vikramjit
Forum Newbie
Topics: 4
Posts: 15
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hi all,
there's this concept i couldnt really 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,
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| study_ing
Forum Fanatic
Topics: 180
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i dont know a proper source for this but what i thought was..
in the adrenal gland, administration of dexa or cortisol exogenously will lead to decreased endogenous cortisol
similar is the thyroid gland. the exogenous t3 wud represent les sneed of the body for t4 which eventually exerts its efects via t3.
maybe transiently and at a local level there is a high t4 as less is converted to t3.
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| tolito
Forum Fanatic
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addendum (edited)
t4 is the one that determines tsh level and not t3. that is why it is possible to have normal levels of t3 and low levels of t4 which would result in incr TSH production. this scenario is seen in the formation of a goitre from iodine defficiency where the thyroid preferentially produces t3 over t4. the thyrotrophs in the ant pit get negative feedback from t4. it is the t4 in them that converts to t3 and this causes a negative feedback
i hope it is clearer
Edited by tolito on 06/11/06 - 03:49 PM. Reason: typo
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