Thyroid Gland Flashcards
What is colloid made of?
newly synthesized thyroid hormones attached to thyroglobulin
What is the general reaction to make T4 or T3?
tyrosine + I2 –> MIT (one I) or DIT
DIT + DIT –> T4
DIT + MIT –> T3 (outer ring w/ 1 I)
What is the difference btw T3 and reverse T3?
reverse T3 = inactive, minor product of rxn
2 I atoms on outer ring of rT3
1 I atom on outer ring of T3
What is the major secretory product of the thyroid?
T4
converted to T3 in periphery
What occurs in TH synthesis when the availability of iodide is restricted?
formation of T3 is favored
What is the I trap?
I- is brought into follicle cells from blood w/ 2 Na
I- moves against its electrochemical gradient - powered by Na/K ATPase pump on basolateral membrane
What does pendrin do?
on lumen side of follicle cell
pumps out I- into lumen and Cl into follicle cell
peroxidase simultaneously turns I- to I2 –> Iodide pumped out to lumen to be added to TH
What does TSH stimulate specifically in a follicle cell?
Stimulates it to pinocytose TG bound to T4 and T3
(then is secreted to blood)
What occurs when there are high levels of I-?
inhibits organification and addition of I2 to MIT and DIT
*Wolff-Chaikoff effect
What do perchlorate and thiocynate do?
inhibit the NIS pump –> I- cannot be pumped into follicle cells to synthesize T3 and T4
What does PTU do?
inhibits the peroxidases that turn I- to I2 on the lumenal membrane of follicle cells –> can’t add I to thyroxine
Where is most of iodine in the thyroid gland located?
most stored iodinated as tyrosines of thyroglobulin
8000 ug total
600 ug is T4 and T3
How much new thyroid hormone is secreted a day?
60 ug
enough stored as iodinated TG in colloid to last the body 2-3 months
What does radioactive I123 allow you to see?
activity of thyroid gland
highest in hyperthyroidism
slightly lower in graves (idk why though)
normal = 25%
depressed in hypothyroidism
How is Thyroid hormone found in the blood?
equilibrium btw bound and free T3 and T4
TBG = main binding protein, higher affinity for T4
TTR and albumin minor binding protein
How does the affinity of TBG for T4 and T3 relate to their half-lives?
higher affinity for T4 –> contributes to its longer half life
T4 = 6 days
T3 = 1 day
What does the T3 resin uptake test do?
measures circulating levels of TBG
unbound and labeled T3 added –> binds to unbound TBG spots –> anti T3 Ab/resin absorbs leftover labeled T3
higher the T3 uptake –> lower the TBG
What occurs to TBG and and T3 uptake levels during pregnancy?
increased TBG = increased total T4, FT4 usually normal
decreased T3 uptake
What occurs to T3 uptake levels in hypothyroidism?
decreased T4
decreased T3 uptake
What occurs to TGB during hepatic failure?
decreased blood levels of TBG
transient increase in FT3 and FT4 –> followed by negative feedback and inhibition of sythesis –> levels normalize
How does TSH regulate the thyroid gland?
TSH regulates growth and secretion of thyroid hormones
regulated by TRH form hypothalamus and - feedback of FT3
TSH secretion occurs at steady rate
What type of receptor does TSH bind on follicle cells?
Gs GPCR –> increases adenylyl cyclase –> cAMP –> increases synthesis, secretion, and growth of cells
What occurs to Na/K ATPase in response to T3?
synthesis of Na/K ATPase increased in most tissues
How does T3 affect cells?
binds to thyroid hormone receptor-retinoid X receptor complex in the nucleus –> stimulates synthesis of new proteins
