Download hormones 4

Hormonal Coordination 4
• Hormones produced by
thyroid glands
• their physiological effects
At the end of this lecture you should be
able to
• explain how thyroid hormones are
synthesized, stored & released
• discuss the physiological effects of
hormones produced by thyroid glands
Thyroid gland
Location:
• just inferior to
larynx,
• lateral lobes lie on
either side of
trachea,
• isthmus lies
anterior to trachea
Structure
• Spherical sacs (thyroid follicles), inside
filled with a colloid
• Walls of follicles contain two types of
cells:
Follicular cells -extend to lumen of follicle
Parafollicular cells -cells present between
the follicles
• Follicular cells produce thyroid
hormones
– Thyroxine
– Triidothyronine
• Parafollicular cells produce
– Calcitonin
Thyroid gland is the only endocrine
gland that stores its secretary products
in large quantities (100 days supply)
Formation, & storage of thyroid hormones
1. Iodide trapping from blood by
follicular cells through active
transport ( Conc. of iodide inside
follicular cells is 20-40 times of
plasma)
Formation, & storage of thyroid hormones
2. At the same time, Synthesis of
thiroglobulin (TGB, contain tyrosine
amino acid) within follicular cells &
packed in secretary vesicles & release to
lumen through exocytosis (material
accumulated in lumen is the colloid)
3. Oxidation of iodide within follicular cells
2 II2
to colloid
4. Iodination of tyrosine in TGB, forms
monoiodothyrosine(T1) & diiodothyrosine(T2)
5. Coupling of T1 & T2
T2 + T2
T4
T2 + T1
T3
Thyroid hormone stored as part of TGB
within the colloid
Release of thyroid hormones
6. Pinocytosis & digestion of colloid
droplets of colloids reenter the follicular
cells, combine with lysosomes, digestive
enzymes cleaves T3 & T4 from TGB
7. T3 & T4 diffuse through plasma
membrane & enter the blood
8. Transport in blood through thyroxine –
binding globulin,
Effects of thyroid hormones
1. Regulate oxygen use & basal
metabolic rate
(increase BMR by stimulating oxygen use
to produce ATP, more heat is given off ,
body temperature rises)
Effects of thyroid hormones
2. Regulate cell metabolism
(stimulate protein synthesis, increase
use of glucose for ATP production,
increase lipolysis, reduce blood
cholesterol, increase cholesterol
excretion in bile)
Effects of thyroid hormones contd..
3. Regulate growth & development
Together with hGH, insulin, thyroid
hormones increase body growth,
especially the growth of nervous
tissue
Effects due to deficiency of thyroid
hormones
• During fetal development – fewer &
smaller neurons, defective myelination of
axones, mental retardation
• During early years of life – small status,
poor development of brain
Abnormalities related to the thyroid
glands
• Cretinism (congenital hypothyroidism)
– Hyposecretion of thyroid hormones at birth,
severe mental retardation
• Myxedema (hypothyroidism in adults)
– Lethargy, weight gain
Goiter (hypothyroidism, hyperthyroidism)
When dietary iodine
intake is low
• low levels of thyroid
hormone in blood
• Stimulate secretion
of TSH.
Thyroid gland
enlargement
Calcitonin
• Help to maintain homeostasis of ionic
calcium & phosphates in the blood
• Lowers the amount of blood Ca2+ &
phosphate by inhibiting breakdown of
bone matrix & accelerates uptake of Ca2+
& phophates to bone matrix (inhibit action
of osteoclasts)
Regulation of calcitonin
• high blood Ca2+ stimulates secretion, low
blood Ca2+ level inhibit secretion
summary
• Hormones produced by thyroid glands
• Physiological effects of these hormones