thyroid hormone (th) structures iodine synthesis secretion metabolism actions hypo- & hyper-...
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Thyroid Hormone (TH)
Structures
Iodine
Synthesis
Secretion
Metabolism
Actions
Hypo- & hyper- thyroidism
Regulation
Regulation of Thyroid Hormone Concentrations
Hypothalamus
Cold
(neonates) +
+
Anterior Pituitary
+ (1)
Thyrotrophin Releasing hormone (TRH)
Thyroid Gland
Thyroid Stimulating Hormone (TSH) = Thyrotrophin
Thyroid stimulating immunoglobulins (TSI)
Thyroid hormone (TH)
1. TH synthesis
2. TH release
3. number & size of cells
Metabolism
T4
T3
rT3
di-iodothyronines I– split off
In: liver, kidney, pituitary
33%
45%
**
* Regulation:
T4 T3 inhibition by: propylthiouracil,
propanolol ( blocker)glucocorticoidsburns, trauma, renal failure, infarct food intake
In Blood
TH
(T4, T3)+
protein(thyroid binding
globulin & others)
Protein bound—T4
or T3)
mostfree
Free % small
< 0.5 %
gets into cells – biologically relevant
Thyroid Hormone Effects
4–5 times
T3 > T4 >> rT3
inert
Mechanism: steroid hormone-like
intracellular receptor
modulation of RNA synthesis
slow on-off effects
Thyroid Hormone Effects
1. Calorigenic actions
effect on intermediary metabolism
CHO / fat / protein
effect on growth & development
2. Sympathomimetic effects
facilitates actions of catecholamines
(adrenalin, noradrenalin)
Calorigenic actions
Heat producing
e.g. basal metabolic rate
heat production
energy use
O2 consumption
number & size mitochondria
MetabolismGlucose: rate absorption
uptake to cells
Fat: clearance lipoproteins
turnover
Protein: amino acid uptake
protein synthesis
• If T4 or T3 is too high, then protein catabolism
Vitamins: B12 absorption
carotene Vit. A
• If T4 or T3 is too high, then vitamin deficiencies
Sympathomimetic EffectsRelationship to catecholamines:• some similar effects• T4 / T3 adrenergic receptors
(number & affinity)• some effects of T4 / T3 can be treated with blockers
Excess T4 / T3
jumpy, anxioustremorpulse rate bounding pulse, systolic P
diastolic P peripheral vasodilation
Hypothyroidism:
• thyroid disease
• I– deficiency
Fetus:
impaired CNS development
Child: height, metal retardation
& cold intolerance, impaired mental function, coarse hair, dry skin, husky voice, reflexes
Goitre= size thyroid gland
• euthyroid
• hyperthyroid e.g. TSI
• hypothyroid e.g. I– deficiency,
autoimmune thyroiditis
I– deficiency T4 / T3
negative feedback pituitary
TSH secretion
number & size thyroid cells
Summary• T4, T3, (rT3)
• synthesis in colloid
• negative feedback regulation
pituitary & hypothalamus
• regulated metabolism
• free concentrations low but biologically relevant
• steroid hormone like – slow on-off
• actions: calorigenic on intermediary metabolism
potentiation of catecholamines actions
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