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The Endocrine system
Contents
1. Pituitary hormones and their control by the
hypothalamus
2. Thyroid metabolic hormones
3. Adrenocortical hormones
4. Insulin, glucagon and diabetes mellitus
5. Parathyroid hormones
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1. Pituitary hormones and their control
by the hypothalamus
• Physiologically, the pituitary gland is divided into 2
distinct portions:-the anterior (adenohypophysis)
and the posterior (neurohypophysis) pituitary.
• The anterior pituitary secretes 6 important peptide
hormones which play major roles in the control of
metabolic functions throughout the body.
• The posterior pituitary secretes 2 important
peptide hormones.
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Anterior pituitary hormones
i. Growth hormone (GH):-promotes growth of the
entire body by affecting protein formation, cell
multiplication, and cell differentiation.
ii. Adrenocorticotrophin (corticotrophin):-controls
the secretion of some of the adrenocortical
hormones(ACTH) which affect the metabolism of
glucose, proteins and fats.
iii. Thyroid-stimulating hormone(TSH):- controls
the rate of secretion of thyroxine and
triiodothyronine by the thyroid gland for the
control of most intracellular chemical reactions
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Anterior pituitary hormones
iv. Prolactin:- promotes mammary gland
development and milk production
And the 2 Gonadotrophic hormones-
v. follicle stimulating and vi. luteinizing
hormones:-for the growth of the ovaries and
testes, as well as their hormonal and
reproductive activities
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Posterior pituitary hormones
• The posterior pituitary gland is composed of glial-
like cells called pituicytes. They do not secrete
hormones; they act as a supporting structure for
large numbers of terminal nerve fibers and
terminal nerve endings.
• The nerve endings contain many secretory
granules where they secrete two hormones:-1.
antidiuretic hormone (ADH), also called
vasopressin and 2. oxytocin
• ADH allows re-absorbtion of water to prevent
water loss in the urine
• Oxytocin stimulates contraction of the pregnant
uterus especially towards the end of gestation.
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Thyroid gland
• The thyroid gland, located immediately below the
larynx on each side of and anterior to the trachea
and weighs 15-20 grams in adults.
• The thyroid gland secrets two major hormones-
thyroxine (T4) and triiodothyronine (T3) which
increases the matabolic rate of the body.
• Thyroid secretion is controlled primarily by thyroid-
stimulating hormone (TSH) secreted by the anterior
pituitary gland.
• The thyroid gland also secretes calcitonin an
important hormone for calcium metabolism.
• Iodine is required for formation of thyroxine
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Diseases of the Thyroid gland
Hyperthyroidism
• Most common cause is Grave’s disease (an
autoimmune disease) where the gland is increased
to two or three times’ the normal size.
• It is characterized by exophthalmos, a high state of
excitability, intolerance to heat, increased sweating,
mild to extreme weight loss, varying degrees of
diarrhea, muscle weakness, nervousness, extreme
fatigue and tremors of hands.
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Diseases of the Thyroid gland
Hypothyroidism
• Hashimoto disease (an autoimmune disease)
which destroys the gland rather than stimulate it.
• Endemic goiter (caused by iodide deficiency) which
leads to enlargement of the gland e.g mountainous
parts of the country
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Adrenocortical hormones
• The adrenal cortex secretes the corticosteroids.
• Two major types of adrenocortical hormones, the
mineralocorticoids and the glucocorticoids are
secreted by the adrenal cortex.
• The mineralocorticoids affect the electrolytes of the
ECF, especially Na+ and K+. E.g aldosterone
• The glucocorticoids increase blood glucose
concentration. They have additional effects on both
protein and fat metabolism. Main glucocorticoid is
cortisol (also called hydrocortisone) and to a lesser
extent corticosterone
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Diseases of adrenals
• Hypoadrenalism (Addison’s disease) results from
inability of the adrenal cortices to produce sufficient
adrenocortical hormones as a result of primary
atrophy or injury of the adrenal cortices. E.g
Mineralocorticoid deficiency (lack of aldosterone)
Glucocorticoid deficiency (loss of cortisol secretion)
• Hyperadrenalism (Cushing’s syndrome)-hyper-
secretion by the adrenal cortex
• Primary Aldosteronism (Conn’s syndrome); a
small tumour of the adrenal cortex causing
secretion of large amounts of aldosterone
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Insulin, Glucagon and Diabetes mellitus
• The pancreas in addition to its digestive functions,
secretes 2 important hormones-insulin and
glucagon that are crucial for normal regulation of
glucose, lipid and protein metabolism.
• The pancreas also secretes hormones, such as
amylin, somatostatin, and pancreatic polypeptide.
• Insulin was first isolated from the pancreas in 1922
by Banting and Best leading to a positive outlook in
the management of diabetes mellitus.
• Insulin affects fat and protein metabolism; which
leads to arteriosclerosis and wasting of the tissues;
as much as it does carbohydrate metabolism.
