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NBME 24 Answers

nbme24/Block 3/Question#38 (26.7 difficulty score)
A 55-year-old man comes to the physician ...
Serum free thyroxine (FT4): increased;
Serum free triiodothyronine (FT3): increased;
Thyroidal iodine uptake: decreased
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 +20  upvote downvote
submitted by m-ice(272),
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Tehy itclyxleip etsta ttah het tptanei ash bene itagkn ecssex fo ihs nvxortoihleye e.tincidamo irohxeLvoyent is eth xsuooeegn fmor of 4.T fer,ohreTe rfee T4 usmt eb e.eltvdea T4 is dnterveoc to 3T at sotm eearprhlpi usss,tie so 3T wlli laos be eelad.tev eauceBs teh bdoy sha emro diohryt monroeh athn ne,ddee sesl THS lwil be aem,d dna hte yohtdir lilw be sles eciatv, tgnkia pu SSEL o.dinie

procrastinator  I forgot that T4 is converted to T3 :( +  



Why wouldn't the body down-regulate the conversion of FT4 into FT3? Is that conversion just constitutively activated? Since FT3 is more potent than T4, it would make sense for the body to turn that conversion down...that was my reasoning...obviously not correct, but idk why that wouldn't be the case. Anyone have insight?

maria_danieli  i thought the same... i remember that T3 conversion is somehow regulated but evidently not in this case +  
sars  Peripheral conversion of free T4 to T3 is done by 5-deiodinase. From what I know, only way to decrease this conversion is via b-blockers, glucocorticoids, propylthiouracil, and potassium iodide (lugols). I believe this was mentioned in the sketchy pharm vid as well. +  
sars  Peripheral conversion of free T4 to T3 is done by 5-deiodinase. From what I know, only way to decrease this conversion is via b-blockers, glucocorticoids, propylthiouracil, and potassium iodide (lugols). I believe this was mentioned in the sketchy pharm vid as well. +  
sars  4th blocking agent isn't lugols, its iodinated radiocontrast dye! Sorry for that mistake. +  
cassdawg  "T3 is derived from peripheral conversion of T4... normal plasma T3 levels are obtained in athyreotic patients treated with sufficient T4 to achieve high-normal plasma (F)T4 levels. Administration of T4 to hypothyroid rats to achieve normal plasma T4 levels results in subnormal plasma T3 levels not only because of the lack of T3 secretion but also because of a decreased T3 production by D1 in peripheral tissues, since this enzyme is under positive control of T3 itself". (https://www.ncbi.nlm.nih.gov/books/NBK285545/) i.e. because he is taking SUPRATHERAPEUTIC T4 his T3 is NORMAL. If he was taking NORMAL T4 then he would have DECREASED T3. Pretty sure the normal T4/decreased T3 thing is in another NBME test or UWorld somewhere. +  
cassdawg  ^Sorry to add, her is suprasupratherapeutic so his T3 is elevated. Essentially in a normal hypothyroid patient they are given supratherapeutic T4 to get normal T3 since T3 is the hormone with the action. Apologies if anything is confusing. +  



 +1  upvote downvote
submitted by sweetmed(120),
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Tiakgn eesscx Leeotnxiorvhy will cu,ase rcIn f,T4 rcnI Tf3, ecdr ,THS decr olTydiarh eodnii tkuea.p esExcs yiorihnneolt itknea llwi cuaes icnr ,T3 Drec 4,T redc r,T3 Drce HTS, becesau 4T sgte dtevoernc to T3 adn Tr3 in .rhripeepy