thyroid blood test timing and fasting - Thyroid UK

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thyroid blood test timing and fasting

luthien profile image

Thanks to all those whom have been following my posts and offering support / advice. Sorry you've had to put up with all my questions.

So my GP has finally booked a blood test for repeat TSH (it was high in December) admitting they didn't test T4, she has also requested B12 and folate, plus "antibodies" but not said which, frustratingly. She said they don't test Vitamin D (they being the NHS), but over the counter vitamin D will help. I have ferritin tested with my annual bloods and that's usually okay. I will ask at my actual blood test what they're testing that day so I can make a note.

I have ordered a test from Thriva which is the Advanced Iron Profile to cover any they'll have missed; it covers everything all you guys have mentioned.

My blood test is at 11:50 on Friday 19th Feb, I asked about fasting as it's almost afternoon, they said I don't need to. However I think I should, would it be wise to fast anyway? How long before with it being that late in the morning? I don't want to end up fainting when there? Usually all my fasting ones are booked between 8:30 and 9:30 this one is odd. I've stopped all my vitamins I usually take from Friday just gone so that should be fine in preparation. I'm not on any prescribed medication.

15 Replies
SlowDragon profile image

If your looking to get diagnosed and needing TSH as high as need early morning blood test

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”

TSH levels showed a statistically significant decline postprandially in comparison to fasting values. This may have clinical implications in the diagnosis and management of hypothyroidism, especially SCH.

Looking at other posts, if looking TTC then TSH must be under 2.5

Obviously you need to test vitamin D

How much vitamin D are you currently taking

How long on this dose?

Vitamin D NHS postal kit

Test twice year, aiming for vitamin D at least around 80nmol and around 100nmol maybe better

What other vitamin supplements are you currently taking

luthien profile image
luthien in reply to SlowDragon

Thanks for your reply, I can call to request a test in the morning but that'll mean it could be weeks waiting, and although I do have levothyroxine on prescription I have been told not to even start to take any until after the GP consultation after the blood test. So we could be looking at a month from now.

Below is what I usually take, not in any order just a list from the vitamin packs:

Vitamin D2 50ug

Thyroid support - advised by GP in December 2020

L-Tyrosine 300mg

Kelp (0.1 to 0.05 % Iodine) 100mg

Zinc (as oxide) 8mg

Manganese (amino acid chelate) 2mg

Selenium (amino acid chelate) 200ug

Copper (as oxide) 0.2mg

Iodine (amino acid chelate) 150ug

Vitamin B12 (as methylcobalamin) 100ug

Molybdenium (amino acid chelate) 50ug

Sleep aid - Advised by GP in November 2018

Tryptophan 100mg

5-HTP 90mg

Vitamin B5 2.4mg

Vitamin B9 0.2mg

Vitamin B7 0.05mg

Other natural herbs and extracts - Montmorency Cherry Extract, Chamomile Powder, Ashwagandha root extract

I will look at the postal kit you've put up the link for.

I'm not sure there's much in the above vitamins or the right combinations to even have any affect, so happy not to take them if they aren't needed / won't work / better things out there

SlowDragon profile image
SlowDragonAdministrator in reply to luthien

You don’t want to take something called thyroid support

Are you vegetarian or vegan?

Generally never supplement iodine unless tested as deficient

SeasideSusie has details of correct iodine test

Most thyroid issues are autoimmune thyroid disease also called Hashimoto’s

Kelp and iodine often make Hashimoto’s considerably worse

Looking at your forum name ....wondering if you have dark skin. BAME population in U.K. often extremely vitamin D deficient

Vitamin D 50ug is only 1000iu.

If deficient that won’t be high enough dose

If your last TSH was 13 was this after starting on “Thyroid support” ? Kelp and iodine can make results go haywire

luthien profile image
luthien in reply to SlowDragon

I eat meat, so normal but due to a caesin sensitivity I do not have any dairy hence the suggestion of Iodine supplementation. The GP hasn't yet seemed to realise about Hashimoto's.

I have tanned skin, so yeah I would need a higher dose of Vitamin D

The TSH of 14.2 was 1st December 2020 with my GP where he said take the Thyroid Support. He had ordered a Levothyroxine prescription but it got "lost".