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Insulin and its functions
• Insulin secretion by the beta cells of the islets of
Langerhans is associated with energy abundance-
when there is increase of energy-giving foods in the
diet (eg carbohydrates), insulin secretion increases.
• Insulin plays an important role in storage of excess
carbohydrates as glycogen in the liver and muscle.
• Under the stimulus of insulin excess carbohydrates
that cannot be stored as glycogen are converted
into fats and stored in the adipose tissue.
• Insulin promotes amino acid uptake by cells and its
conversion into protein. In addition it inhibits the
breakdown of the proteins that are already in the
cells.
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Glucagon and its functions
• Glucagon is a hormone secreted by the alpha cells
of the islets of Langerhans when the blood glucose
concentration falls. An action opposite to insulin.
• Glucagon is referred to as a hyperglycaemic
hormone because it elevates the blood glucose
concentration
• Glucagon increases gluconeogenesis by increasing
the rate of amino acid uptake by the liver cells and
then the conversion of the amino acids to glucose.
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Diabetes mellitus (DM)
• D M is a syndrome of impaired carbohydrate, fat
and protein metabolism caused by either lack of
insulin secretion or decreased sensitivity of the
tissues to insulin. There are 2 types:
Type 1 diabetes, also called insulin-dependent
diabetes mellitus (IDDM) is caused by lack of
insulin secretion
Type 2 diabetes, also called non-insulin dependent
diabetes mellitus (NIDDM) is caused by decreased
sensitivity of target tissues to the metabolic effect of
insulin
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Parathyroid hormone (PTH)
• Controls extracellular Ca++ and PO4
concentrations by regulating intestinal reabsorption,
renal secretion, and exchange between the ECF
and bone of these ions.
• Excess activity of the parathyroid gland caused
rapid absorption of calcium salts from the bones,
with resultant hypercalcaemia in the ECF,
conversely, hypofunction results in hypocalcaemia,
often with resultant tetany.
• The active form of vitamin D, 1,25 dihydroxychole-
calciferol has several effects on the intestines,
kidneys, and bones that increase absorption of
Ca++ and PO4 into the ECF
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Parathyroid hormone (PTH)
• Parathyroid hormone increases Ca++ and PO4
absorption from the bone
• Parathyroid hormone decreases Ca++ excretion
and increase PO4 excretion by the kidneys
• Parathyroid hormone increases intestinal
absorption of Ca++ and PO4
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Calcitonin
• Calcitonin is a peptide hormone secreted by the
thyroid gland and decreases plasma Ca++
concentration and, in general has effects opposite
to those of PTH.
• Increased plasma Ca++ concentration stimulates
calcitonin secretion
• Calcitonin decreases plasma Ca++ concentration.
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Medical importance
Rickets
• Rickets occurs mainly in children and is due to
Ca++ or PO4 deficiency in the ECF, usually caused
by lack of vitamin D in the diet.
• The 7-dehydrocholesterol in the skin, on exposure
to sunlight, is converted to vitamin D which prevent
rickets. So expose children to sunlight.
• Rickets weakens the bones.
Osteomalacia (adult rickets) –
• can occur in steatorrhoea (failure to absorb fat)
which causes vitamin D and Ca++ to pass into the
faeces leading to deficiency of vitamin D
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Medical importance
Osteoporosis
• Is the most common of all bone diseases in adults
especially in old age and is due to diminished
organic bone matrix. Common causes are
Lack of exercises
Malnutrition that results in decreased protein matrix
Lack of vitamin C-necessary for the formation of
osteoid by the osteoblasts
Postmenopausal lack of estrogen which decreases
the number and activity of osteoblasts
Old age-decreased growth hormone
Cushing’s syndrome
21. Medical importance
Testicular feminization syndrome
• These are men with (X Y chromosomes) but are
resistant to male hormones called androgens.
• This person is genetically male have an enzymatic
defect in the synthesis of testosterone or have absence
of testosterone receptors..
• Testosterone promotes closure of the epiphyseal
joints and plates of the long bones. Hence they help in
the control of height.
• People with this syndrome grow unusually tall and
have some physical traits of a woman.
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Questions
Question 1
a.Why would males with testicular feminization syndrome
be unusually tall?
b. A black market for growth hormone abuse exists among
weightlifters and other athletes. What actions of growth
hormone would induce a full-grown athlete to take
supplemental doses of this hormone?
c. What are the potential detrimental side effects?
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Questions
Question 2
• Name two endocrine glands (or regions) that are
important in the stress response, and explain why
they are important.
• The posterior pituitary is not really an endocrine
gland. Why not? What is it?
Question 3
• A colloidal, or endemic, goiter is not really the
result of malfunction of the thyroid gland. What
does cause it?
• List some problems that elderly people might have
as a result of decreasing hormone production.
