Question on timing of TFT blood draw and not having breakfast or medication beforehand. Any evidence to help my cause?

[hashihol]

As a kind of add on to my last post, I may be getting a new TFT done tomorrow at my midwife appointment. However, I'm worried they will take the blood draw at around midday (at the time of the appointment) and I know this will show a false low TSH. I'm struggling to find evidence to show them that it makes a difference if I do this test later in the day after breakfast. Last time I tried to arrange a blood test for first thing in the morning the GP told me it doesn't matter what time the blood draw is taken. I want to show them something to say that it does matter but I can't find anything. Does anyone know of any studies or guidelines that explain this? I just looked for NHS advice on Patient Access page and it says 'You don't need to fast before the blood test. And it doesn't matter if you have taken your thyroid medicine just before the blood test', which is quite unhelpful for me!

[Lalatoot]

The difference in TSH is not massive. For those whose GPS dose by TSH alone it might make a difference and mean a reduction in dose so many follow that protocol.My surgery does not do early morning blood tests for thyroid so I always have mine at around 2pm so that I am always comparing like for like. Obviously I don't fast either.

[hashihol in reply to Lalatoot]

Ok, good to know. Thank you.

[Jazzw in reply to hashihol]

Perhaps best not to take your thyroid hormone before the test though, I’d say. In case that point was in any doubt.

[hashihol in reply to Jazzw]

Yes, I'll take it with me and take it as soon as i've been tested!

[Jazzw in reply to hashihol]

Just read your post from yesterday and now completely get why you’re worried. You have a numpty GP. 😩

Fingers crossed. But I’d definitely stick to my guns in insisting on staying on 150mcg in your position. x

[Lalatoot in reply to hashihol]

Forgot to add. Alter the timing of your levo dose the day before so you take it 24 hours before your test. Then take your next dose after your test.

[jgelliss in reply to Lalatoot]

I was told that if you have no choice the way you test is the right way Lalatoot. Consistency is very important.

[SeasideSusie Remembering]

hashihol

It's not a false low TSH as such. TSH has a circadian rhythm and it happens to be that TSH is highest between about midnight and 4am and lowest between 1 and 4pm. See first graph here:

thyroidpatients.ca/2020/07/...

We aways suggest an early blood draw, eg 9am, because that's the highest your TSH will be during the daytime and if you are looking for an increase in dose of Levo or to avoid a reduction then that's the best time to do the test. Of course, none of this would matter if doctors would look at the actual thyroid hormones - FT4 and FT3 which give the true picture of our thyroid status rather than the TSH which is a pituitary hormone.

[SlowDragon Administrator]

TSH daily variation healthunlocked.com/thyroidu...

Testing in morning

researchgate.net/publicatio...

“According to the current TSH reference interval, hypothyroidism was not diagnosed in about 50% of the cases in the afternoon.”

“Further analysis demonstrated inadequate compensation of hypothyroidism, which was defined in 45.5% of the morning samples and in 9% of the afternoon samples”

If delaying taking levothyroxine until after blood test, you likely want to not eat or drink anything apart from water before taking levothyroxine

It may (or may not) make a difference to TSH if fasting ….opinions differ

ncbi.nlm.nih.gov/labs/pmc/a...

ncbi.nlm.nih.gov/pubmed/252...

NHS England Liothyronine guidelines July 2019 clearly state on page 13 that TSH should be between 0.4-1.5 when OPTIMALLY treated with just Levothyroxine

Note that it says test should be in morning BEFORE taking levothyroxine

Also to test vitamin D, folate, B12 and ferritin

sps.nhs.uk/wp-content/uploa...

Graph showing median TSH in healthy population is 1-1.5

web.archive.org/web/2004060...

[Buttercupsareyellow in reply to SlowDragon]

Interesting as when you read the conclusion of the Sviridonova article above they talk about TSH levels being effectively abnormally elevated in the morning and ‘spontaneously returning to normal’ in the afternoon. I worry the medical field would interpret that as a reason to test in the afternoon to get a ‘true result’ in their world view, rather than accepting the morning measure might be more reflective of a level…..

[humanbean in reply to Buttercupsareyellow]

CONCLUSION

The interpretation of TSH levels in patients with SH and in patients taking L-thyroxine

replacement therapy requires considering many different factors, including blood

sampling time. This study has shown that circadian TSH variability can lead to

significant elevation of TSH level in the morning hours with spontaneous normal-

ization in the afternoon.

I wonder why these authors consider the TSH in the afternoon to be more "normal" than the TSH in the morning? It suggests that they just "prefer" the results in the afternoon because it reduces the number of people they need to treat. Getting a TSH test done at midnight is just as "normal" as TSH at noon.

[hashihol]

Thanks so much for your help everyone. This is all brilliant stuff. I've printed off pages to take with me!