Test 3: lecture 7 Flashcards
___ fills a thyroid follicle
colloid
two types of thyroid hormone
Thyroxine (T4)
Triiodothyronine (T3)
Thyroxine (T4)
Thyroxine (T4)
formed and secreted solely by the thyroid (93%)
longer half-life than T3
less potent than T3
Triiodothyronine (T3)
formed and secreted by the thyroid (7%)
mainly formed by deiodination of T4 in peripheral tissues
more active on a cellular level than T4
shorter half-life than T4
T3 or T4 have a longer half life
T4
___ is the precursor for thyroid hormones
thyroglobulin
a protein hormone (a glycoproteins made by two subunits)
made by the ER and the Golgi → stored in colloid
>100 tyrosyl residues; 25-30 get iodinated; 4-8 end up in T3 and T4
T3 and T4 are synthesized by iodination of tyrosyl residues that are part of thyroglobulin
T3 and T4 remain part of thyroglobulin until secreted into blood
how are T3 and T4 made
some of the tyrosyl residues in thyroglobulin are iodinated
what happens to injected iodine
thyroid gland takes most of it
some is recycled
rest of it is excreted in urines or in stool
synthesis, storage and secretion of T3 and T4 by the thyroid gland
iodide pump
moves iodine into follicular cells (Na/I co transporter)
moves Iodide against its chemical and electric gradient
TSH stimulates iodide trapping
why is thiocyanate not used for HTN medication any more
blocks iodide pump leading to decrease of iodide in follicular cell of the thyroid → hypothyroidism
how does iodide turn into I2
I(thyroid peroxidase + hydrogen peroxide) → nascent iodine/ I2
I is oxidized into I2
occurs at the apical membrane of thyroid cell (near colloid)
this reaction is inhibited by anti-thyroid drugs and too much iodide
___ will inhibit the oxidation of iodide into I2
some anti-thyroid drugs and by too much iodide
how do MIT and DIT form
iodination of tyrosyl residue
(add iodide by adding I2 and iodinase)
MIT has 1 iodide
DIT has 2 iodide
what does the coupling reaction due in the thyroid
converts DIT + DIT→ T4 and alanine
converts MIT + DIT → T3 and alanine
how are T3 and T4 released from the thyroglobulin
T3 and T4 are stuck of thyroglobulin in colloid
this is eaten by follicular cell and broken down by lysosomal and enzymes
T3 and T4 is then released
MIT and DIT have their I recycled to be used to make more thyroglobulins
what happens to the I on the thyroglobulin after the T3 and T4 have been cleaved?
MIT and DIT have their I recycled to be used to make more thyroglobulins
how is T3 and T4 made and released
I taking into cell by Na/I co-transporter
I is oxidized into I2
I2 is used to turn Tyrosyl into MIT and DIT (iodination- MIT has one I, DIT has 2 I)
coupling results in T3 and T4 and alanine (DIT + DIT→ T4 and alanine. MIT + DIT → T3 and alanine)
DIT, MIT, T3 and T4 all stuck together on thyroglobulin in the colloid
this is taken up and broken down by lysosomal enzymes
T3 and T4 transported out of cell
DIT and MIT have their I cleaved to be reused
how long does colloid last
3 months of thyroid T3 and T4
99% of T3 and T4 in blood is ___
bound to proteins made by the liver
free or bound thyroid hormones effect cellular activity
free hormones less than 0.3% of all thyroid hormones
will also inhibit pituitary secretion of TSH
___ will inhibit pituitary secretion of TSH
free T3 and T4
less then 0.3% of all thyroid hormone in the blood
____ binds to T3 and T4 in the blood and lasts for 5 days
thyroxine-binding globulin (TBG)
(a type of thyroid hormone binding protein made by the liver)
highest affinity for T3 and T4
most T3 and T4 bound to TBG
half-life – 5 days
___ is specific for T4 in the blood
thyroxine-binding prealbumin
(a type of thyroid hormone binding protein made by the liver)
specific for T4
intermediate affinity and capacity
half-life – 2 days
___ binds to T3 and T4 and lasts 13 days
albumin (a type of thyroid hormone binding protein made by the liver)
lowest affinity for T3 and T4
high capacity due to its abundance in blood
half-life – 13 days
what turns T4 into T3
5’ deiodinase
overfeeding stimulates this
illness and fasting inhibits this
where is the receptor for T3
in the nucleus → will result in transcription of genes
if you have a high Basal metabolic rate, you have ___ thyroid hormones
high
