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NBME 20 Answers

nbme20/Block 2/Question#26 (53.5 difficulty score)
A male newborn is found to have a decreased ...
Thyroid hormone-binding globulin deficiencyπŸ”,πŸ“Ί

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submitted by monoloco(141),
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eercaseDd ,ttoal mlaonr feer u(u)ondnb = rdhyTio erdih-nbonnogmi loinglbu nciiedfyec

submitted by sugaplum(409),
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hTsi is aypraptlne liotncgane ydroiht giidnnb ullniogb cicienyfde

Tndhnior"gnexyib-i olunligb eednciyicf β€” bnriTxgynnodeh-ii bnguioll G(BT) enicefcydi si dzacacethirer yb low sumre aotlt 4T but almonr eref 4T dna T;SH het sindioasg is foierdmnc yb iergusanm TBG ehseT nnfstia vahe amnrol hroydit uoinnftc nda do nto rireque tternt.m"ae - ttadnt*a pecou' dinf in FA, beyma it is in ehert reemow?esh

hhsuperhigh  The only thing I can relate to this is FA P331 " TBG in pregnancy, OCP use (estrogen increases TBG) increases total T3/T4", so here is the opposite situation, which TBG decreases, and total T3/T4 decreases... +16  
jawnmeechell  Goljan talks about this (around 33 mins into his endocrine lecture) in relation to increased androgens causing decreased TBG +2  
sarahs  why isnt it maternal antithyroid ABs? +2  
sexymexican888  @sarahs I think if it was antithyroid ABs (hashimotos) TSH would be high +  
srmtn  @sarahs because it would be cretinism (+ physical findings) = thyroid disgenesis, ectopy or hypoplasia. the thyroid of the baby would not be able to make thyroid hormones at all. this baby has decreased T3 and T4 and normal free T4. +  

submitted by step1soon(46),
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heT 3 iman retopisn atht rrcay teh tayomirj g9%t(5)&; of orhiyTd nmoeHors TH() rea hdtxrbiogy-nnniei iglbluon T()B,G nyrnrhtitesat R(T,T or m,in)pablrue adn muiab.ln A mrion trooirnppo of the HsT is bonud no ruesm nl.tprospeoii yerV rerl,ay and in the otxnetc fo i-HTant anbodsiiet in tonmueimua drhyoit ssie,aed nouslimgbnomuli saol amy bdni TH. TH iidgnbn to BTG si ichztacrdeera by olw ypactiac but ghih t;iaidyv hte ecvsoern is retu, ei, hhgi ypcacati ubt olw ativyid, orf TH dningbi ot TRT nda amn.ubil

A icyeefnicd ni Hdi-ibnTgn rsopetni is scdtspuee hwen lyoarmnbla wol eumsr tatlo TH srnettinonccao ear ueenodntcre in clllynciia toryehdui etsscujb in teh rcpeesen fo noraml eurms yhtoirnrtpo (,ei aildisumhott-tgiynr remnhoo )T[H]S. Moer lpceiacf,ylis owl BGT is sdeggtsue seeubca hits nrpeito cresria het ymtaijro of the emsru .TH

eeSlvar ssaett fo idyifencec of stih reotpin avhe eben cdiseerbd ttah aer reehit tireiendh or ecdu.qari ydihoTr untcfnoi stste ()TTFs in intpates whti BTG cidifyneec wosh laronm TSH dan eefr T,4 but owl tlato T4 dna, lisaclyaoocn, olw olatt T3 urmes onsitecanortc.n eTh stmo ttipmnrao ciliancl pacste fo GTB fcieeicnyd aetsts si tcnrgineioo of etshe dessrdoir nad dvaeciona fo usrsneneacy dna nltploieaty arhmulf TH rpltneamcee yharpe.t

submitted by waterloo(89),

If you think about it this way, his free T4 is normal and his TSH is normal. Would any other option allow this to happen?

A step further would be, why is his free serum T4 okay but his T3 and T4 dec. His total T4 must be decreased then, which one of those can cause that while keeping free T4 normal.

submitted by hhsuperhigh(41),
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shTi is hwo ym niarb feradt elwhi I saw ngido ihts qt..iuo.ens. I tandwe to ohcseo BGT diyfce,icen tbu I ktpe inikthng hatt if BTG is i,fecdntie thta maesn ehetr rea sels or no nnibigd sitoprne ni hte l.bodo dnA ohw anc eht feer 4T eb mloran? n'tSldohu efer T4 neiascer fi ereth reew ssle 4T bniigdn reip?ton ...

adong  free T4 wouldn't increase because it would be sensed by the pituitary and TSH would drop until free T4 normalizes +2  

submitted by whossayin(27),
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hyw 'ctna iro"ignoncatifa edtfec in T3 dna "4T eb the nwesra?

sugaplum  I think if it was organification defect you wouldn't have a normal T4 level in the serum. +17  
divya  because there would be an overall decrease in serum T3, T4 and increase in serum TSH levels. +  

submitted by alimd(18),

about 70% of the circulating thyroid is bound to thyroid-binding globulin (tbg). the remainder of the bound protein is attached to thyroxine-binding prealbumin (transthyretin) and albumin. Large variations in tbg do not normally affect the free form. a rare congenital deficiency or excess of tbg drastically alters the bound fraction but because the free fraction is normal, the individuals are all euthyroid.