Transcript Thyroid hormonesx

Thyroid hormones
Physiological effects of thyroid hormones
Thyroid hormone synthesis
Regulation of thyroid function
Hypothyroidism
Hyperthyroidism
Parathyroid glands
Parathormone (PTH)
Hyperparathyroidism
Hypoparathyroidism
Calcitonin (CT) (Thyrocalcitonin)
Thyroid hormones
 The thyroid gland is a small gland situated in the neck,
wrapped around the trachea just below the larynx (voice box).
 It secretes hormones that increase the metabolic rate and
oxygen consumption and that are necessary for proper
growth and development.
 The thyroid also secretes calcitonin (hormone that
participates in regulation of plasma Ca2+concentration by
inhibiting bone resorption.
 The thyroid hormones thyroxine (T4) 3,5,3',5‘
tetraiodothyronine and 3,5,3'triiodothyronine (T3)
Physiological effects of thyroid hormones
1) Thyroid hormones stimulate protein synthesis.
2) Promotes intestinal absorption of glucose and may cause
hyperglycemia.
3) Have tendency to lower the serum cholesterol.
4) Increase both the rate of secretion of digestive juices and
the motility of the GIT.
5) Increase the rate of depth of respiration.
6) In males lack of thyroid hormones is likely to cause
complete loss of libido (sexual desire), whereas on the
other hand much more excess of thyroid hormones causes
impotence.
7) In female, excess of thyroid hormones causes menorrhoea
or polymenorrhoea which means excessive and frequent
menstrual bleeding, respectively. Lack of thyroid hormones
may cause irregular period or even total amenorrhoea.
Thyroid hormone
synthesis
 Thyroxine (T4) 3,5,3',5‘ tetraiodothyronine and small amounts of
tri-iodothyronine (3,5,3'triiodothyronine) (T3) and reverse T3 (rT3)
(3, 3',5'triiodothyronine) are all synthesised in the thyroid gland.
 Their immediate precursors are monoiodotyrosine (MIT) and
diiodotyrosine (DIT), these compounds do not appear in plasma.
 Neither MIT nor DIT has any hormonal activity.
Thyroid hormones production
1) Entry of inorganic iodide ion into
the thyroid cell.
2) I- oxidized with help of a
peroxidase enzyme system.
3) Activated iodine react with
tyrosyl residues in protein
thyroglobulin. MIT and DIT are
produced.
4) MIT and DIT are oxidatively
coupled by enzyme system to
form T3 , T4andrT3 residues
attached to thyroglobulin.
5) T3 , T4, rT3 , MIT and DIT are
formed by the action of protease.
6) MIT and DIT are enzymatically
deiodinated and the I is recycled
within the gland.
7) T3 T4 and rT3 diffuse through the
thyroid cell wall into plasma
where they are quickly bound by
thyroid-binding globulin (TBG)
Regulation of thyroid function
 The most important regulator of
thyroid homeostasis is TSH. The
production of TSH is controlled by
a stimulatory effect of the
hypothalamic tripeptide, TRH,
mediated by a negative feedback
from circulating FreeT3 and
FreeT4.
 It is thought that the hypothalamus,
via TRH, sets the level of thyroid
hormone production required
physiologically, and that the pituitary
acts as a ‘thyroid-stat’ to maintain the
level of thyroid hormone production
that has been determined by the
hypothalamus.
Thyroid hormones reference values
Free T4 (reference range 10–21 pmol/L);
Free T3 (reference range 2.6–6.2 pmol/L).
Total T4 (reference range 70–150 nmol/L);
Total T3 (reference range 1.2–2.8 nmol/L).
Euthyroidism means level of thyroid
hormones are normal
Hypothyroidism
• (FreeT4 low; TSH high)
 It is state in which there is not sufficient development of
thyroid gland, in the embryonic life the child become a dwarf
or cretin.

 A cretin grows slowly (stunted growth) and has a low
mentality and low I.Q. his hair is scanty and coarse, and his
skin is thick and dry.
 His basal metabolic rate (BMR) is low
 If the gland is removed or if it becomes subnormal in activity
in adult, myxoedema develops.
Myxoedema
 In myxoedema the skin becomes thick and dry and
fall out.
