The endocrine system consists of glands that secrete hormones directly into the bloodstream to regulate organs and cells throughout the body. There are two main types of hormones - steroid hormones like sex and stress hormones which are derived from cholesterol, and non-steroid protein and peptide hormones like insulin. Hormones bind to specific receptor cells and have various effects like stimulating growth or metabolism. Disorders can result if there is too much or too little of certain hormones, such as stunted growth from growth hormone deficiency or gigantism from its excess. The hypothalamus and pituitary gland regulate many other glands to maintain hormone balance.

1. Endocrine
system
By
Dr. Israa H.Mohsen

2. The endocrine system consists of a variety of glands and
specialized cells throughout the body. These glands and cells
secrete chemicals (called hormones) that influence almost
every cell and organ in the body.

3. Endocrine glands are ductless glands: they secrete
hormones directly into the bloodstream. (In contrast,
exocrine glands secrete hormones into ducts, which,
in turn, lead to a surface in some other location such
as the body's surface—as in sweat glands—or the
digestive tract—as in digestive enzymes.) Once in the
bloodstream, each hormone travels throughout the
body. In the process, the cells of many different
organs are exposed to a particular hormone;
however, only cells having receptors for that hormone
(called target cells) will respond.
In other words, a hormone acts only on cells with
receptors specific to that hormone; this is called
specificity.

5. Hormones can be classified as steroid or non-steroid.
1- Steroid hormones are synthesized from cholesterol; they
include male
and female sex hormones as well as aldosterone (secreted by
the adrenal cortex).
2-Nonsteroid, or protein-based, hormones are synthesized from
amino acids.
They can be further divided into
1- protein hormones (such as insulin),
2-peptide hormones (such as antidiuretic hormone),
3- or amino acid derivative hormones (such as epinephrine and
norepinephrine).
Hormones travel through the bloodstream to stimulate specific
target cells. The target cells may lie anywhere in the body: close
to the gland or far from it. Once a hormone reaches the target
cell, it binds with a receptor to trigger changes within the cell.
How this occurs , though, depends upon whether the hormone is
a steroid or protein-based hormone.

10. Growth hormone abnormalities
The secretion of too much growth hormone
(hypersecretion) during a child’s growth years will trigger
rapid, excessive skeletal growth, resulting in a condition
called gigantism. If the epiphyseal plates have already
fused when the hypersecretion occurs, cartilage will form
new bone, causing the hands, feet, face, and jaw to
enlarge—a disorder calledacromegaly.
In contrast, a deficiency of growth hormone
(hyposecretion) while a child is still growing will result in
stunted growth. This is sometimes called pituitary
dwarfism. The condition is usually treated with growth
hormone injections.

11. the important hormones released by the hypothalamus. Each acts
on the anterior pituitary to release, or suppress, a particular
hormone. For example, thyrotropin-releasing hormone stimulates
the anterior pituitary to release of thyrotropin, also called thyroid-
stimulating hormone; in turn, this hormone stimulates the
thyroid to secrete thyroid hormone, or TH.
• Gonadotropin-releasing hormone: Promotes secretion of FSH
and LH
• Thyrotropin-releasing hormone: Promotes secretion of TSH
• Corticotropin-releasing hormone: Promotes secretion of ACTH
• Prolactin-releasing hormone: Promotes secretion of prolactin
• Prolactin-inhibiting hormone: Inhibits secretion of prolactin
• Growth hormone–releasing hormone: Promotes secretion of GH
• Somatostatin: Inhibits secretion of GH and TSH

14. Pineal Gland
The pineal gland produces melatonin, a hormone that rises at night, when sunlight is
absent, and falls during the day. High melatonin levels trigger sleepiness, making it a
key factor in the sleep-wake cycle. Although yet to be proven, scientists speculate that
the pineal gland may also regulate the timing of puberty.

15. Seasonal affective disorder
Low levels of melatonin have been linked to mood disorders,
particularly seasonal affective disorder (SAD).
Occurring during winter months, when exposure to sunlight is
limited, people with SAD complain of sleepiness, depression,
irritability, and carbohydrate cravings. Exposure to special high-
intensity lights for several hours each
day often relieves symptom.s.

16. Thymus
The thymus secretes thymosin and thymopoietin, two hormones having a role in
the development of the immune system. Although it secretes hormones, making it a
member of the endocrine system, the actions of the hormones make the thymus part
of the immune system. (For more information on the thymus gland
In children, the thymus gland is large. Beginning in puberty, though, the gland starts
to shrink. By the time one reaches old age, the gland consists of mostly fat and
fibrous tissue.

18. The abbreviations T3 and T4 refer to the number of iodine atoms
in each hormone: T3
has three iodine atoms while T4 has four.
TSH (released from the anterior pituitary) stimulates the release
of thyroid hormone (TH) from the thyroid gland. The overall effect
of TH is to increase th body’s metabolic rate, which, in turn,
increases heat production. (This explains why TH is released
when the body is exposed to cold.) Just a few of the other effects
of TH are increased rate and strength of heart contractions,
increased respiratory rate, and increased appetite. TH is also
crucial for growth and development: it promotes the development
of bone; the nervous system; and skin, hair, nails, and teeth.

19. Thyroid disorders
A number of significant disorders develop from hyper- or hyposecretion of
thyroid hormone. If a child is born without a thyroid gland, the lack of thyroid
hormone (TH) leads to cretinism: a condition characterized by retarded growth
and sexual development, a low metabolic rate, and mental retardation.
If the hyposecretion develops later in life, a lowered metabolic rate causes
weight gain, hair loss, and fatigue. If hypothyroidism is severe or prolonged, a
condition called myxedema may result, which involves swelling and firmness of
the skin.
Hypersecretion of TH stimulates the body’s metabolism, resulting in a condition
called Graves’ disease. Thought to be due to an autoimmune disorder, Graves’
disease causes unexplained weight loss, increased heart rate, nervousness,
and exophthalmos—a protrusion of the eyeballs due to swelling of tissue
behind the eye.
An enlarged thyroid gland due to a dietary deficiency of iodine is called simple
goiter. Iodine is the basis for TH. Without enough iodine, the production of TH
falls. Consequently, the anterior pituitary never receives a negative feedback
message to stop producing TSH. Not only that, because TH levels are low, the
anterior pituitary tries to stimulate the thyroid even more by secreting greater
levels of TSH. Instead of triggering the production of TH (which the thyroid
gland can’t do without enough iodine), the TSH stimulates growth of thyroid
tissue, leading to thyroid enlargement and goiter

20. Parathyroid Glands