THYROID GLANDS

1. 1
Thyroid Gland
Introduction
• Thyroid is an endocrine gland situated in the
lower part of the front and the sides of the neck.
(on each side of the thyroid cartilage of the
larynx on the lateral side of trachea.) It has
two lobes,which are connected in the middle by
an isthmus.Normally,the weight of thyroid is
about 20-40gm in adults,variations occure under
the physiological conditions.Thyroid is larger in
females than in males.

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Hormones of Thyroid Gland.
• Thyroid gland secretes three hormones.
• Thyroxine or tetra-iodothyronine(T4).90%
secreted by follicular cells.
• Tri-iodothyronine(T3).10% secreted by
follicular cells.
• Calcitonin,secreted by parafollicular cells.

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Biosynthesis of thyroid hormone.
• The synthesis of the thyroid hormones tetra
iodothyronine(T4),triiodothyronine(T3) takes
place in the following steps:
Trapping of iodide by the thyroid gland.
• First stage in the formation of thyroid hormone is
transport of iodides from extracellular fluid into
the thyroid glandular cells and follicles.

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• The basal membrane of thyroid cells has the
specific ability to pump the iodide activety to the
interior of the cell.This is called iodide trapping
or iodine pumping.
Oxidation of iodide
• In this stage of T3 and T4 formation,iodide ions
are converted to an oxidized form of iodine.The
oxidized iodine is then capable of combining
directly with amino acid tyrosine.This process is
accelerated by enzymes such as peroxidase.

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Thyroglobulin synthesis.
• The endoplasmic reticulum and golgi
apparatus of thyroid follicular cells
synthesize and secrete into the follicles a
glycoprotein,called thyroglobulin. Each
molecule of thyroglobulin contains 140
tyrosine amino acids,the major substrate
that combine with iodine to form
hormones.

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Iodination of Tyrosine.
• The combination of iodine with tyrosine is
known as iodination.It take place in the
follicle within thyroglobulin.In this stag,the
oxidized iodine combine with tyrosine in
the molecule of thyroglobulin and form
thyroid hormones,T3 and T4.This process
is accelerated by iodinase.

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• The stage are as follows,
• 1-Tyrosine + iodine-------- Mono
iodotyrosine.(MIT)
• 2-MIT + iodine ---------Di-iodotyrosine.(DIT)
• 3- MIT + DIT----------Tri-iodothyronine.(T3)
• 4- DIT + DIT--------Tetra-iodothyronine(T4).

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Coupling Reactions / Coupling of
iodotyrosyls.
• The coupling of two DIT molecules to form T4 or
of one MIT and DIT to form T3 occurs within the
thyroglobulin molecule.A separate coupling
enzyme has not been found.
Mechanism of action.
• Thyroid hormones act through gene medistor.

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Storage of Thyroid Hormones.
• After synthesis,the thyroid hormones remain in
the form of vesicles with thyroglobulin.Each
thyroglobulin molecule contains 5 or 6 molecules
of thyroxine.There is also an average of 1 tri-
iodothyronine molecule for every 10 molecules
of thyroxine.In combination with
thyroglobulin,the thyroid hormones can be
stored for several months.
.

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• It is the only endocrine gland that can
store its hormones for a long period of
about 4 months.So,when the synthesis of
thyroid hormone stop,the signs and
symptoms of deficiency do not appear for
about 4 months

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Release of Thyroid Hormones from the Thyroid
Gland.
• Plasma membrane of follicular cell sends foot like extensions called
pseudopods,which close around the thyroglobulin-hormone
complex.
• The pseudopods convert thyroglobulin-hormone complex into small
pinocytic vesicles.
• Lysosomes of the cell fuse with the vesicles.
• Lysosomes contain some digestive enzymes(proteinases) which
digest thyroglobulin and release the hormones.
• Hormones diffuse through the base of the follicular cell and enter the
capillaries.
• Only T3 and T4 are released into the blood.