 There is disinclination towards work either physically
or mentally.
 There becomes a tendency to put on weight.
 The metabolic rate is reduced, nitrogen metabolism
gets lowered, and the body temperature is
subnormal .
 Ossification of bones is delayed
Salient characteristics of hypothyroidism
 Tiredness
 Cramps or
inflammation in
muscles
 Tingling in fingers.
 Dry skin and
intolerance of cold
 Yellow skin.
 Dry hairs or fall of hair
 Stress
 Decrease of heart beating
 Hoarseness of voice
 Goiter
 Loss of memory
 Loss of concentration
 Sterility or termination of
pregnancy
 Myxoedema
 Irregular or heavy menses
Hyperthyroidism
• (FreeT4 and / or FreeT3 high; TSH low)
 It is a state in which the thyroid is too active, as a result of
which condition known as exophthalmic goiter occurs.
 A common symptom of this disease is the bulging of the
eyes. The disease also known as Grave's or Basedow's
disease.
 The symptoms of exophthalmic goiter are the opposite
of myxoaedema which mean that :
symptoms of exophthalmic goiter
1)
2)
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4)
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8)
The basal metabolic rate (BMR) is high.
Nitrogen metabolism is increase.
The hair fine.
The body temperature is above normal.
The patient is nervous and irritable.
His mental is above rather than below normal.
He or she is under weight.
His/her heart beat is generally fast and irregular.
Salient characteristics of hyperthyroidism
 Increase in heart beating
(palpitations).
 Disappointment/dull/
irritation.
 Breathing problem.
 Irregular menses or
complete loss of menses.
 Trembling of hand
 weakness
Fall of hairs.
 Intolerance of heat.
 Increase of hunger.
 Increase of motility
of intestine.
Loss of weight.
Hot and damp skin.
To watch anything
gazingly.
Parathyroid glands
 There are 4 parathyroid glands, 2 are embedded in the
posterior surface of thyroid gland ; and 2 lie close to and
behind the thyroid.
 They are yellow-brown, egg shaped bodies.
 2 hormones are known to be secreted by parathyroid
glands: parathormone (PTH) and calcitonin (CT).
 Both PTH and CT are responsible for controlling calcium
and phosphate metabolism in the human beings in a
balancing manner.
Parathormone (PTH)
 PTH is protein with molecular weight of 9500,
consists of 83 amino acid residues composed of 17
different amino acids.
 Regulated by the level of circulating calcium.
 Low ionic calcium level stimulates whereas high ionic
level inhibits PTH.
 PTH secretion is regulated by both calcium and
magnesium.
Hyperparathyroidism
 The main cause of hyperparathyroidism is the development
of tumor.
 Hyperparathyroidism causes a syndrome known as osteitis
fibrosa cystica which characterized by :
1) Hypercalcemia.
2) Hypophosphatemia.
3) Raised serum activity of alkaline phosphatase.
 Excessive demineralization of bone causes pain in them,
deformities and spontaneous fractures.
 As a result of high calcium level in serum calculi may be
formed in the kidney.
 other symptoms of this disorder also include : muscular
asthenia, peptic ulcer, and constipation
Hypoparathyroidism
 Hypoparathyroidism is more common than excessive
secretion. It is caused by injury or removal of parathyroid
glands during surgery of thyroid.
 Less secretion of PTH causes hypocalcaemia leading to
muscle spasm, lenticular cataracts and calcification of
the basal ganglia of the brain.
Calcitonin (CT) (Thyrocalcitonin)
 Parathyroid hormone is the single and most important factor
in fine regulation of blood calcium level ( extracellular and
intracellular).
 Calcitonin is lowering calcium hormone.
 It is secreted by the parafollicular or “C” cells of the
thyroid gland (hence the name thyrocalcitonin).
 Calcitonin is 32 amino acids with molecular weight 3000.
Calcitonin has been synthesized in lab.
 The only known stimulus of CT secretion is a rise in
plasma calcium.
 Level secretion is also stimulated by glucagon and cyclicAMP
 Raised in serum CT level is found in medullary thyroid
carcinoma.
 CT may be administered in treatment of Paget's disease.
 There is no clinical syndrome of deficiency of CT.