When (if?) to take levo before blood tests please? - Thyroid UK

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When (if?) to take levo before blood tests please?

MaisyGrace profile image

I'd be grateful for some guidance on whether I take my levothyroxine just before a blood test please and if so, how much time beforehand.

Thank you for your help.

Maisy Grace

12 Replies
SlowDragon profile image

Never take Levothyroxine before a blood test

Recommended on here that all thyroid blood tests should ideally be done as early as possible in morning and fasting. Last Levothyroxine dose should be 24 hours prior to test, (taking delayed dose immediately after blood draw). This gives highest TSH, lowest FT4 and most consistent results. (Patient to patient tip, best not mentioned to GP or phlebotomist)

Getting full Thyroid and vitamin testing recommended. Low vitamin D, folate, ferritin and B12 are extremely common

Are you taking any vitamin supplements?

Thank you. Just replied to Shaws below. Yes I am taking Vit D, plus Selenium, but nothing else as I do have a non processed organic diet.

Thanks so much for taking the time to reply and share your thoughts.

Really appreciate it.

MG x

shaws profile image

No MaisyGrace you never take thyroid hormones before a blood test otherwise your results will be skewed and you may not get the increase you may need.

Some hints:-

Always get the very earliest blood test.

It should be a fasting test although you can drink water.

You take thyroid hormones, with one full glass of water, on an empty stomach and allow a gap of 24 hours between dose and test and take it afterwards.

Also ask for B12, Vit D, iron, ferritin and folate to be tested. Everything has to be optimal.

The aim is a TSH of 1 or lower with Free T4 and Free T3 in the upper part of the ranges.

MaisyGrace profile image
MaisyGrace in reply to shaws

Brilliant. So grateful. Thank you.

Yes. this is a re-test as the last one only had TSH and T4 as 'measurements', so I've requested the (almost) full Monty as you recommended above.

Then I'm transitioning onto NDT - self-medicating in the absence of no support from 2 endos, but a pliable GP who confesses to:

'I know nothing!' in Fawlty Towers style.

Thanks so much for your guidance. Wll no doubt be posting results next week for some more astute and insightful forum observations. Have a great day!!

MG x

shaws profile image
shawsAdministrator in reply to MaisyGrace

One of our Advisers (now deceased) only took one blood test for the initial diagnosis and thereafter patient was titrated slowly on NDT (he'd never prescribe levo) until symptoms were relieved. This is a link which I think you'll find helpful. He treated 'the person' not the blood test.


MaisyGrace profile image
MaisyGrace in reply to shaws

Yes. I've just been reading an article to that effect how, before Levo, everyone was on NDT and was treated as a person, ie; how they felt vs their 'results'. Thank you for sharing. x

m7-cola profile image
m7-cola in reply to MaisyGrace

Biotin is often used by labs in the assay of thyroid hormones. So if you are taking any supplements containing it you need to stop 3 to 5 days before testing otherwise the results become distorted/ inaccurate.

MaisyGrace profile image
MaisyGrace in reply to m7-cola

Thanks for that. Very useful,

fortunately I'm not taking anything with biotin.


MG x

SlowDragon profile image
SlowDragonAdministrator in reply to MaisyGrace

If you have Hashimoto's, then it's very likely you have list vitamins regardless of how good your diet is

It's not what we eat, but how little nutrients we can absorb

Getting all four vitamins optimal first BEFORE trying NDT

Also if you have Hashimoto's, it's important to retest antibodies before and after starting NDT. Some Hashimoto's patients can get increase in antibodies on NDT and may do better adding small dose of T3


Levo plus T3 is more flexible if you find you need to adjust ratio of T4 to T3

Supplementing a good quality daily vitamin C can help support adrenals

Selenium supplements can help improve conversion of FT4 to FT3

Obviously if you have Hashimoto's it's recommended to try strictly gluten free diet too, ideally before changing from Levo

Only ever name one change at a time or add one supplement at a time or you won't know which is helping

Hashimoto's affects the gut and leads to low stomach acid and then low vitamin levels

Low vitamin levels affect Thyroid hormone working

Poor gut function can lead leaky gut (literally holes in gut wall) this can cause food intolerances. Most common by far is gluten. Dairy is second most common.

According to Izabella Wentz the Thyroid Pharmacist approx 5% with Hashimoto's are coeliac, but over 80% find gluten free diet helps, sometimes significantly. Either due to direct gluten intolerance (no test available) or due to leaky gut and gluten causing molecular mimicry (see Amy Myers link)

Changing to a strictly gluten free diet may help reduce symptoms, help gut heal and slowly lower TPO antibodies

Ideally ask GP for coeliac blood test first or buy test online for under £20, just to rule it out first

Trying gluten free diet for 3-6 months. If no noticeable improvement then reintroduce gluten and see if symptoms get worse







But don't be surprised that GP or endo never mention gut, gluten or low vitamins. Hashimoto's is very poorly understood

Yes. I agree and am aware of this. I appreciate you sharing this and the links. Thank you.

Adam10 profile image
Adam10 in reply to shaws

I have realised that I inadvertently leave 36 hours between my last Levo dose and my blood test - I take last dose evening of say Sunday night then I miss the next Levo dose on Monday evening and give blood on Tuesday morning. That is 36 hours not the 24 hours I intended.

I just received a TSH result of 0.06 micU / mL (range 0.27 - 4.2). What would it be if I had left only 24 hours not 36 hours?

What are the units micU / mL? Is this new (first time I’ve seen it).

Any comments appreciated.

SeasideSusie profile image
SeasideSusieAdministrator in reply to Adam10

Timing of Levo doesn't affect TSH. Levo is thyroid hormone - T4 - so it's FT4 result that is affected by the gap between last dose of Levo and blood test. Take your Levo too close to blood draw and you get a false high FT4, take it longer than 24 hours and you get a false low FT4.

Don't worry about the units of measurement, look to see where within (or outside) the range the result is.

The range of 0.27-4.2 is used by Medichecks and Blue Horizon and the unit of measurement is put as mIU/L. The hospital lab that does my GP tests uses a range of 0.35-5.5 and their unit of measurement is put as mU/L.

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