THYROID GLAND
THYROID GLAND
o
Thyroid Gland is located
immediately below the larynx on its either side and anterior to trachea.
o
It weighs about 15-20gms
o
Thyroid gland has number of
follicles filled with colloid and lined by Cuboid epithelial cells.
o
The major component of
colloid is a large glycoprotein Thyroglobulin which contains the thyroid
hormones within it.
o
Thyroid gland secretes 2
major hormones
Ø Tetra-Iodo Thyronine(or)Thyroxine (or) T4
Ø Tri-Iodo Thyronine(or)T3
THYROID HORMONES
o
The principle hormones
secreted by the follicular epithelial cells are T3 and T4
o
About 93% of metabolically
active hormones secreted by thyroid gland is T4 and 7% are T3.
o
T3 is more potent than T4
o
T3 has short half-life than
T4
Chemistry
o Both Thyroxine and Tri Iodo thyronine are formed from Tyrosine of Thyroglobulin by Iodination.
T3 (Tri Iodo thyronine)
o
Molecular weight -650
Daltons
T4 (Thyroxine)
o
Molecular weight-770 Daltons
Synthesis Of Thyroid
Hormones
o
Synthesis of thyroid
hormones requires 2 raw materials
(i) Thyroglobulin
(ii) Iodine
(i)Thyroglobulin
o
It is a dimeric glycoprotein
containing 140 Tyrosine residues
o
It is synthesized and
secreted by follicular cells into colloid
o The tyrosine residues in the thyroglobulin combine with iodine to form thyroid hormones
(ii) Iodine
o
To form normal quantities of
thyroid hormone, about 1mg/week of Iodine is ingested in the form of Iodides
are required.
o
Ingested Iodides orally are
absorbed form GIT into the blood
Steps Involved in
Thyroid Hormones Synthesis
I.
Iodine
Trapping (Iodide Pump)
o
The first step in the
synthesis of thyroid hormones is transport of Iodides from the blood into the
thyroid follicular cells.
o
The basal membrane of the
thyroid cell has specific ability to pump the Iodide actively to the interior
of the cell.
o Iodide pumping requires ATP as it is actively transported against concentration gradient.
II.
Oxidation
Of Iodide Ion
o
The iodide trapped is Oxidized
to I° or I3¯ by the enzyme Hydrogen Peroxidase (on the membrane) in presence of
H2O2
o
The Oxidized Iodide is
capable of combining directly with the amino acid Tyrosine in Thyroglobulin.
III.
Organification
Of Thyroglobulin (Iodination of Tyrosine and formation of Thyroid Hormones)
o
The binding of Iodine with
the Tyrosine of Thyroglobulin is called as Organification of thyroglobulin.
o
This is carried out by the
Iodinase enzyme.
o
Iodination of the Tyrosine
residues in thyroglobulin occurs first in 3¹ position in aromatic nucleus
forming Mono Iodo Tyrosine (MIT)
o
MIT is next iodinated in the
5¹position to form Di Iodo Tyrosine (DIT)
IV.
Coupling
Of Iodo Tyrosines
o
Two molecules of DIT undergo
an Oxidative condensation, by the enzyme thyroperoxidase to form Thyroxine (T4)
with the help of ATP, with the release of Alanine which then forms Pyruvate and
NH3.
o
Similarly, condensation of
MIT and DIT results in the Tri Iodo Thyronine (T3) formation by the same enzyme
thyroperoxidase and ATP with the liberation of Alanine.
o
Synthesized thyroid hormone
is stored inside thyroid till it receives signal from TSH
o
Normal T3 secretion
-80-180ng/dl
T4 secretion
-4.6-12µg/dl
Functions of Thyroid
Hormones
o
The binding of thyroid
hormones to their nuclear receptors leads to the transcription of genes and
synthesis of proteins, leading to following
ü Increases
metabolic activities of almost all tissues
(i)
Increases protein synthesis
(ii)
Increases BMR
(iii)
Increases Lipolysis
(iv)
Increases Glycogenolysis
(v)
Increases Glyconeogenesis
(vi)
Increases Glucose absorption
(vii)
Increases O2 consumption
(viii)
Increases Mitochondria
(ix)
Increases Na K -ATPase
ü Has
effect on Cardiovascular system
(i)
Increases Cardiac output
(ii)
Increases Heart rate
(iii)
Increases Respiration
ü Has
Great Effect on Growth –(i.e) Enhances Growth.
ü Helps
in CNS development
Regulation of Thyroid
Secretion:
o
Thyroid Hormone secretion is
controlled by TSH of anterior pituitary
o
TSH is under the control of
TRH of Hypothalamus.
o
Increased thyroid hormone
level has feedback effect on Anterior Pituitary and Hypothalamus.
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