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DR. JITENDRA PATEL (MBBS, MD)
Medical Educator & Researcher
Assistant Professor, Department of Physiology
Email: [email protected] Web: www.esphys.weebly.com
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FROM GREEK thyreoeides= SHIELD SHAPE
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OUTLINE
 Introduction
 Thyroid Hormones
 Physiological Action
 Regulation of Hormones
 Applied Aspect
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Physiological Anatomy
 Thyroid gland is the largest endocrine gland in
the body (weighing about 15−25 g in adults).
 It consists of two lobes joined together by a
narrow isthmus and is located on either side of
the trachea just below the larynx.
 It receives high blood supply (400–600 mL/100
g/min).
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Imp Functions
 Regulates BMR
 Stmulates somatic and mental growth
 Plays important role in calcium metabolism
Synthesis
 Iodine –raw material for thyroid hormone synthesis
 Ingested iodine converted to iodide before absorption
 15O μg of iodine required for normal thyroid function
of which 120μg enter thyroid at normal rates of
hormone synthesis and secretion
 Reduced intake of iodine causes endemic goiter
Steps of thyroid hormone synthesis
 Iodine trapping
 Oxidation
 Iodination
 Coupling
 Storage
 Release
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Potency
T3
T4
8 times more
Less
Activity
More active
Circulation
Less in
circulation
Inactive In target
tissue
Main circ.
Hormone
Duration of
action
Short
Long
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Storage
 Once TG has been iodinated it is stored in the
lumen of follicle as colloid for several
months (1-3)
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Actions
1. Effects on growth and tissue development.
2. Effect on the metabolic rate in general (BMR)
3. Effect on carbohydrate metabolism
4. Effect on fat metabolism
5. Effect on protein metabolism
6. Metabolic effects through other hormones
7. Effect on vitamin metabolism
8. Effect on water and electrolyte balance
Actions…
9. Respiratory effects
10.Cardiovascular effects
11.Effects on nervous system
12.Effects on gastrointestinal tract
13.Effects on reproductive system
14.Effects on other endocrine glands
15.Effects on kidney
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Autoregulation of thyroid gland
• The secretions of thyroid gland are regulated by food
iodine contents.
• If there is deficiency of iodine content in the diet then
the iodine trapping mechanism of the follicular cells
becomes super efficient and vice versa is also true.
• In this way, iodine availability for thyroxine synthesis
remains constant and this phenomenon is called
autoregulation of thyroid gland.
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“Either be selected or be elected”
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