Mehl. Pregnancy and thyroid 03-03 (1) Flashcards
for pregnancy, what arrows?
no change TSH,
no change free T4,
incr. total T4 for women who have no thyroid symptom.
estrogen effects on TBG?
increase production by liver
function of TBG?
TBG is the protein carrier molecule for thyroid hormone in the blood.
NBME Q asks for which hormone causes the TBG in pregnancy?
answer = estrogen, not progesterone.
active form of thyroid hormone?
Free T4 is the physiologically active form of thyroid hormone.
T4 protein-bound to TBG (99%) has minimal effect. total T4 ,,formule”?
Free T4 + TBG-bound T4 = total T4.
TBG will mop up free T4, causing …. what T4 levels and TSH?
free T4 to transiently decrease and TSH to rise (less negative feedback).
This rise in TSH will stimulate more production of T4 by the thyroid gland, making total T4 go up. The absolute amount of free T4 will increase back to normal, thereby suppressing TSH back to normal. But the total amount of T4 is now increased – i.e., free T4 is normal again, but TBG-bound T4 is higher.
T3 levels?
normal
T3 is normal because …..?
free T4 is normal.
- T3 is normal because free T4 is normal.
.
Free T4 is peripherally converted to T3. I’ve never seen anything about “free T3” on NBME material and I wouldn’t worry about it.
A student might say, “Wait, but why are you giving the above bold arrows if you just gave me all sorts of transient changes in the arrows based on TBG?.” It’s because the bold arrows are what the USMLE wants. Pregnant women who are euthyroid will have normal free T4 and increased total T4, and their TSH will be normal. The changes due to TBG rising are likely synchronous and slow enough that the patient’s TSH and free T4 stay within reference ranges.
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Postpartum (silent) thyroiditis can result in …… post labor?
Postpartum (silent) thyroiditis can result in either hypo- or hyperthyroidism following parturition
Postpartum (silent) thyroiditis can result in either hypo- or hyperthyroidism following parturition. These arrows are unrelated to the aforementioned ones.
The highest yield point you need to know is that 131I uptake into the thyroid gland is low, even if the patient is hyperthyroid. This is the same for deQuervain and drug-induced thyroiditis, where uptake is always low. This is because with thyroiditis conditions, there is merely increased spacing between the cells of the thyroid gland due to inflammation, allowing thyroid hormone to leak out into the blood. The gland itself is not excessively producing thyroid hormone. Then we have negative feedback causing low TSH, and in turn less stimulation of the thyroid gland, which is why uptake is low.
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postpartum thyroiditis, uptake?
the highest yield point you need to know is that 131I uptake into the thyroid gland is low, even if the patient is hyperthyroid.
If USMLE asks you about levothyroxine dosing during pregnancy, the answer is ??
increase dose by 50%
what drug avoid in first trimester?
Avoid methimazole in first trimester (teratogenic).
if thyroid storm in pregnancy, what drug???
PTU is the answer for thyroid storm during pregnancy, even though longer-term use in 2nd and 3rd trimesters isn’t considered ideal because of hepatic toxicity risk.
