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physio > Thyroid > Flashcards

Thyroid Flashcards

1
Q

what does the thyroid gland regulate

A

tissue metabolism and growth and development

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2
Q

what are the thyroid hormones

A

T3 and T4

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3
Q

where are t3 and t4 sythesized, and stored

A

synthesized in the follicular cells of the thyroid and stored in iodinated thyroglobulin in extracellular colloid compartment

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4
Q

how does iodine relate to t3 and t4

A

it is enesential for t3 and t4 synthesis

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5
Q

by what hypothalamus and pituitary hormone is the thyroid stimulated and regulated by

A

TRH and TSH (thyroptropin)
- there is feedback regulation of TSH and TRH by t3 and t4

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6
Q

which thyroid cells secrete calcitonin

A

C cells

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7
Q

what is the active throid hormone how does it occur

A

t4 is deiodinized into t3 ( active)

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8
Q

what is the steady state level of iodine in the body where is the majority located, whats the percentage

A

20-50mg , large proportion (25%) is in the thyroid

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9
Q

what form is unincorporated iodine in?

A

in the form of iodide (I-) not iodine (I2)

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10
Q

what is the daily need of iodine

A

150ug

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11
Q

how much iodine does the thyroid use? what is the majority used for

A

the thyroid gland uses 120ug a day, mostly for the synthesis of t3 and t4

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12
Q

what happens to iodine when there is thyroid hormone degredation?

A

iodine and iodide are unused and are excreted through the urine

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13
Q

what does iodine insuffiecent cause

A

hypothyroidism

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14
Q

what are the steps in the synthesis and release of thyroid hormones (7)

A

1) active transport of iodide by follicular cells
2) synthesis and internal secretion tyrosine containing precursor protein (thyroglobulin) in the colloid
3) iodiniation of thyroglobulin tyrosine residues within the colloid
4) coupling of 2 iodotyrosine residues of thyroglobulin
5) cellular uptake of iodinated thyroglobulin by reabsorption from the colloid
6) proteolytic release of iodinated thryronines in the lysozymes
7) release of t3 and t4 into the blood

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15
Q
  • how does iodine accumulate in follicular cells of the thyroid
  • what type of transport is this
A
  • through a I-/Na symport
  • secondary active transport
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16
Q

how does iodine get from the follicular cell to the colloid

A

via facilitated diffusion

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17
Q

what is thyrogloblulin

A

protein precursor that contains tyrosine residues used to to synthesize t3 and t4

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18
Q

where is thyroblobulin secreted

A

in the ER of follicular cells

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19
Q

what is thyroglobulin secreted in to the colloid with, what is its function?

A

with thyroid peroxidase ( TPO) catalyzes its iodination

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20
Q

what is the function of tpo

A

catalyzes the oxidation of iodide to iodine and its binding to tyrosine on thyroglobulin

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21
Q

what are the 2 steps of the iodiniation of thyroglobulin

A

first step yields 3-monoiodotyrosine (MIT) and then 3,5 diodotyrosine (DIT)

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22
Q

what is the coupling of the 2 iodotyrosine residues catalysed by

A

TPO ( thyroid peroxidase)

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23
Q

what does DIT + DIT give rise to?
- MIT +DIT?

A

t4,t3, and small amounts of inactive/ reverse t3 (rT3)

