Pg. 135
1
Thyroid Gland
• The thyroid gland has two lateral lobes and lies on either
side of the larynx, over the trachea
•About 2 inches long, with lobes joined by strands of thyroid
tissues called the isthmus
•Rich blood supply
• It secretes three hormones:
• T4 (thyroxine)
• T3 (triiodothyronine) *5 to 10x more active than thyroxine*
• BOTH control rate of metabolism, stimulate protein
synthesis, stimulates breakdown of liver glycogen to
glucose
• Calcitonin-controls calcium level in bloodstream
2
Structure of the Gland
Larynx
Thyroid
gland
Colloid
Follicular
cell
Follicular cells
Colloid
Isthmus
(a)
Extrafoll
icular
cell
Extrafollicular
cells
© Fred Hossler/Visuals Unlimited
3
Thyroid Hormones
OH
I
OH
I
I
O
I
O
I
CH2
I
I
CH2
NH2CHCOOH
NH2CHCOOH
Thyroxine (T4)
Triiodothyronine (T3)
4
© Mediscan/Visuals Unlimited
© Mediscan/Visuals Unlimited
© Mediscan/Visuals Unlimited
5
Parathyroid Glands
• The parathyroid glands are on the posterior surface of the
thyroid gland
• There are typically four tiny parathyroid glands the size
of grains of rice
• It secretes one hormone:
-PTH (parathyroid hormone or parathormone)-also
controls concentration of calcium in bloodstream;
stimulates increase in number and size of osteoclasts
6
Structure of the Glands
Pharynx
Secretory cells
Thyroid
gland
Capillaries
Parathyroid
glands
© R. Calentine/Visuals Unlimited
Esophagus
Trachea
Posterior view
7
Parathyroid Hormone
Foods
Cholesterol
Intestinal enzymes
Provitamin D
Ultraviolet light in skin
Vitamin D
(Cholecalciferol)
Also obtained directly
from foods
Liver
Hydroxycholecalciferol
Kidney
Stimulated by PTH
Dihydroxycholecalciferol
(active form of vitamin D)
Controls absorption of
calcium in intestine
Ca+2
8
Ca+2
Ca+2
Parathyroid glands (on
posterior of thyroid gland)
Release into
bloodstream
Decreased blood calcium
stimulates parathyroid
hormone secretion
Stimulation
–
Increased blood
calcium inhibits
PTH secretion
Inhibition
PTH
Bloodstream
PTH
Ca+2
+
Bone
releases Ca+2
PTH
+
Ca+2
Kidneys
conserve Ca+2 and
activate Vitamin D
Ca+2
Intestine
absorbs Ca+2
Active
Vitamin D
9
10
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