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Thyroid GlandThyroid Gland
• HistologyAcini / follicles ( active & inactive gland)
Cells
Thyroid cells –
amine hormones- T3,T4, and rT3
C - cells (Para follicular cells)
protein hormone – Calcitonin)
Thyroid HormonesThyroid Hormones
• Triiodothyronine (T3) By Thyroid Follicular Cells
• Thyroxine or Tetraiodothyronine (T4) By Thyroid Follicular Cells
• Reverse Triiodothyronine - rT3 By Thyroid Follicular Cells
• Calcitonin By Thyroid Para Follicular C Cells
SYNTHESISSYNTHESIS
ColloidBloodThyroid
Cell
I-
ATP
Na
I- I
MIT
Tyrosine
Na I
DIT
Thyroglobulin Mol
MIT DIT
DITDIT
T3
T4
Peroxidase
TGPeroxi
EPRExo
++
IODIDE TRAPPINGOXIDATION OF
IODIDE ION
IODINATION OF TYROSINE
COUPLING
SECRETIONSECRETION
Colloid Blood Thyroid
Cell
DIT
Thyroglobulin Mol
MIT DIT
DITDIT
T3
T4
MIT DIT
T3 T4
Lysopro
MIT DIT
T3
T4
I Tyrosine
Deiodinase
I
Endo
TSH FROM ANTERIOR PITUITARY
Production of Thyroid Production of Thyroid HormonesHormones
• Iodide (I-) actively transported into the follicle and secreted into the colloid.
• Oxidized to iodine (Io).• Iodine attached to tyrosine within
thyroglobulin chain.– Attachment of 1 iodine produces
monoiodotyrosine (MIT).– Attachment of 2 iodines produces
diiodotyrosine (DIT).
• MIT and DIT or 2 DIT molecules coupled together.
Production of Thyroid Hormones Production of Thyroid Hormones (continued)(continued)
• T3 and T4 produced.
• TSH stimulates pinocytosis into the follicular cell.– Enzymes hydrolyze T3 and T4 from
thyroglobulin.
• Attached to TBG and released into blood.
T3 T4Normal Plasma level 1.2 – 3.1 n mol/L 60-160 n mol/L
Free 0.3 ng 0.15 ug
Bound 99.8% 99.98%
Binding Proteins Thyroxin Binding Globulin
Thyroxin Binding Globulin
Duration of action 1-2 days 4-6 days
Site of formation/ Source
Thyroid Cells + T4 Thyroid Cells
Potency 4-5 times More potent Less potent
Mechanism of ActionMechanism of Action
T3,T4 – Genomic action – formation of new proteins
TRH – Activation of Phospholipase-C System
TSH – Activation of cAMP system
FEEDBACK FEEDBACK CONTROL CONTROL
OF THYROIDOF THYROIDSECRETIONSECRETION
T3 T4T3 T4
Pituitary Pituitary TSHTSH
HYPOTHALAMUSHYPOTHALAMUS
TRHTRH
PHYSIOLOGICAL EFFECTS OF PHYSIOLOGICAL EFFECTS OF THYROID HORMONETHYROID HORMONE
Regulation of metabolism – (O2 consumption)
Mental development Normal growth
potentiate somatomedin Normal secretion of GH.
Skeletal maturation ossification of cartilage & growth of teeth
Normal menstrual cycle Conversion of carotene to vitamin A.
Actions of TActions of T33
• Stimulates protein synthesis.• Promotes maturation of nervous system.• Stimulates rate of cellular respiration by:
– Production of uncoupling proteins.– Increase active transport by Na+/K+ pumps.– Lower cellular [ATP].
• Increases metabolic heat.• Increases metabolic rate.
– Stimulates increased consumption of glucose, fatty acids and other molecules.
Effects of Hypersecretion Effects of Hypersecretion
• Effects of hypersecretion are due to
increased O2 consumption
Catabolism of carbo; proteins; & fats.
Enzyme synthesis
No. and size of mitochondria
ATPase activity
Effects of HypersecretionEffects of Hypersecretion
• Caloregenic Effects
• Effects on CVS
• Effects on CNS
• Effects on GIT
• Effects on Sympathetic system
• Effects on Growth
Caloregenic effectsCaloregenic effects
• It involves the effects on – Carbohydrate Metabolism– Protein Metabolism– Fat Metabolism
Increased Catabolism
EFFECTSEFFECTS ON CVSON CVS
Increases • Heart rate
• Cardiac output
• Systolic pressure
• Pulse pressure
• Mean pressure
Effects on CNSEffects on CNS
Rapid cerebration Psychoneurotic tendency Tremors Time for stretch reflex is reduced Decreased sleep
Features of HyperthyroidismFeatures of Hyperthyroidism
• Increased – BMR, and Heat production
– Heart rate – palpitation
– Weight loss
Diarrhea
Exophthalmos & corneal ulcers
GRAVE’S DISEASE
Features of HypothyroidismFeatures of Hypothyroidism
• Decreased BMR and Pulse rate• Easy fatigability• Slow reflexes• Slow mentation• Slow speech• Increased weight• Increased sleep• Myxedema (water, chondriatin sulphate, and
hyaluronic acid)
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