# Somewhat stupid question....



## teri2280 (Feb 7, 2012)

If someone could explain something to me in layman's terms, that'd be great.

Info I've gathered from research:

Our thyroids make both T3 and T4.

Synthroid/Levothyroxine is T4, while Cytomel is T3, essentially.

T4 is converted to T3 in our bodies, outside of the thyroid.

So..........
My question: Why do we go off the T4 (Synthroid/Levo) and on the T3 (Cytomel) in preparation for RAI??? Wouldn't keeping someone on the Levo have the same result, since it's converted to T3 anyways?


----------



## joplin1975 (Jul 21, 2011)

It has to do with half-life.

The T4 can be stored in your body and then converted to T3 when needed, so it stays in your body longer and, therefore, takes longer to leave the body. Whereas T3 gets used up quickly, so by putting someone on T3 meds, they can minimize the effects of hypo and stop it closer to the RAI date without effecting the results of the RAI.


----------



## teri2280 (Feb 7, 2012)

That makes sense. Didn't realize it was stored before use. Just thought it was an automatic thing. Thanks!


----------

