Why does T4 go down? (Sorry I put it on posts by mistake)

[sheenah]

Ultimately, what causes T4 to go down in the system?

[Aurealis]

I'm not sure I understand your question - do you mean that your measured T4 is less than it used to be? I suppose this would be due to your thyroid producing less than previously, or to your Levothyroxine dose being reduced.

[eeng]

T4 is converted to T3 in your system which is in turn used by the cells in your body (this is where my understanding stops!) so you need to either produce or take enough T4 each day for your body's needs (unless you are taking T3 of course).