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NBME 17 Answers

nbme17/Block 2/Question#13 (18.9 difficulty score)
A 28-year-old woman at 18 weeks' gestation ...
Free T4🔍,📺

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submitted by cassdawg(1381),

In agreement with the other post: (see FA2020 p331)

You would want to check FREE T4 because pregnancy increases Thyroid binding globulin. It is possible she might have increased overall T4, but NOT have hyperthyroidism because the free T4 is normal (i.e. her increases amount of thyroid binding globulin has bound more T4, and since our bodies respond to the concentration of free T4 only, the hypothalamus should ensure that the free T4 is kept constant; this would appear as increased overall T4)

Another way of thinking of this:

  • Overall T4 = bound T4 + free T4

If we increase bound T4 and keep free T4 the same, we would still increase overall T4. Thus to know if she truly has hyperthyroidism we must look at free T4 concentration.

lpp06  Does Overall T4 stay the same because TBG+T4 can last longer in circulation? Cause I always struggle thinking that free T4 is low because its being bound by the extra TBG +  
covid_19  I'm not sure if I'm interpreting your question correctly, but I thought overall T4 increases in pregnancy? The way I think of it is that early in pregnancy, β-HCG (acting like TSH) → ↑ free T4 → ↓ TSH via (-) feedback → TSH back to nml as ↓β-HCG during pregnancy (i.e. gestational thyrotoxicosis). If the mom really has hyperthyroidism, then free T4 remains high and TSH low even as β-HCG ↓ and ↑TBG. Later in the pregnancy, estrogen → ↑TBG → ↓ free T4 → TSH release → ↑ free T4 to regain equilibrium. (bound T4: free T4) 2:2 (nml) → 3:1 → 3:3 (new nml where there's a relative increase in free T4 AND overall T4). What are your thoughts on this? +  
topgunber  Overall t4 increases, but when t4 is bound to thyroid binding globulin it is not active, therefore someone would not show characteristics of hyperthyroidism if they had high thyroid binding globulin and high total t4. Basically, free t4 is the actual amount of thyroid hormone that can cause a physiologic effect. As the first comment says, total thyroid hormone and thyroid binding globulin are increased in pregnancy. Thus to check for hyperthyroid we are concerned only with free t4 (if it is low then the patient would be hypothyroid) +  

submitted by kbizzitt(6),

Estrogen increases Thyroid Binding Globulin (TBG). (FA2020 - 331).

TBG binds free T4 --> increased total T4 and decreases free T4.

TSH rises to compensate for decreased free T4 to return to normal states.

Therefore in pregnancy total T4 is increased but free T4 is normal. If T4 were to be increased --> hyperthyroidism.