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Rate of Secretion.
• The rate of secretion of thyroid hormones
per day:
• Thyroxine 80-90ug
• Tri-iodothyronine 4-5ug
• Reverse T3 1-2ug

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Transport.
• Thyroid hormones are transported in the
blood by three types of the proteins.
• 1-Thyroxine Binding Globulin.(TBG)
• 2-Thyroxine binding prealbumin.(TBPA)
• 3-Albumin.

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Actions of Thyroid Hormone.
• The important actions performed by
thyroid hormone are as follows:
1- Calorigenic Action.
• Thyroxine stimulates metabolism in the
tissues and increases oxygen
consumption and heat production.It also
increases the size of the cells.

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2- Effects on Carbohydrate
Metabolism.
• Increases absorption of glucose from GIT.
• Increases insulin secretion.
• Increases glycolysis in the liver.
• Increases breakdown of glycogen into
glucose.

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3- Effects on Protein Metabolism.
• Thyroid hormone increases the synthesis of
protein in the cells,
• By increasing the translation of RNA.
• By increasing the transcription of DNA and RNA.
• By increasing the activity of Mitochondria.
• By increasing the activity of cellular enzymes.

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4-Effects on Fat Metabolism.
• Thyroxine increases the free fatty acid level in
blood.
• Decreases phospholipids,triglycerides,and
cholesterol in blood.
5- Effects on Vitamin Metabolism.
• The metabolism of fat soluble vitamins is also
affected by thyroid hormone.T4 is necessary for
the synthesis of vitamin A from carotene and the
conversion of vitamin A to retinine.

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6- Effects on water and mineral
Metabolism.
• Promotes demineralization of bones.
• Increases excretion of calcium and
phophates in the urine.
• Increases k+ excretion.
• Causes diuresis.

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7- Effects on CNS.
• Increases the synaptic activity.
• Cerebration becomes very rapid.
• Development and maintenance of normal
function of the CNS.
8- Effects on CVS.
• Heart rate increased.
• Cardiac output increased.
• Systolic blood pressure increased.
• Diastolic blood pressure decreased.

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9- Effects on Respiration.
• Causes increased utilization of oxygen and
carbon dioxide.
• Increases pulmonary ventilation.
• Increases rate and depth of respiration.
10- Effects on GIT.
• Increases appetite.
• Increases food intake.
• Increases secretion of digestive juices.
• Increases motility of GIT.

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11- Effects on Blood.
• Increases the production of RBCs.
• Increases the blood volume.
12-Effects on Body Weight.
• Maintaining weight of the body.
• Increase in thyroxine secretion decreases the
body weight and fat storage,and decrease in
thyroxine secretion increase the body weight b/c
of fat deposition.

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13- Effects on Muscle.
• A muscle weakness occurs in most
• patients with hyperthyroidisms called thyrotoxic
myopathy.
• When hyperthyroidism is severe and
prolonged,the myopathy may be severe.
14-Effects on other endocrine glands.
• Increased thyroid hormone increased the rates
of secretion of most of the other endocrine
glands.

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Regulation of thyroid hormone secretion.
• Thyroid function is regulated primarily by variations in the
circulating level of pituitary (THS) and the availability of
iodine to thyroid gland.
A-Control of thyroid stimulating hormone
secretion.
• Thyrotropic releasing hormone is released from the
hypothalamus.This hormone is transported to the
anterior pituitary by hypothalamo-hypophyseal portal
vessels.

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• It stimulates the secretion of THS into the
blood.THS inturn increases the secretion of
thyroid hormones from the thyroid gland.
Feed Back Mechanism.
• When thyroid hormone secretion rises to about
1.75 times the normal level,this inhibits (THS)
secretion from the anterior pituitary gland.Thus,it
decreases the thyroid hormone secretion.

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B- Availability of Iodine.
• Iodine is an essential part of thyroid
hormones.The lack of iodine inhibits the
formation of hormones.
• Hypoactivity of thyroid hormone can be
corrected by administration of iodine.

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Factors affecting T3 and T4.
• Stimulating factors.
• THS.
• Cold.
• Low BMR.
• Inhibitory factors.
• Excitement.
• Anxiety.
• Lack of iodine.