Download Thyroid Gland

Thyroid Gland
Prepared by Yousaf khan
Anatomy of Thyroid Gland
- The thyroid gland is situated in the neck in front of
the larynx and trachea.
- The thyroid gland is shaped like a butterfly,
- just inferior to the larynx.
- At the level of the 5th, 6th and 7th cervical and 1st
thoracic vertebrae
- Thyroid hormone secretion is controlled by TSH
secreted by the anterior pituitary gland.
Microscopic Structure of Thyroid
Gland
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Gland consists of numerous follicles
Walls of follicles are lined by cuboidal epithelium
Each follicle contains “ colloid”
The colloid consists of mainly iodinated
thyroglobulin.
• Parafollicular cells are present b/w the thyroid
follicles.
Histology of thyroid gland
Hormones Secreted by thyroid Gland
Three hormones are secreted
1. Thyroxine (T4) is secreded by follicular cells
2. Triiodothyronine (T3) is secreted by follicular cells
3. Calcitonin is secreted by parafollicular cells.
Secretion of thyroid hormones
• Thyroid hormones secretion is controlled by TSH
secreted by anterior pituitary gland.
• If TSH increase the thyroid secretions increase,
• If TSH decrease thyroid hormones decrease.
Elements required for synthesis
• Elements required for synthesis of T3 and T4
A: Iodine
B: Thyroglobulin
Formation of thyroid hormones:
About 90% of the hormone secreted by the thyroid
gland is thryoxine and 10% is triiodothyronine.
Difference b/w T3 and T4
• The function of these two hormones are qualitatively
the same, but they differ in rapidity and intensity of
action.
• Triiodothyronine is about four times as potent as
thyroxine, but it is present in the blood in much
smaller quantities and persists for a much shorter time
than does thyroxine
Stages for the formation of T3 & T4
• Absorption of Ingested Iodides
Iodine is absorbed from G.I.T into the blood in the
form of iodides.
A: Iodide Trapping
B: Oxidation of Iodide ion
C: Formation and secretion of Thyroglobulin
D: Iodination of Tyrosine
E: Storage of Thyroglobulin
F: Release of T3 & T4 from the thyroid gland
Why is Iodine Important in Thyroid
Hormone Production?
• Thyroid hormones are unique biological molecules in
that they incorporate iodine in their structure.
• Thus, adequate iodine intake (diet, water) is required
for normal thyroid hormone production.
• Major sources of iodine:
- iodized salt
- iodated bread
- dairy products
• Minimum requirement: 75 micrograms/day
Transport of hormones
• In the blood T4 & T3 combine with plasma protein,
thyroid binding globulin (TBG) and are transport from
one place to another in the blood.
• TBG is more important not only for transport of
thyroid hormones but also that it serves as reservoir
of hormone.
Effects of thyroid hormone on specific
bodily mechanisms
1: Calorigenic Action
(it maintains B.M.R)
2: Effects on carbohydrate metabolism
(increase glucose absorption form GIT, Peripheral
utilization of glocose, glycolysis in the liver,
gluconeogenesis, and uptake of glucose by cells)
3: Effects on protein Metaboloism
( stimulate protein synthesis in ribosomes, normal
range it increase protein anabolism while high conc. It
causes protein catabolism, increase level of amino acid
in the blood)
Effects of thyroid hormone on specific
bodily mechanisms
4: Effects on lipid Metabolism
(increase mobilization of lipid from their sources,
increase oxidation of free fatty acid, decrease
cholesterol, phospholipids and triglyceride level of
blood)
5: Effects on vitamin metabolism
(T4 is necessary for conversion of Beta- carotene to
vitamin A so in hypothyroidism blood carotene level
increase to give a yellow color to skin. High B.M.R in
hypothyroidism increase the requirement for vitamin
B group, particularly thiamine (B2).
Effects of thyroid hormone on specific
bodily mechanisms
6: Effects on water and mineral metabolism
( promotes demineralization of bone,
increase excretion of calcium and phosphates
in the urine, it causes diuresis, increase K
excretion)
Specific effects
• Effects on C.N.S
1: increased rapidity of cerebration
2: increased synaptic activity
3:hyperthyroidism is likely to cause extreme
nervousness, anxiety and extreme worry.
4: in growing children decreased activity of thyroid
hormone causes mental retardation
• Effects on Heart
1: increase cardiac output
2:increase heart rate
3: increase strength of heart beat
4: increase peripheral blood flow
• Effects on blood volume
1: slight increase in blood volume
Specific effects
• Effects on GIT
1: Increase the rate of absorption
2: increase the rate of secretion
3: increases the motility of GIT
4: increases the appetite
• Effects on Respiration
1: causes increased utilization of O2 and formation of co2
2: increase pulmonary ventilation
3: increase the depth of respiration
Specific effects
• Effects of Gonads
1: essential for normal reproduction
2: secondary sexual characters do not develop with out
this hormones
3: fertility is reduced in hypoactivity of hormone
• Effects of mammary gland
This promotes galactopoiesis ( formation of milk)
Specific effects
• Effects on muscle
1: muscular weakness occurs in both hypo and
hyperthyroidism
2: slight increase in thyroid hormone usually makes the
muscle react with vigor
• Effects on sleep
1: hyperthyroid patients often have a constant feeling of
tiredness due to excitable effects on synapses, it is
difficult for patient to sleep. So in hyperthyroidism
there is insomnia.
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