NBME 22 Answers : Block 2 : #33 = A 29-year-old woman comes to the physician because ...

+2 upvote downvote
submitted by bubbles(45),

Just to be crystal clear (because I've gotten more thyroid axis questions wrong than I should):

T4 --> T3 is possible but T3 --> T4 isn't?

meningitis Exactly. I know there are papers saying there is some conversion of T3 to T4 but I try to keep it simple and think of it as once you break it apart (T4->T3), you cant put it back together. Only thyroglobin etc can put another I on it, so any T3 cant become T4 because you need it to be done in thyroid. +5

angelaq11 I honestly don't know about this, but the way I reasoned this was: she is taking a whole lot of T3, so on top of already having hypothyroidism, she is just making things worse, so TSH is going to be decreased because of feedback inhibition, and hence T4 (Which is the main one produced by the thyroid) is also going to be decreased. I think the high T3 is the exogenous T3. +