinar cells – secrete digestive enzymes
– Endocrine cells – pancreatic islets (islets of Langerhans)
• About one million islets – scattered throughout the pancreas
• Secrete insulin
• Secretes glucagon
• Glucagon
• Signals liver to break down glycogen into glucose
• Raises blood sugar
• Insulin
• Signals most body cells to take up glucose from the blood
• Promotes storage of glucose as glycogen in liver
• Lowers blood sugar
DIABETES
1. DIABETES INSIPIDUS: is when the pituitary gland does not secrete antidiuretic
hormone, or the kidney does not respond to the hormone. It can be caused by
damage to the pituitary or kidney damage.
2. DIABETES MELLITUS tends to be hereditary, caused by lack of insulin
secretion in the pancreas, or resistance to insulin by the body’s cells.
a. Type I diabetes (insulin dependent, develops in children) is more serious.
It is caused by destruction of pancreatic islets by autoimmune disorders.
They must have insulin injections daily throughout life.
b. Type II diabetes is much more common, usually appears after age 40, and is a
consequence of obesity. They produce insulin, but their cells are less sensitive to the
effects of insulin. It is initially treated with diet and exercise, but if that fails, oral
medicines or injected insulin may be needed.
The Gonads: Ovaries and Testes
OVARIES
– Secrete progesterone: Prepares uterus for pregnancy
– Secrete estrogen
• Female secondary characteristics
• Stores enough for several months
TESTES
– Secrete androgens (e.g. testosterone)
• Promotes the formation of sperm
• Maintains secondary sex characteristics
• Testes are the primary sex organs in the male, NOT the penis
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DANGERS OF STEROIDS
Steroids that weightlifters take are synthetic testosterone, and they are taken in doses
100x larger than a prescription, so they are dangerous. Although they increase muscle
size, they increase rage and aggression, cause kidney and liver disease, cause impotence
and sterility, and baldness. They also decrease the size of testicles and cause a low sperm
count. Other side effects are that they cause baldness, give women hair on their face and
chest, enlarge the breast of males and decrease the breast in females, cause kidney and
liver disease, cancer, severe acne, high blood pressure and high cholesterol, can stunt
growth in those whose bones have not finished growing yet, and they can shorten the life
span by several decades.
OTHER ENDOCRINE GLANDS
• Many of the glands we talked about have no other function than to make
hormones. But almost all organs are endocrine glands in addition to their other
functions.
• Heart pumps blood and produces hormones
• Liver makes enzymes, produces hormones
• GI tract digests food and produces hormones.
• Kidney: excretes wastes, produces hormones
• Dermis Involved in vitamin D synthesis, makes hormones
• Bones stores calcium and produces hormones.
• Placenta oxygenates and produces hormones.
The only thing that does NOT make hormones are epithelial glands that have ducts
(hormone glands are by definition without ducts).
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