Optimal fasting and draw times for thyroid blood tests

[ZenBear]

Hi,

Just wanted some clarification on the best times to fast to and from and when to get bloods drawn for best/most helpful thyroid results.

So my medichecks says the best time to have bloods drawn is between 8AM-10AM (I’m having loads checked including cortisol so this makes sense). However I’ve also been advised to get bloods drawn no later than 9AM for an accurate/helpful thyroid reading. BUT...this is also to be a fasting blood test and medichecks says to fast between 8-10 hours before bloods. Does this mean that (my bloods are at 9:15AM) it’s best to have an extremely late dinner/snack around midnight? This will mean I’ll have fasted for around 9hours? (Which coincides with the 8-10 hour fasting rule for medichecks). What have others done and what’s best for a good thyroid result? The timing seems a little strange to me.

Thanks.

[greygoose]

I don't think there's any precise time needed to fast. I think most people just have their evening meal at the usual time and leave breakfast until after the blood draw.

Before nine is best. Not because the results will be more accurate, but because the TSH is highest before 9 am. Then it starts to drop throughout the day. As doctors tend to only look at the TSH, if you want a diagnosis or and increase in dose, it's best to get your TSH as high as possible.

[ZenBear in reply to greygoose]

Yes that’s what I meant when I said ‘accurate/helpful’ result. Thank you for clarifying. 🙂 I just wonder if medichecks specifies those specific fasting hours then for some of the others tests? I’m having the ultimate performance test done so there’s an extensive range of bloods, including cortisol which has to be measured at a specific time of day...If it’s not for thyroid then maybe it’s something else that requires those strange fasting specifications.

***Special Instructions.

Ultimate Performance Blood Test: This test must be taken between 8am and 10am. If you are tracking your results over time then it is vital that you always have your blood taken at the same time. Fast 8 - 10 hrs prior to sample being taken. Active B12 levels can be affected by use of vitamin B12 supplements. If you are taking an oral supplement then we recommend not taking it for one week prior to the test to ensure an accurate measure of your B12 levels. If vitamin B12 is being prescribed to you by your doctor then you should discuss with them how long to omit B12 supplements for.***

[greygoose in reply to ZenBear]

Hmmm… I'm afraid I don't know.

[SeasideSusie Remembering in reply to ZenBear]

Medichecks assured me they would be including information about leaving off Biotin (or B Complex containing Biotin) in their instructions because it can affect results, it would appear that they're not doing so.

As for the fasting time, I would take that as that being the minimum time and I wouldn't change the time of my evening meal the night before. However, you might want to clarify that with Medichecks and perhaps let us know so we can pass the information on if it crops up again.

[ZenBear in reply to SeasideSusie]

Ok, so just called medichecks to clarify that info and I was advised that fasting for less than 8hours or more than 10hours can affect the accuracy of the Liver and Kidney function tests, so that’s why they advise those hours to get the truest view of liver and kidney health. He said 10 and a half hours as a maximum would be okay, but longer than that won’t give a ‘true’ result.

So he said to eat something small around midnight if that fits in with the fasting and blood draw guidelines, but doesn’t have to be anything heavy, just something that will register in the body, so more than just a drink or fruit, but less than a whole meal.

Never knew that either, because usually with fasting bloods, I fast for maybe 15hours, sometimes more depending on what time the blood test is. Good to know.

[SeasideSusie Remembering in reply to ZenBear]

just called medichecks to clarify that info and I was advised that fasting for less than 8hours or more than 10hours can affect the accuracy of the Liver and Kidney function tests, so that’s why they advise those hours to get the truest view of liver and kidney health. He said 10 and a half hours as a maximum would be okay, but longer than that won’t give a ‘true’ result.

Well, that's very enlightening. I have regular liver function tests and have asked at my surgery in the past about fasting and have been told it's not necessary. I always do fast when it's an early morning blood draw but often for my liver test it's an afternoon blood draw so I don't fast then.

I've also recently had to have a kidney test to see if a new medication has affected them and again, no warning about this and it was an afternoon test.

Thank you for this information ZenBear, certainly something I will have to remember for the future with my regular GP tests.

[ZenBear in reply to SeasideSusie]

Sometimes it’s hard to know fact from fiction, but as a general rule I trust medichecks/other private organisations information more than I trust what GPs tell me. Even from GP to GP in the same practice, what they say can vary so widely, it’s so difficult to know the right thing to do. But I’ll definitely be following their advice also.

[ZenBear in reply to SeasideSusie]

Just had bloods taken. My appointment was at 9:15 but she didn’t end up drawing the bloods until 9:35. Will this still be okay?

[SeasideSusie Remembering in reply to ZenBear]

9am at the latest is best but there may not be too much of a difference. You can see how the daily rhythm of TSH fluctuates in the first graph in this post

healthunlocked.com/thyroidu...

[humanbean]

If you take iron supplements in any form it is a good idea to stop taking them for a week before testing.