Download MBBS Physiology Presentations 7 Thyroid Hormones Lecture Notes

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Thyroglossal duct marks the path of

thyroid from tongue to neck

Gland is highly vascularized and has

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one of the highest rates of blood flow

per gram of tissue
Thyroxine (T4)- Greater amount

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released. Biological activity less

Triiodothyronine (T3)- Lesser amount

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released. Biologically more active.

Generated at the site of action in

peripheral tissues by deiodination of

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T4.

Reverse

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Triiodothyronine (RT3)-

Small amounts
Pendrin (Cl/I exchanger)

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Pendred syndrome- Thyroid dysfunction

and deafness

Salivary

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glands, Gastric mucosa,

placenta, ciliary body of eye, choroid

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plexus, mammary glands and certain

cancers of these tisssues express NIS

but the transprter is not affected by TSH

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in these tissues.
T4 and T3 are deiodinated in liver, kidneys

and other tissues

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Pituitary and cerebral cortex has high T3/T4

ratio

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Three deiodinases: D1, D2 and D3- contain

selenocysteine

D1 ? present in liver, kidneys, thyroid and

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pituitary

D2- present in brain, pituitary and brown

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adipose tissue

D3- present in brain and reproductive tissues
T4 and T3 increase oxygen consumption

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of all metabolically active tissues except :

Adult brain
Testes
Uterus

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Lymph nodes
Spleen
Anterior pituitary
o T4 depresses oxygen consumption of

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anterior pituitary because it inhibits TSH

secretion

Thyroid hormones increase the activity

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of membrane bound Na-K ATPase

Some calorigenic effect is due to

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metabolism of fatty acids they mobilize
Nitrogen excretion is increased
Potassium released due to protein

catabolism is excreted in urine

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Increased urinary hexosamine and

uric acid excretion

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Need for vitamins is increased due to

increased BMR

Thyroid hormones are necessary for

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hepatic conversion of carotene to

Vitamin A

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Thyroid

hormones increase the

number and activity of mitochondria

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Thyroid hormones increase active

transport of ions through cell

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membranes.
Stimulation

of

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carbohydrate

metabolism

Stimulates rapid uptake of glucose by

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cells

Glycolysis is increased
Gluconeogenesis is increased

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Increased reabsorption from GIT
Increased insulin secretion

Stimulation of fat metabolism:
Lipid are mobilized rapidly from fat

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tissue

Decreases fat stores of the body
Increases free fatty acid concentration

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in plasma

Increased thyroid hormones decrease

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the concentration of cholesterol,

phospholipids and triglycerides in the

plasma

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Increase cholesterol secretion in bile

Increased thyroid hormones increase

LDL receptors on the liver leading to

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rapid removal from plasma

Increased requirements of vitamins
Increases basal metabolic rate

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Decreased body weight
Increases respiration
Increased gastrointestinal motility
Increased

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activation

of

Reticular

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Activating system (RAS)

Affects brain development
Cerebral cortex, basal ganglia

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and

cochlea are mainly affected

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Deficiency causes during development

causes mental retardation, motor

rigidity and deafness.

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Exerts effects on reflexes- Reaction time

of reflexes is increased in hypothyroidism

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and decreased in hyperthyroidism

In hypothyroidism, mentation is slow

and CSF protein levels increased

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Thyroid

hormones revert these

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changes

Cerebral blood flow, glucose and

oxygen consumption are normal in

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both hypo and hyperthyroidism
Increases blood flow and cardiac

output

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Increases heart rate
Increases heart strength
Pulse pressure increases
Increases expression of - MHC,

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sarcoplasmic reticulum calcium

ATPase, K- Channels

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Inhibits - MHC, Phospholamban,

adenylyl cyclase, nuclear receptors,

Na-Cl exchangers

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Increases

rate and force of

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contractions