Term
| 10 steps of thyroid hormone biosynthesis |
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Definition
1. iodine trappng
2. iodine oxidation of thyroperoxidase (TP)
3. TGB exocytosis
4. iodinations of TGB
5. coupling of iodotyrosines
6. TGB endocytosis
7. hydrolysis of TGB
8. release of T4 & T3
9. deiodination of MIT & DIT
10. recycling of iodine |
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Term
what's the advantage of having a carrier?
what percentage of T4 are in free pools?
what about T3?
what are most of the T4 bound to?
what about T3?
what are the other carrier proteins? |
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Definition
0.03% of T4 are free, 55% are bound to TBG (thyroid binding globulin)
0.30% of T3 are free, 65% are bound to TBG
other carrier proteins: TBPA (thyroxin binding pre-albumin)
Albumin |
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Term
what enzyme converts T4 to T3?
where is most converted? where else?
what converts T4 to inactive T3
which one is released more by the thyroid? |
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Definition
5'-monodeiodonase (5'-MD)
liver & kidney converts 80%, also brain, CNS, placenta have an isoform of the enzyme
alot of T4 is converted to inactive T3 by 5-MD
thyroid releases 10x more T4 |
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Term
| what's a crucial developmental role of thyroid hormone? |
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Definition
development of the CNS, also generalized tissue growth (often along w/ GH)
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Term
| what effect does thyroid hormone have on metabolism? |
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Definition
1. increases energy metabolism & oxygen consumption (calorigenesis)
2. intermediary metabolism (carbs, fats, proteins)
3. sensitizing the CNS to catecholamines |
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Term
how does thyroid hormone work in the cell?
what helps it perform this task?
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Definition
it diffuses through plasma membrane into nucleus
it then initiates transcription of genes for proteins etc.
retinoid X helps it (forms a dimer with T3)
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Term
| how do we get rid of thyroid hormones? |
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Definition
T3 & 4 are deiodinated in the liver & kidney yielding partially iodinated thyronines
can be conjugated to glucoronide & excreted in bile or urine |
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Term
where does the main neg feedback occur from T3 & T4?
where are the peak TSH levels?
what else can inhibit TSH secretion? on what structure does this have an effect? |
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Definition
at the anterior pituitary
at night due to the circadian rhythm
sympathetic stimuli (stress/temperature - hypothalamus) |
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Term
| what causes an iodine deficiency goiter? |
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Definition
low iodine meaning low T3&4 production, thus little negative feedback on the pituitary, thus pituitary secretes more TSH which stimulates thyroid growth.
Also, T3/T4 ratio increases cuz T3 is more potent, need more of it! |
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Term
what's the thyroid's response to increased dietary iodine?
what's this effect called also?
what is this used for clinically? |
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Definition
more iodine - more T3, T4, thus more negative feedback on pituitary.
less TSH - less thyroid growth & less blood supply to thyroid.
Wolf-Chaikoff effect
used for surgery
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Term
| why is the responsiveness of the thyroid hormone system so slow? |
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Definition
long life of the hormones: T3 - 1 day, T4 - 6 days
slow response of target tissues |
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Term
what does the drug thyiocyanate do?
what type of chemical is it? |
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Definition
decreases iodine trapping
it's a goitrogen (can cause goiter) |
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Term
what does percholride do?
what type of chemical is it? |
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Definition
inhibits iodine transport
goitrogen |
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Term
what does propylthiouricil do?
what type of chemical is it? |
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Definition
blocks oxidation & coupling (thyroperoxidase)
goitrogen |
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Term
Grave's disease
what causes it?
is there much TSH present?
what are the symptoms?
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Definition
immune system makes antibodies that stimulate TSH receptors to make excess T3&4
very little endogenous TSH present
nervousness & sweating
hypersensitive to heat
palpitation / tachycardia
weight loss |
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Term
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Definition
decr appetite, weight gain, somnolence (sleepy), fatigue, lethargy
puffy face, hands, feet (skin is infiltrated by mucopolysacherides) = myxedema |
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Term
Hashimoto's disease
what does it cause?
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Definition
1. attack from cytotoxic CD8 lymphocytes on the thyroid
2. autoimmune antibody affects that inhibit thyroperoxidase |
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