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Thyroid Hormone (TH)
Structures
Iodine
Synthesis
Secretion
Regulation
Metabolism
Actions
Hypo- & hyper- thyroidism
follicle
Reverse T3
Activity: T3 > T4 >> rT3
iodine
iodised salt
… no salt sale
in the shops of
Dushanbe
Blood
I–
T3T4
Thyroglobin
90% T4, 9% T3, 1% rT3
Regulation of Thyroid Hormone Concentrations
Cold
(neonates)

+
Hypothalamus
Thyrotrophin Releasing hormone (TRH)

+
Anterior Pituitary
Thyroid
hormone (TH)
Thyroid Stimulating Hormone (TSH) = Thyrotrophin
+ (1)
Thyroid Gland
1.  TH synthesis
2.  TH release
3.  number & size of cells
Thyroid stimulating immunoglobulins (TSI)
Metabolism
33%
T4
45%
*
*
T3
di-iodothyronines
rT3
I– split off
In: liver, kidney, pituitary
* Regulation:
T4  T3
inhibition by:
propylthiouracil,
propanolol ( blocker)
glucocorticoids
burns, trauma, renal failure, infarct
 food intake
In Blood
protein
TH
+ (thyroid binding
(T4, T3) globulin & others)
free
Free %
Protein
bound—T4
or T3)
most
small
< 0.5 %
gets into cells – biologically relevant
Thyroid Hormone Effects
T3 >
4–5 times
Mechanism:
T4
>>
rT3
inert
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
Metabolism
Glucose:  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
Protein
Synthesis
net
too
little
[ TH ]
too
much
Growth and development
in utero
CNS
& neonates
Sympathomimetic Effects
Relationship 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, anxious
tremor
pulse rate 
bounding pulse, systolic P 
diastolic P 
peripheral vasodilation
Thyroid stimulating
immunoglobulins =
Thyroid stimulating
hormone
No negative feedback
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