The TSH 13.81 was 13th Jan 2021 with Thriva. This is shortly before I contact my GP to chase up the prescription, ask you guys about full tests, forum mention of Hashimotos (supported by my endometriosis specialist) and then ask the GP to do full tests. She said keep the supplements up, take Vitamin D OTC and she will book in tests (TSH, T4, vit B12, folate and antibody).

I hope that makes sense

SlowDragon profile image
SlowDragonAdministrator in reply to luthien

Anyone with TSH over 10 is clearly hypothyroid and levothyroxine should be started

See both flow charts

Dairy/Casein intolerance and Hashimoto’s often go together

Often dairy intolerance improves after year or two on strictly gluten free diet

SeasideSusie profile image
SeasideSusieAdministrator in reply to luthien


I'm amazed that a GP has recommended a thyroid support supplement. The amount iodine that contains - 150mcg - is the daily recommended amount for adults. Then let's say the amount of iodine from the kelp is 0.1% of 100mg = another 100mcg, so a possible total of 250mcg iodine. Then add in what iodine one gets from a normal diet from milk, yogurt, cod, haddock, scampi, etc. Then once you start taking Levothyroxine that provides another approx 62mcg iodine per 100mcg Levo. So all in all not a good idea to supplement iodine unless you have tested and know you are deficient.

Best iodine test is the non-loading urine test provided by Genova Diagnostics, this test establishes existing iodine levels and can also be used for monitoring when supplementing. Details of how to order:

Your sleep aid contains B7 - biotin, this must be left off for 7 days before any blood test, including thyroid.

Ashwagandha is an adaptogen and is known to lower cortisol, it would be prudent, if not already done, to test your cortisol plus DHEA levels with a 24 hour saliva test from Regenerus or Genova Diagnostics.

luthien profile image
luthien in reply to SeasideSusie

Thanks, I shall take a look at all your info.

As mentioned before; I usually stop all vitamins etc for 7 days before any blood test to eliminate false readings :)

Just checking i've got this right :-A private Endo asked GP to test thyroid.

you had a TSH of 13.8 in December . but no fT4 or antibodies were done.

They prescribed 50 mcg Levo ....but then told you not to take it yet , and you are not doing ?

You aren't (knowingly!) suffering any hypothyroid symptoms , it came to their attention because of fertility issues?

This is a repeat NHS blood test for thyroid function including antibodies.

So , you need to be aware that the TSH part of the test will be influenced by the time of day .

It is highest in the middle of the night and falls to its lowest around 2pm, so if you take a test at around midday it will be nearly at it's lowest.

This may not present a problem for you since your TSH is so high over the range, and you have already been diagnosed hypothyroid and prescribed Levo ....

The reason we want to show the highest TSH level on a blood test is to help with getting a diagnosis if TSH is only just over the range, or to help with getting the dose increased.

With regard to fasting.... This doesn't make as much difference to the TSH result as the time of day does, in fact there is not much evidence that if makes much difference at all. But if it does have an effect , it is that eating breakfast may lower the TSH slightly.

What i don't understand , is what they want to do a repeat test for at this point, rather than just telling you to start taking the Levo ,and then re-testing after you've been on 50mcg for 6 weeks, since they have already diagnosed you with hypothyroidism.

luthien profile image
luthien in reply to tattybogle

All correct, sorry it's all been a bit everywhere.

Ah right, so really I should push for a morning test? I shall email / call in a bit.

I'm not sure regarding repeat testing; I think it's because they realise they didn't make the connection between a high TSH and needing the T4 included in the test and the connection between Endometriosis and thyroid issues. So I guess they want to backtrack to make it look like they diagnosed it correctly. I did specifically ask if I should start my levothyroxine, but they very clearly said not until the GP has seen your new set of blood results.

tattybogle profile image
tattybogle in reply to luthien

Not all NHS doctors bother including the fT4 test to be honest, but your over range TSH would usually have triggered the lab to do it, But they don't technically need it to diagnose hypothyroid, they can diagnose that simply by the fact that your TSH has gone over 10.

It is usual to retest out of range thyroid tests after 3 months to rule out a temporary issue.

But they seem to have prescribed Levo just on the 13.8 result , is it possible you have got previous over range TSH result on record ?

