The role of thyroid hormones in the regulation of metabolism.

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The role of thyroid hormones in the regulation of
metabolism.
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THYROID GLAND

• Thyroid gland synthesizes two kinds of hormones
  iodine containing hormones and calcitonin.

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Hormones of two types

• 1. Iodine containing thyroxin
  (tetraiodthyronin) and triiodthyronin
  derivatives of tyrosine
• 2. Calcitonin - peptide

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Synthesis of iodine containing hormones is
regulated by thyrotropic hormone, which in turn
is stimulated by thyroliberin
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Thyrotropic hormone (TTH). Chemical structure
glicoprotein. This hormone is necessary for the
normal functions of thyroid glands.

• Thyrotropic hormone promotes
• accumulation of iodine in thyroid
• including of iodine into the tyrosine
• synthesis of thyroxine and triiodothyronine.

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In difference to hormones of protein and
peptide nature, receptors for steroid hormones
are located within the cells - in the cytoplasm.
From cytoplasm the hormone-receptor
complexes is translocated into the nucleus
where they interact with DNA of nuclear chromatin
causing the activation of genes for respective
enzyme proteins. So, if hormones of the
first group cause the activation of existing
enzyme molecules, the acting on the target cells
of steroids and thyroid hormones results in the
biosynthesis of new enzyme molecules.
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CHEMISTRY STRUCTURE OF THYROID HORMONES
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Thyroxine and triiodthyronine are iodinated
derivatives of amino acid tyrosine.
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Formation and Secretion of Thyroglobulin by the
Thyroid Cells.
The thyroid cells are typical protein-secreting
glandular cells. The endoplasmic reticulum and
Golgi apparatus synthesize and secrete into the
follicles a large glycoprotein molecule called
thyroglobulin, with a molecular weight of about
335,000.
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Each molecule of thyroglobulin contains about 70
tyrosine amino acids, and they are the major
substrates that combine with iodine to form the
thyroid hormones.
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Transport of thyroxine and triiodothyronine to
tissues
They combine mainly with Thyroxin-binding
globulin lt90 Thyroxin-binding pre-albumin Thyroxi
n-binding albumin.
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MEHANISM OF ACTION OF THYROID HORMONES
The thyroid hormone receptors are either
attached to the DNA genetic strands or located in
proximity to them. The thyroid hormone
receptor usually forms a heterodimer with
retinoid X receptor (RXR) at specific thyroid
hormone response elements on the DNA.
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MEHANISM OF ACTION OF THYROID HORMONES
On binding with thyroid hormone, the
receptors become activated and initiate the
transcription process. Then large numbers of
different types of messenger RNA are formed,
followed within another few minutes or hours by
RNA translation on the cytoplasmic ribosomes to
form hundreds of new intracellular proteins
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• Functions of iodine containing hormones

Necessary for normal growth, differentiation, sex
and mental development Regulate the speed of
metabolism
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• Effect of thyroxine and triiodthyronine on the
  protein metabolism
• In physiological concentration stimulate the
  synthesis of proteins and nucleic acids.
• In the increased concentration activate the
  protein decomposition.

Effect on lipid metabolism Activate the exit of
lipids from depot, its decomposition and
oxidation
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Effect on carbohydrate metabolism Accelerate the
absorption of carbohydrates in the
intestine Activate the decomposition of glycogen.
Effect on energetic metabolism In excess
thyroxin uncouples respiration and
phosphorilation, decreases the ATP formation and
increases the heat formation
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Hypofunction of gland (occurs in the deficit of
iodine in water, soil, air)

• Hypofunction in adults mixedema
• Edema of mucosa

• Decrease of metabolism
• Decrease of body temperature

mixedema
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Hypofunction of gland (occurs in the deficit of
iodine in water, soil, air)

• Decrease of metabolism
• Decrease of body temperature
• Hypofunction in childhood - cretinism
• Growth inhibition
• Unproportional body development
• Disorders of mental development

cretinism
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Endemic goiter (occurs in the deficit of iodine
in water, soil and air)
Connective tissue is enlarged in gland and it is
increased in size markedly
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Hyperfunction of gland diffuse toxic goiter
(thyrotoxicosis, Graves disease)

• Goiter hyperplasia of gland
• Hypermetabolism
• Increase of body temperature
• Sweating, muscle weakness
• Weight loss with good appetite
• Tremor, emotional lability, insomnia
• Exophtalm

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