effects of thyroid hormone
- catabolic hormone: increases basal metabolic rate
- increases the metabolism of almost all active tissues
- increases the number and activity of mitochondria → increases amount of ATP
- increases membrane-bound Na+-K+ ATPase activity (because there is more ATP available
– increased heat production
5.stimulation of carbohydrate metabolism
increases Glucose uptake, glycolysis, and gluconeogenesis
6.stimulation of fat metabolism
increases free fatty acids concentration in the blood. decreases cholesterol & phospholipid concentrations in the blood
why does blood cholesterol decrease with thyroid hormone
thyroid stimulates LDL
will cause cholesterol to be taken up into the cell
why will thyroid increase heat production
causes more mitochondria→ more ATP
= more activity of Na/K ATPase pump → more heat
what are some other effects of thyroid hormone
- increased cardiac output, heart rate & strength
- increased respiration, gastrointestinal motility and secretion
- excitation of central nervous system
- essential for growth
- essential for development of brain
how will thyroid effect HR
hypo- decreased HR
hyper- increased HR
hypo or hyper thyroidism causes constipation
hypo
hyper thyroid leads to increase in GI motility → diarrhea
thyroid is essential for development of the ___
brain
hyperthyroidism
increased metabolism → weight loss
intolerant of heat
very anxious → increased sympathetic innervation
diarrhea → increased GI mobility
always hungry but never gaining weight
hypothyroidism
lethargic
weight gain- slow metabolism
cold intolerance
constipation
if you inject thyroxine what happens
slow onset and long duration
TSH
Thyroid-stimulating hormone (TSH or thyrotropin)
- TSH is a glycoprotein (protein hormone)
- increased activity of the iodide pump
- increased iodination of tyrosine
- increased secretory activity of thyroid cells
- increased proteolysis of thyroglobulin
- increased number and size of thyroid cells
how does TSH work on thyroid
TSH is a protein hormone made by the anterior pituitary
binds to surface of follicular cell and causes release of cAMP to act as second messenger
TRH
Thyrotropin-releasing hormone (TRH) from the hypothalamus
- TRH is a tripeptide amide – pyroglutamyl-histidyl-proline amide (peptide hormone)
- binds to TRH receptors in the pituitary cell membrane
- activates the phospholipase second messenger system
- stimulates pituitary secretion of TSH
TRH uses ___ second messenger system
phospholipase
(Thyrotropin-releasing hormone (TRH) from the hypothalamus)
negative feedback regulation of hypothalamopituitary- thyroid axis
what happens to TSH if there is no iodide?
increases
I is needed to make T3 and T4 to turn off TSH, no I means no T3 and no T4
results in free hormones, endemic goiter (hypothyroidism), TSI (thyroid-stimulating immunoglobulin) (hyperthyroidism
goiter is caused by
lack of iodide in the diet
hypothyroidism
I is needed to make T3 and T4 to turn off TSH, no I means no T3 and no T4
TSI
TSI (thyroid-stimulating immunoglobulin)
binds to TSH receptors on thyroid glands and stimulates thyroid to make T3 and T4 → hyperthyroidism
no feed back control
treated by removing thyroid gland
what happens to thyroid hormone synthesis with stress
decreases
stress inhibits hypothalamus
____ decrease iodide trapping and causes a goiter
Thiocyanate ions (an anti-thyroid substance)
___ inhibits the peroxidase enzyme and causes a goiter
1.Propylthiouracil (an anti-thyroid substance)
___ decrease all phases of thyroid activity and decrease thyroid gland size.
1.Iodide in high concentration (100x)
Hyperthyroidism is a common disease of middle-older aged cats.
What would the major clinical signs be?
increased metabolism → weight loss
intolerant of heat
very anxious → increased sympathetic innervation
diarrhea → increased GI mobility
always hungry but never gaining weight
exhausted easily
. Hypothyroidism is a common disease of adult dogs. What would the major clinical signs be?
lethargic
weight gain- slow metabolism
cold intolerance
constipation
What is the best way to diagnose hyperthyroidism in cats?
T4 and TSH blood tests: High T4, low TSH.
What is the best way to diagnose hypothyroidism in dogs?
T4 and TSH blood tests: Low T4, high TSH.