24
Q

what step does the release of thyroid hormone to the blood start with

A

the reuptake of iodinated thyroglobulin via endocytosis

25
what inititaes proteolytic degredation of iodinated thyrogloblobulin into t3 and t4
colloid vacuoles fusing with lysozymes
26
what areresidual MIT and DIT degraded by
iodotyrosine deiodinase
27
what is mostly released what is converted to ?
t4 mostsly released and converted to t3 and rT3 in other tissues
28
what enzymes regulate T4 and T3 what are the active and inactive forms
- 5' deiodinase converts t4 to active t3 - 5 deiodinase convers t4 in to the inactive rT3 - T3 is inactivated further by deiodination
29
what are thyroid hormones bound to in the blood how does this affect free concentrations compared to the total
bound to plasma proteins so that the free concentration is lower that the total concentration
30
what percentage of t4 and t3 is free. - what form bound or free is active
0.02% t4 is free and 0.2% of t3 is free. on the free form is biologically active
31
which thyroid hormone is bound more weakly to plasma proteins
t3
32
what is the plasma protein that binds t3 and t4 which binds more strongly
thyroxine-binding globulin , binds t4 more stongly
33
what are the other two plasma binding proteins which is present in higher amounts
transerythrin and albumin (present in higher amounts)
34
advantages of thyroid hormone binding proteins
1) enhances distribution to all body 2) provides a resivoir of readily availible hormones 3) protects from degredation, increasing 1/2 life in circulation
35
what hormone increases sythesis and secretion of t3 and t4
TSH
36
what regulates TSH and TRH
negative feedback from free t3 and T4
37
what is used to test endocrine function and why
TSH bc of the close realtionship between t3 and t4 and tsh
38
what other system regulated TRH
the thermosensory system, at low temps TRH is increased, increasing TSH and t3 and t4 and thus increasing body temp
39
what are all the affects of TSH on the thyroid
- release of preformed t3 and t4 - iodide uptake - tyrosine iodination and coupling - endocytosis of colloid - thyroglobulin proteolysis - increase in cell height
40
how do the physiological affects of t3 and t3 differ and how are they similar
- similar physiological affects t3 is more potent bc it has a higher affinity for receptors and acts more rapidly bc it has a lower affinity for plamsa proteins
41
how do thyroid hormones act on a cell
- bind to intracellular receptors and activate transciprion of a gene and increases the synthesis of specific proteins
42
how are the affects of thyroid hormones on growth and development
- enhance bone growth and epiphyseal closure - stimulate GH - brain development especially the cortex - proper formation of the cochlea
43
what are the effects of thyroid hormones on adult metabolism
- elevate BMR - increased heat production - activate mitochondrial metabolism - enhance absorpition of carbs in the intestine - break down of carbs and lipids - enhance sensitivity of catecholamines
44
45
what is cretinism, symptoms, and what are the causes
- hypothyroidism during pre and post natal development - mental retardation, dwarfism, deafmutism, -caused by maternal iodine deficency, fetal congenital abnormalities to the thypothalmus, pituitary or thyroid, materanl antithyroid antibodies
46
what occurs when when thyroid diffeciancy is prenatal how can this affect later development?
maternal t3 and t4 can compensate and subsequent treatment can be given after birth
47
what is the cause of adult hypothyroidism what is it charecterized by
- hypothalmin and pituitary dysfunction and defective TRH and TSH or direct disease to the thyroid gland - low metabolic rate, cold intolerance, drying and yellowing of the skin, thin hair
48
how can hypothalmic or pituitary hypothyroidsm be identified
by normal t3 and t4 in response to TSH injection
49
what is iodine deficant goiter
when iodine is low there can be an enlargemnt of the thyroid bc low thyroid output leads to elevated TSH which causes hypertrophy of the thyroid
50
what is hashimotos thyroditis, what is a symptom and why
an auto immiune disease that destroys the thyroid gland follicles and reduces t3 and t4. goiter is a symptom due to lymphocyte infiltration
51
what are the charecterisitcs of hyperthyroidism
- increased BMR - increased food intake - weight loss - heat intolerance - sweating - nervousnes
52
what is graves disease - what are the symptoms
hyperthyroidsm caused by autoimmune antibodies that stimulate TSH receptors leading to unregulated t3 and t4 release - goiter and protrusion of the eyes due to TSH receptors in the orbit - increase in follicular cell height
53
what is the result of TSH secretinf pitutiary tumor
hyperthyroidsm - goiter
54
how can we use radioactive iodine
- since large amount of ingested iodine is in the thyroid gland radioactive iodine can be used to monitor thyroid function
55

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