Thyroid UK
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FT4 peak vs. baseline level when on levothyroxine after TT

Whenever I go for a checkup, I take my Euthyrox in the morning and the blood sample is drawn 1-2 hours after intake. 

I've recently read that FT4 is at its peak 2hours after levothyroxine intake. This makes me think the result is actually the FT4 at its highest.

Is there a way to know what is  the FT4 baseline level other than taking a blood test without ingesting Euthyrox before ?

Is there a sort of formula or range to calculate the baseline level? 

Could there be big variations between peak and baseline?

Please note I had a Total Thyroidectomy, so Euthyrox is my only source for FT4.

3 Replies


Levothyroxine peaks in the blood stream for up to 6-8 hours after ingestion which is why members are advised to take their Levothyroxine after the blood draw.



Thank you for your answer. Do you have any idea about the difference between the peak value and the baseline ?



No,  but if you compare your previous result with your next test results taking Levothyroxine after your blood draw, you'll be able to see whether there is much fluctuation.


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