Page 1 of 1: physio query regardin T4 control

drviks · Credits: 1146 My Scrapbook

hi all,

there's this concept i couldnt comprehend, would be very grateful if someone could sort it out for me:

On one hand Kaplan states that "as long as circulating free T4 remains normal, changes in circulating T3 have minimal effects on TSH secretion"

and then in the endocrino q's , the effect of administration of large doses of T3 apparently decrease TSH, and subsequently T4.

i found these contradictory, which way do we go?

thx,

Siyaa · Credits: 8577 My Scrapbook My Reading List 1 Books

Dr Viks i am giving you one adaptation of Harrison :-
The major negative FEEDBACK action of thyroid hormones is at pituaitary level.modified by binding of hormones to TRH( Thyrotropin releasing Hormone) in the nucleus of Thyrotropic cellsresulting in reduced expression of genes for alpha and beta subunits of TSH.
tHE PRINIPAL ARBITER OF OF tHYROID HORMONE ACTION IS WITHIN THE PITUITARY IS T3 WHICH IS GENERATED LOCALLY FROM INTRAPITUITARY T4 AND SERUM.
To what extent T4 is effective in pituiatry is uncertain , but other factors mosify the secretion.

Siyaa · Credits: 8577 My Scrapbook My Reading List 1 Books

This simply means that the it is serum T3 a principla FEEDBACK provider.
In my view ( i Hope it is nt chhota muh badee baat)
the following line is misprinted.

On one hand Kaplan states that "as long as circulating free T4 remains normal, changes in circulating T3 have minimal effects on TSH secretion

It shud be as follows

"as long as circulating free T3 remains normal, changes in circulating T4 have minimal effects on TSH secretion.

aNYWAYS SUGGESTIONS ARE WELCOME AND I SHALL DO SOME RESEARCH FROM HIGHER BOOKS AS WELL

drviks · Credits: 1146 My Scrapbook

thanks a lot siyaa, even i'll look into it...

though kap does say that free T4 diffuses into the pituitary, converts to T3 which decreases TSH sensitivity to TRH..
confusing..
just hope they dont ASK this on our exams!.

thanks again

drviks · Credits: 1146 My Scrapbook

i googled it , for lack of better things to do, and came up with this:
If the laboratories notice a raised TSH then automatically T4 is measured. If the laboratories notice a suppressed TSH then T4 and T3 are measured. Why both? Because there are some patients whose thyroid oversecrete only T3 called T3 Toxicosis and both hormones need to be measured to detect this form of thyrotoxicosis; it is usually seen in those who have had previously a thyroidectomy or radioactive iodine for thyrotoxicosis in the past.

the article does imply that T3 has a negative FEEDBACK on TSH secretion, further comments welcome.

neo · Credits: 904 My Scrapbook

well dr viks its T4 which determines the relese of tsh from pituitary ..... T4 gets converted to T3 locally in the pituitary and then acts on the pituitary gland ...... though t3 ACTS DIRECTLY ON THE PITUITARY ITS t4 WHICH ACTUALLY gets converted to t3 ......
secondly if there si low T4 it leds to increased tsh which canbe seen in the case of thyroid goitre because of iodine deficiencey\
were t3 levels are more or less normal

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