Lecture: Thyroid gland Flashcards
Thyroid hormone synthesis
Cells actively accumulate iodide and iodinate tyrosine residues to form T3 and T4- most likely to get into the cell
Iodinated thyroglobulin enters lumen by exocytosis
Stored thyroglobulin re-enters follicle cells by endocytosis
Lysosomal enzymes release T3 and T4
Most (~90%) are “bound” by binding proteins in plasma
“Free” fraction of T3 and T4 can enter target tissues
Thyroid stimulating hormone
Thyroid hormone production absolutely requires TSH
+ Iodine uptake from blood by stimulating expression of the ATP-dependent sodium/iodide transporter
+ TH synthesis by stimulating expression of thyroid peroxidase
+ Thyroglobulin internalization and breakdown by lysosomal proteases
Actions of thyroid hormones
90 % of released hormone is T4
70-75% of both T3/T4 are bound by thyroid binding globulin, rest by thyroid binding prealbumin
Only unbound T3 (0.3%) and T4 (0.03%) can enter target tissues
Most of physiological effects of thyroid hormones are due to T3
Mitochondrial effects
++ size and number
++ ATP production
Nuclear receptor increases transcription and translation via TRE
Effect is generalized increase in enzyme synthesis
Nearly all cells have TH receptors - widespread effects
Physiological effects of TH
Cardiovascular
increased cardiac output
positive inotropic
vasodilation
increased blood volume
increased pulse pressure
Respiration
increased depth
increased rate
Muscle
increase in tension
increase force of contraction
GI tract
increased secretions
increased motility
CNS
increased excitability
increased speed of thought
Body weight
decrease body weight
increased appetite
Hyperthyroidism- TH excess
Body weight
decreased body weight- change
increased appetite
increased food intake
Cardiovascular
cardiac output
increased force of contraction
increased rate of contraction
increased blood volume
vasodilation
atrial arrythmias
congestive heart failure
Trophic effects
excess hair growth
seperation of fingernails
hair loss
Muscle
weakness
tremor
CNS
nervousness- change
irritability-change
emotional instability
sleeplessness
GI tract
increased bowel
movements
Other symptoms
fatigue
myedema
exopthalmus
amennorhea
heat intolerance- change
sweating
moist skin
Signs/Symptoms of hyperthyroidism
“goitre”
(swelling of the thyroid
“exophthalmos”
(bulging eyes)
Hypothyroidism- TH deficiency
Body weight
increase body weight-change
decreased appetite
Cardiovascular
decreased cardiac output
decreased force of contraction
decreased rate of contraction
decreased blood volume
heart enlargement
arteriosclerosis
Trophic effects
decreased hair growth
loss of hair
scaly skin
brittle finger nails
Muscle
weakness
stiffness
swelling
CNS-change
mental sluggishness
fatigue
somnolence
GI tract
constipation
Other symptoms
husky voice
myxedema
decreased fertility
increased menstruation
cold intolerance- change
regulation of secretion
in the hypothalamus TRH( thyrotropin releasing hormone) and somatostatins
produce thyroid stimulating hormone in the anterior pituitary
in the thyroid uptake of iodine produces T3 and T4
in the blood T3 and T4 are
GH and cortisol negative
cold and sympathetic activation positive
Thyroid Hormones
Major homeostatic regulators
Growth and development
Stimulate protein, carbohydrate and lipid metabolism
Regulate energy metabolism
Body temperature
Regulation of nervous system, cardiovascular, musculo-skeletal and reproduction
TH receptors
Regulate gene transcription
Thyroid hormone affects BMR
T3/T4 join with TH receptors
mitochondria
Increased size, number and
surface area of mitochondria
Increased rate of ATP production
Increased heat
production
Increased
BMR
sodium potassium pump
increase in
Na-K ATPase
Increased heat
production
Increased
BMR
TH stimulates protein metabolism
T3/T4 join with TH receptors- nucleus
Increased protease synthesis to
Increase in protein breakdown to
increased energy expenditure
to
increased BMR- basal metabolic rate
TH stimulates carbohydrate metabolism
T3/T4 join with TH receptors
general increase in enzyme
synthesis
Increased glycogenolysis
Increased gluconeogenesis
increased glucose
uptake by GI tract, liver
and muscle
Increased release
of insulin
what does the TH receptors stimulate
fat metabolism
carbohydrate metabolism
protein metabolism
