Thyroid UK

FT4 Levels

Can anyone please advise if there is an optimal time of day to be tested for FT4 ? My last result was 15.9 ( 12 - 22) and my doc said if it was closer to 12 she would have given me medication...

I understand TSH levels are at their highest first thing on a morning so that's the best time for testing.

I knew nothing about this a month ago and am learning from you amazing people out there.

I just want to get my next blood test right as I feel rubbish most of the time and it seems the GP's tend to go on results rather than how someone feels.

Many thanks in advance 😘

2 Replies


TSH fluctuates throughout the day and is higher in the early hours which is why we recommend early morning testing and fasting (water only) as TSH drops after eating and drinking.

FT4 barely fluctuates so there is no ideal time for a blood test.

See the graph in the link below to see how TSH, FT3 and FT4 fluctuate.


Many thanks Clutter . 😊


You may also like...