Blood test information

[5219]

Can you tell me the rationale for fasting and no medication for 24 hours before a blood test? Because I need to be properly informed when I see my Gp. If my medication was increased to make my tests optimal would my weight go up or down?

[shaws Administrator]

Patients are free to take blood tests as they think fit. We don't demand that they follow a certain routine - only to give advice as to how to prevent a reduction in dose by the GP or Endo.

The rationale for fasting and no medication is that doctors/endocrinologists only take notice of the TSH and probably T4. If the TSH is low they will adjust dose of levo to bring it up to within the range. This we do not want.

TSH is highest early A.M. and then drops throughout the day and food also causes it to drop. Therefore, if you don't want an unnecessary reduction in dose, follow the advice - which endos seem to be unaware of and also the GPs.

TSH is highest when we get up. Food interferes with the uptake and also reduces the TSH.

[SeasideSusie Remembering]

Can you tell me the rationale for fasting and no medication for 24 hours before a blood test?

As mentioned by SlowDragon in reply to your previous post, there are certain patient to patient tips that we don't discuss with doctors or phlebotomists.

When a patient is looking for a diagnosis of hypothyroidism, or an increase in their dose of Levo, or to avoid a reduction, the highest possible TSH is needed. To achieve the highest TSH then the blood draw has to be as early as possible in the morning, see first graph in the picture here

healthunlocked.com/thyroidu...

As you can see, TSH is naturally highest between 12 midnight and 3.30am but we can't get blood drawn then. Then there is a decrease in level until 8am then it rises again till 9am, so 9am is a good time to do the test.

Fasting is useful for two reasons:

1) There may be other tests done at the same time which they don't tell you about which may require fasting, so it's always a good idea to fast before any blood test, obviously only in the morning, no-one would expect anyone to fast all day. Fasting means have your evening meal as normal the night before then nothing but water until after the early morning blood draw.

2) In this thread by Diogenes (Dr John Midgely, scientist, thyroid researcher and advisor to ThyroidUK) he mentions in one of the responses that "TSH falls after eating, but FT4 not affected. Don't know about FT3 but suspect it will behave like FT4." so again, if a high TSH is important then it's sensible to delay eating until after the blood draw.

healthunlocked.com/thyroidu...

As for not taking Levo for 24 hours before the test (it's 8-12 hours for T3 and NDT), the reason for this is if you take your Levo too close to the blood draw then you get a false high FT4, if it's too high then a doctor might use that to lower dose of Levo. Taking Levo longer than 24 hours before blood draw gives a false low FT4.

If my medication was increased to make my tests optimal would my weight go up or down?

Who knows. I'm on my optimal dose of thyroid meds and my results are optimal for me, however, my weight stays stable as long as I eat sensibly. I am a chocoholic, biscuit/cake addict and could stuff myself silly and put on weight and being optimally medicated wouldn't shift it, so I just don't have them in my house.

[5219 in reply to SeasideSusie]

Thank you. Sorry if you are repeating yourself but there is so much to take in and can be confusing at times. I am a chocoholic too and have a very sweet tooth that does help. Like you I don’t have a lot of stuff like that in the house.

[silverfox7]

I was once asked by the nurse why I had fasted on my last test as the hospital always ask and put it on the form/results so I said I once forgot to fast when booked for a glucose test so all my bloods are now fasted so I don't get it wrong again! Got a pat on the back for being thoughtful!!!

[helvella Administrator in reply to silverfox7]

That is part of my argument in favour of fasting.

Fasting versus eating might have little impact of thyroid tests, but by fasting every time you know even that potential small impact is consistent. Rather than having to fast, or not fast, depending on other tests.

They should tell us of any test requests which have a significant impact from eating, but by fasting we avoid even minor impacts.

[silverfox7 in reply to helvella]

Totally agree helvella but I thought saying I had once mucked it up might have appealed to her more!!!!