NBME 21 Free T4 (NBME Answers)

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submitted by ∗assoplasty(108)

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I think the concept they’re testing is the increased TBG levels in pregnancy, and not just hyperthyroidism in general.

When screening for hypo/hyperthyroidism, TSH levels are ALWAYS preferentially checked because they are more sensitive to minute differences in T3/T4. Often times TSH levels can demonstrate a change even when T3/T4 levels are in the subclinical range. The only exception to this would be in pregnancy (and I guess maybe liver failure? I doubt they would ask this though). High estrogen levels prevents the liver from breaking down TBG, leading to increased TBG levels in the serum. This binds to free T4, decreasing the amount of available free T4. As a compensatory mechanism, TSH levels are transiently increased and the RATE of T4 production is increased to replenish baseline free T4 levels. However the TOTAL amount of T4 is increased.

The question is asking how to confirm hyperthyroidism in a pregnant woman --> you need to check FREE T4 levels (because they should be normal due to compensatory response). You cannot check TSH (usually elevated in pregnancy to compensate for increased TBG), and you cannot check total T4 levels (will be increased). You got the answer right either way but I think this is a different reasoning worth considering, because they can ask this concept in other contexts of hyper-estrogenism, and if they listed “TSH” as an answer choice that would be incorrect.

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hungrybox Extremely thorough answer holy shit thank u so much I hope you ACE Step 1 +9

arkmoses great answer assoplasty, I remember goljan talking about this in his endo lecture (dudes a flippin legend holy shit) but it kinda flew over my head! thanks for the break down! +3

d_holles https://step2.medbullets.com/obstetrics/120352/physiologic-changes-in-pregnancy +4

whoissaad you mean total amount of T4 is "not changed"? 2nd para last sentence. +1

ratadecalle @whoissaad, in a normal pregnancy total T4 is increased, but the free T4 will be normal and rest of T4 bound to TBG. If patient is hyperthyroid, total T4 would still be increased but the free T4 would now be increased as well. +3

maxillarythirdmolar To take it a step further, Goljan mentions that there are a myriad of things circulating in the body, often in a 1:2 ratio of free:bound, so in states like this you could acutally see disruption of this ratio as the body maintains its level of free hormone but further increases its level of bound hormone. Goljan also mentions that you'd see the opposite effect in the presence of steroids and nephrotic syndromes. So you could see decreased total T4 but normal free T4 because the bound amounts go down. +2

lovebug Amazing answer! THX +1

an_improved_me Just to add: Pregnancy is not an exception to using TSH in suspected hyperthyroid pregnant patients (not sure in hypothyroid); you would still get a TSH first, and if its unusually low, you would then proceed to measure T4 (free, total), and so on. https://www.uptodate.com/contents/hyperthyroidism-during-pregnancy-clinical-manifestations-diagnosis-and-causes?search=hyperthyroidism%20in%20pregnancy&source=search_result&selectedTitle=2~150&usage_type=default&display_rank=2#H994499 +1