Thyroid Gland Flashcards
(34 cards)
Thyr fx
Maint metab level optimal for normal fx
Ths stim o2 consimption by cells
Reg lipid and cho metab and are essential for normal g and maturation
Thyr dev
From invag of pharynx
2 lobes connected hy isthmus
Good blood supply from cran carotid a and rich symp n supply
Drain via caud thyr vv to int jug v
Capsule
Surround thyr gland
Sends smaller ct pieces into gland to surround follicles
Patches of lymphocytss sometimes seen
Follicle structure
Simple epi surrounding a gel like material called colloid
Principle cells
Principle cells secr Ths and thyroglob
Ths stored Ec-ly as thyroglob
Incr gland act cause incr principle cell thickness so decr colloid stored
Squamous rel inact columnar v act
Para Follicular cells
In epi lining or in grps in ct between follicles
Large clear cells dont stain
Secr calcitonin
Th struc
Derived from tyrosine and cov bound to I2 (no and pos determine bio act)
T4 2 tyr 4 I2
T3 2 tyr 3I2
Rev T3 2 tyr 3i2
Thyroglob fx
Used to sunth T3&4 by iodination and condensation
T3&4 bound to thyroglob until secr
When secr colloid is ingested by principle cells, peptide bonds hydrolysed and free T2&4 is discharged into capsule
Principle cell fx
Collect and transport I2
Synth thyroglob
Remove Ths from thyroglob and secr into circ
I2 processing
Ingestd I converted to I2 and abs
Efficient uptake active against conc grad
I2taken up from blood by Prin cells by I2 traps on outer plasma mem
Inside I2 transported into follicle lumen with thyroglob and oxidised to I
Thyroid peroxidase
Ox I2 to I and bind to tyr ag prin. cell lumen surface
Iodination of tyr (oxidative) and synth of T3&4 from 2 iodotyrosines (coupling)
T4:3 ratio
5:1
Th storage
Stored as thyroglob colloid i. Follicles via T perpxidase
Cont iodinated tyr and thyronine detivatives
Th release
2% of thyronine daily
TSH secr by ant pit cause thyroglob to be endocytosed by prin cells and proteolysed releasing iodinated derivs
Th synth stim
Bind TSH to prin cell recs
Stim I transporter, T peroxidase an thyroglob synth
Magnitude of signal set rate of colloid endocyt
Dog ratios and half life
T4 free in plasma 0.1-0.3% 10-16hr
T3 1-3% 5-6hr
Man half life
T4 7d
T3 24hr
Th binding
Free Th bind to spec high aff thyroxine binding glob in plasma (and plasma pros and albumen)
Th poorly soluble so 99% bound to TBG
Bound h is reserve and buffers loss from plasma so less Th lost in urine
Only free can diffuse into target cells
Exogenous Th
Exogenous T4 slow effect and long lasting as more bound
Exogenous T3 more rapid and harder to control as less bound
Th conversion
T3 5-10x active as t4
Most T4 deiodinated to t3 by 5 monodeiodinase in target tissue
Enz in liver release much T3 back into plasma
Conversion failure lead to hypothyroid state
Other deiodinating enz prod inact metabolites eg rev T3 that blocks recs
Large change in secr or admin rate
Cause little immediate effect
IC rec and IC action
Enter by act transport
Rec mostly T3 spec
Normal plasma lecels 50% rec occupation - most by T3
Similar to steroid h recs
Effects on tissue dec and metab take 1-2 d to appear
Generally stim activity in most tissues
Hypot ant pit thyr axis and acute results
Most stimuli esp cold impunge on thyrotropin RH neurones to cause secr
TRH act on thyrotrophs cause TSH secr
TSH bind rec
Secr incr within 2 mins
Acute- incr blood flow and I uptake from plasma and synth iodination and proteolysis of thyroglob
Reg by -ve feedback
Metab effects
Incr metab heat prod except gonads and brain
Incr bmr food intake and gluc uptake
Stim fat mobilisation and cho metab
Potentiate metab effects of catechol glucagoncortisol and GH
Chol level inversely correlated