Regarding re booking the test . it's most likely that the NHS will not even know /accept that time of day affects TSH result. (it absolutely does , there is no question there, but they are not taught about it, so be prepared to be told " it doesn't matter")

If you are not suffering hypothyroid symptoms and you want to be sure of your untreated results, before you start on Levo then there is no harm in doing the test as it is booked, but you will have to bear in mind that the TSH would have been higher at 9am, The fT4 would not be affected by time of day.

The only danger in doing test at midday is if your TSH was lower enough for them to "un-diagnose" you , which is unlikely to happen since it was 13.8 and top of the range for TSH is around 4/5 and TSH doesn't change by that much over the day.

What time of day was the December blood taken at , can u remember ?

I also notice GP has told you to take a supplement with iodine & kelp in.... this is odd, and i'm sure others will have comment's about the wisdom or not of that. But it seems highly unusual that an ? NHS GP would suggest this.


The thyroid antibodies won't be affected by the time of day, and they use these to confirm if the cause of hypothyroidism is autoimmune (it usually is)

luthien profile image
luthien in reply to tattybogle

Perhaps that's why they're okay with the test being at 11:50 as they may primarily want to focus on T4 and antibodies and assume within two months that TSH on no medication won't have changed?

luthien profile image
luthien in reply to tattybogle

Previous TSH result was way back in 2017 at 5.4 (2.2 to 5.5) so was in range, no further action needed.

I have requested an earlier blood test appointment I shall see what they come back with.

The December blood test was taken at 8:30am. I had to check my calendar!

I'm not sure on the NHS GPs reasoning I think he just picked something. It's difficult to clarify anything currently as all appointments are over the telephone currently so they tend to just tell you what's going on, what's happening and that's that. It's usually not the same doctor for each call, so I've not actually spoken with my GP as there's only certain ones on phone consultations each day.

SeasideSusie profile image


When doing thyroid tests, if TSH is important (as in this is the test result that the GP adjusts dose by) then to avoid a reduction in dose, or to get an increase in dose, or for a diagnosis of hypothyroidism, we need the highest possible TSH. TSH is highest early morning and lowers throughout the day.

You can see from the first graph in this post:

that TSH is highest between midnight and 3am and lowest about 1pm. So you can see why we always suggest testing no later than 9am when the TSH result is important.

Eating can lower TSH, and coffee and other caffeine containing drinks can affect TSH, so we suggest not eating or drinking anything but water before the test. Another reason to have an early appointment.

Doctors don't tend to know about all this, these are patient to patient tips we share and don't discuss with doctors or phlebotomists, they will say it doesn't matter.

So if TSH is an important result for you, then your 11.50am appointment wont do you any favours and you may want to try and get this changed.

If the TSH result is not important, and your GP tends to adjust dose of Levo by FT4 and FT3 results, then don't worry, your appointment time is fine.

She said they don't test Vitamin D (they being the NHS), but over the counter vitamin D will help.

An over the counter D3 supplement will help if you need it. However, besides knowing if you need it, you will also need to know what dose you need to take, and this is where lack of training in nutrients shows with the medical profession. D3 is a fat soluble vitamin and any excess is stored and will lead to toxicity, therefore it is important to know our level before supplementing, then base our dose on the current level and what we want to achieve.

The Vit D Society and Grassroots Health both recommend a level of 100-150nmol/L.

I recommend that you do a private fingerprick blood spot test if your GP wont do it. There is an NHS lab which offers this test to the general public for £29:

Come back with your Vit D result and I will point you in the right direction for the dose of D3 that you need and I will give you details of the important cofactors needed when taking D3 (another thing doctors don't know about).

I have ordered a test from Thriva which is the Advanced Iron Profile to cover any they'll have missed; it covers everything all you guys have mentioned.

When doing iron tests, we need to leave off iron supplements for 7 days and we need to fast for 12 hours before the test (water allowed), so this is another test that is best done early morning.

luthien profile image
luthien in reply to SeasideSusie

Thanks for your reply.

I will request to rebook my blood test for the morning, as what you're saying makes sense to me.

Vitamin D3 - the only thing they asked is if I'm tanned as I'll need more than what I can absorb from being outside. but not more. I shall look at the link you've provided.

Thriva - I may leave it say a few weeks anyway as I'm currently still recovering from my endometriosis excision operation and on my period so may not be the best time to test anyway. I do all my at home tests in the morning and usually all my blood tests are in the morning too, this seems to be an odd one.

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