Thyroid Physiology Flashcards
What hormones does the thyroid gland make?
thyroxine (T4) and triiodothyronine (T3) and calcitronin
How is the thyroid visualized? to check for enlargement or nodules
Pt given radioactive iodine which is taken up by the thyroid
Describe the cellular structure of thyroid tissue
Follicular cells surround colloid
Colloid contains thyroglobulin
Rich blood supply
Parafollicular cells that make calcitonin
Briefly describe how thyroid hormone is synthesized
- Na/I transporter takes up iodine
- pendrin proves iodine into colloid/follicular lumen
- Tg made in follicualr cell and transported out via secretory vesicles
- Tg iodinated in colloid by thyroid peroxidase
- endocytosis of iodinated Tg into follicular cell
- cleavage of I-Tg into T4 and T3
- MIT and DIT recycled
anion exchanger on apical side of follicular cell that move I+ into the colloid /follicular lumen
Pendrin
made in the follicular cell and packaged into vesicles
thyroglobulin
coats luminal side of secretory vesicles containing Tg
thyroid peroxidase
mediates binding of iodine to select tyrosine residues of thyroglobulin
thyroid peroxidase
Internal rearrangement of 2 iodinated tyrosyl residues forms
iodothyronine (via thyroid peroxidase)
Only ___% of the tyrosine residues on Tg are iodinated
~20%
is endocytosed by follicular cell
Iodothyronine
hydrolysis of iodothyronine forms …
T3 and T4
What is the first thing to happen ti T3 and T4 once they enter the blood stream?
immediately bound by plasma proteins
What are the plasma binding proteins for T3 and T4?
Thyroxine-binding globulin (TBG) > albumin and transthyretin (TTR)
_____ will increase the amount if TBG in blood. How will the inc in TBG affect the levels of free T4?
Pregnancy, hepatitis, heroine abuse
will increase TOTAL T4 but free T4 will not change
Enzyme that metabolizes T4 to T3? T4 to rT3?
T4 to T3: 5’/3’ mondeoiodinase
T4 to rT3: 5/3 mondeoiodinase
difference between T3 and rT3?
rT3 is inactive because 2 Iodides are on the outer ring, and there needs to be 2 iodides on the inner ring for activity
T0 aka
thyronine
Enzymes that metabolize T3 –> T2 –> T1–>T0
5’/3’ mondeoiodinase
5/3 mondeoiodinase
is primarily responsible for forming the T3 in the blood
type 1 5’/3’ mondeoiodinase
is primarily responsible for forming the T3 that regulates the production of TH
type 2 5’/3’ mondeoiodinase
Where is type 1 and 2 5’/3’ mondeoiodinase found?
1 = liver, kidney and thyroid (forms T3 in blood) 2= pituitary, CNS, placenta (forms T3 that regulates TH production)
Describe the mechanism of TH when it reaches target cells
- transported into the cell
- Bind THR (thyroid hormone receptor)
- THR dimerizes with RXR (retinoid X receptor)
- Activate transcription (sometimes will inhibit transcription)
Thyroid hormone that acts like_____ hormone because it binds to an intracell/nuclear receptor
steroid
effect of hyper- and hypo- thyroidism on…
basal metabolic
inc - dec
effect of hyper- and hypo- thyroidism on…
carb metabolism
inc - dec
effect of hyper- and hypo- thyroidism on…
protein metabolism
inc - dec
effect of hyper- and hypo- thyroidism on…
lipid metabolism
inc - dec
effect of hyper- and hypo- thyroidism on…
thermogenesis
inc - dec
effect of hyper- and hypo- thyroidism on…
blood glc levels
no change - no change
effect of hyper- and hypo- thyroidism on…
serum cholesterol
dec - inc
Futile cycle with hyperthyroidism
breakdown of lipids for carb generation in liver + protein metabolism to supply carb in liver opposes use of aa and glycerol from breakdown of protein an lipids for carbs in liver
*breaking down something just to re-build it in the same place (all in liver)
describe the negative feedback regulation of TH production
inc T4 and T3 inhibits the release of TSH…
- Indirectly by decreasing the amount of thyrotrophin RH receptors on thyrotrophs
- Directly by downregulating the transcription of TSH (in thyrotrophs)
Is there positive feedback for TH production? explain
yes,
Decrease in T3 and T4 activate the release of TSH by… (reverse of neg reg)
- Indirectly by increasing the amount of thyrotrophin RH receptors on thyrotrophs
- Directly by upregulating the transcription of TSH (in thyrotrophs)
In __________ of hypothalamus, _______ RH is made → to ________ → activates ______ production in thyrotrophs → ________ activates T3 and T4 production
In arcuate nucleus of hypothalamus, thyrotrophin RH is made → to anterior pituitary → activates TSH production in thyrotrophs → TSH activates T3 and T4 production
TSH exerts ________ effects on thyroid cells
growth factor
TSH aka thyrotrophin
also increases the activity of every step in production of TH…
•TSH increases activity of the Na/I symporter
•Stimulates the iodination of Tg in the colloid/follicular lumen
•Stimulates the conjugation of Iodinated tyr to make T3 and T4
•Increases the endocytosis of iodinated Tg
•Stimulates the proteolysis of iodinated Tg in lysoendosomes
•Increases the secretion of T4 and T3
