Thyroid function Flashcards
1
Q
The thyroid gland secretes
A
T3 -active form - binds to receptors and generates a response
- T4 (pro hormone)- 4 iodine residues, cleaved by peripheral deiodinases to yield active T3
2
Q
T3 stands for
A
tri-iodothyronine
3
Q
para-follicular (C) cells are involved in
A
- Ca homeostasis
- minority of cells
- secrete calcitonin to regulate serum Ca levels
- inhibition of osteoclasts to prevent bone resorption
4
Q
iodine is high in which foods?
A
- sea weed
- fish
- dairy products
- vegans are deficient
5
Q
transport of iodine into the follicular cells
A
- RLS in T hormone biosynthesis
- active transport, against concentration gradient
- mediated by NIS (Na iodide symporter) , secondary AT (via ATPase)
- located on BLM
6
Q
iodide enters the follicular lumen via
A
- Pendrin transporter (anion)
- apical membrane
- diffusion
7
Q
TPO is involved in?
A
- oxidation of iodide to iodine to facilitate the iodination of Tg tyrosine residues
- coupling of iodotyrosines to form thyroid hormone
(MT + DIT)= T3
8
Q
Tg
A
- serves as a scaffold for T synthesis + storage
- 132 Ty residues - store for iodine
9
Q
final steps of TH synthesis
A
- iodinated Tg stored in follicular lumen as colloid
- colloid endocytosed (pinocytosis) from lumen into vesicles through AM
- fusion of vesicles with lysosome
- Tg digested by proteolytic enzymes to release T3/4
- secreted into bloodstream via MCT8 on BLM
10
Q
control of TH synthesis
A
- HPT axis
- TSH from ant pit gland
- negative feedback
11
Q
effects of TSH
A
- binds to TSHr on BLM of follicular cells
- cAMP pathway/ CREB/ T of genes
- localisation of NIS / pendrin in their membranes
- increase expression of NIS/ TPO enzyme and Tg
- endocytosis of colloid into cell
12
Q
T3/T4 binding proteins
A
- both hydrophobic
- require transport proteins in blood
- 70% - TBG
- 20%- transthyretin
- 15%- albumin
- free unbound H enters and binds to Target cells
13
Q
further levels of hormone regulation
A
- presence of transporters MCT8, take up T3/4 into tissues
- T4-T3 vis deiodinases
- 20% of T3 produced by gland
- HPT axis - negative feedback
- 80% generated by DeI of T4 in periphery
- TH rs
14
Q
D1
D2
D3
A
inner/outer ring - activate/inactivate TH, secondary activator of T3
outer ring- main T3 activator
inner ring - inactivates T3/4
15
Q
peripheral effects of T3 include
A
- increased carb metabolism (glycogenolysis)
- increased fat metabolism
- mobilization of lipids/ FA
- reduced circulating cholesterol
- regulates HR, contractility
