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Endocrine DisordersPhysiology of the Endocrine System!
Hypothalamic-Pituitary-Thyroid Axis!
HPT axis is required for hormone system control. !
TRH: thyrotrophin-releasing hormone; secreted in the
hypothalamus and travels via portal system to pituitary where
it stimulates production of TSH by the thyrotrophs!
TSH: thyroid-stimulating hormone; secreted into circulation;
stimulates increased thyroidal iodine uptake and synthesis and
release of T3 and T4. TSH also stimulates peripheral conversion
of T4 to T3!
T3 and T4 enter cells, bind to nuclear receptors and promote
increased metabolic and cellular activity. !
Levels of T3 sensed by pituitary and hypothalamic receptors. If
T3 too high, TRH and TSH synthesis is suppressed and thus T4
and T3 secretion is reduced. Peripheral T3/T4 levels drop to
normal.!
If T3 and T4 levels are too low, increased production and
secretion of TRH and TSH results in the thyroid increasing its
production of T3 and T4. This restores the plasma levels of T3
and T4, however may occur at the expense of TSH (thus get
high TSH levels; compensated euthyroidism). !
!
Central Control of Endocrine
Function!
Many peripheral hormone systems are controlled by the
hypothalamus and pituitary.!
Hypothalamus: situated at base of brain around the third
ventricle and above the pituitary stalk which carries the
hypophyseal-pituitary portal blood supply.!
Optic chiasm: lies just above the pituitary fossa - therefore,
any lesions expanding from the pituitary/hypothalamus may
compress the chiasm and cause visual defects. !
Suprasellar expansion: upwards expansion of pituitary gland
!
through the diaphragma sellae!
Hypothalamus: contains many vital centres for functions, e.g.
appetite and thirst, thermal regulation and sleeping/walking.
the hypothalamus acts as an integrator of many neural and
endocrine inputs to control the release of pituitary hormone-
releasing factors. !
Role in circadian rhythm, menstrual cyclicty, responses to
!
stress, exercise and mood. !
Hypothalamic Physiology!
Hypothalamic neurones secrete pituitary-releasing and
inhibiting factors and hormones into the portal system, which
descends down the pituitary stalk to the pituitary. The
hypothalamus also contains large amounts of other
neuropeptides and transmitters, e.g. Neuropeptide Y, VIP and
NO, which can also alter pituitary function. !
!
!
!
ENDOCRINE DISORDERS SIOBHAN PAGE 1
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