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Thyroid UK
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Do I take thyroxine before I have blood test

My Dr begrudgingly is sending me for a thyroid blood test, I've been on thyroxine for years, currently 125 a day, do I take it before the blood test as usual or wait till after, I usually take it at half 7 then take my blood pressure and painkillers an hour later, I have to take the blood pressure meds at the same time every day, it's very high, can anyone help so I can get an accurate thyroid test, it says on the blood form -(TFGPRX) thyroid (on T4) that's all it says

22 Replies

Wait till after!!


Definitely delay taking Levothyroxine until after blood test.

It's helpful to get test as early as possible in morning and fasting too


Leave at least four hours between taking levothyroxine and having blood taken. Fasting has a slight effect, giving a higher TSH which is a trick to get more levothyroxine prescribed. I wouldn't fast, just makes life difficult and has little effect.


Only 4 hours?


jimh111 has different ideas about testing the thyroid compared to everyone else on the forum. He muddies the waters for everyone and confuses newbies.

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Muddled thinking muddles the waters !!! Do we want accurate results or do we end up with unreliable results which we can't compare because the blood has been taken early morning when TSH is changing rapidly. If you do this you can't compare todays results with the ones you had a month or two ago, or year or two ago.

Sticking strictly to the question, fasting makes very little difference at all if any. Enjoy your breakfast.

If you really want to fiddle the figures you have two options:-

1. (preferred) Knock up the numbers you want in a word document.

2. If you are 'lucky' enough to be a mensurating woman, have the blood taken mid-cycle, this will have a far greater effect than fasting or taking the blood in your sleep.

I don't like faking the blood test results because it messes up your health record and more importantly reinforces doctors' view that the blood tests are infallible for diagnosing and treating hypothyroidism. Let them see patients are hypothyroid with normal hormone levels.


We want a method of testing thyroid levels that is consistent, so that we can compare our results from one test to another.

We want a method of testing thyroid levels that is not affected if we also test blood sugar or chlesterol levels or iron levels or anything else at the same time as thyroid levels.

We can also judge our results to a certain (limited) extent if we compare them to someone else's results, although it is of limited usefulness because we aren't all being treated with the same meds, nor do we all take meds at the same time, nor do we all take the same meds for other conditions.

If a group of people are all taking 100mcg Levo per day, one takes it at 4am when they go for a pee, another takes it at 7am, another takes it mid-morning, and another takes it at bedtime. And then they get tested with or without food at 10am or 2pm or 5pm, then how can anyone say anything about their result? To me, consistency is really important. But clearly our ideas about consistency are different.


TSH plateaus during the middle part of the day. There is less variability at this time, the results are more consistent. You're also trying to give too much precision and importance to the blood test.

Purely from the point of view whether to fast or not the answer is it make either little difference or it makes no difference depending on which study you find.


If my doctor tells me to get a thyroid test (she never does, but let's pretend), and I book an appointment for 8.30am, I could get it done with or without breakfast. Let's suppose I always have breakfast.

A few months later my doctor tells me to have a cholesterol test and a thyroid test. Cholesterol testing should always be done fasting. So this time my thyroid test will be done fasting whereas previous ones will have been done following breakfast.

In a way I'm not that interested in whether food affects my thyroid test result or not. What I am interested in is consistency so I can compare results.

And for that reason I will always book tests first thing in the morning having fasted, whatever the test is that is being performed because my breakfasts are not always the same, but when fasting I can control the consistency of my testing conditions to a much greater extent.

And I will always stick to having skipped thyroid meds for a certain number of hours and I will stick to the same number of hours each time, for the same reason - consistency.

Your attitudes to testing are different to almost everyone on the forum. I don't think it is helpful and I think it is confusing.


Four hours is enough to allow the levothyroxine to bind to its transport hormones and so not give a misleading fT4 figure.

In my response to Shaws below I pointed out flaws in the hopeless study she referenced. I found another study by the same team which is better quality ncbi.nlm.nih.gov/pmc/articl... . This study found no difference between the fasting and non-fasting states.

"The mean difference in TSH values performed during fasting and extended fasting state (Delta 1) was 0.59 ± 0.72 (median 0.38 and range of -1.26 to 2.78). The mean difference in TSH values performed during fasting and 2 h postmeal state (Delta 2) was 0.51 ± 0.44 (median 0.41 and range of -0.4 to 1.68). Difference between Delta 1 and Delta 2 was not statistically significant (P = 0.61)."

Thus, there was no difference between fasting and taking the blood two hours after a meal. Four hours should be plenty.

(Even this study was sloppy. They compared fasting and non-fasting on two different days. It's possible that the effects of fasting are transient and so on the second day the effect might be reduced. Had they compared two groups on day one they might have found a small difference between fasting and non-fasting. This shows how important it is to study the full paper and not use abstracts or take the word of 'alternative' doctors who probably haven't studied the papers they reference.)


Fast before the test (you can drink water). Research has shown that eating food reduces the TSH which may make the difference between being diagnosed or not. You want your TSH to be at its highest level which is very early a.m. If you were taking thyroid hormones you'd allow a gap of 24 hours between last dose and the test and take afterwards.


TSH was suppressed in all subjects after food irrespective of the fasting levels. Free T4 values did not change significantly. This resulted in reclassification of 15 out of 20 (75%) subjects as subclinical hypothyroidism (SCH) based on fasting values whose TSH values were otherwise within range in the postprandial sample.

(" postprandially " this means after eating)



Thanks to everyone, so I don't take my thyroxine before and fast as well? Sorry if I sound a bit stupid,, stressed over my blood pressure and trying look after my autistic son who suffers from anxiety and depression and self harm, phew life sure isn't easy for us people with thyroid probs, is no wonder my hair is falling out and I'm constantly anxious, stomach turning like I'm frightened, heart thumping, anyway thanks again to all the lovely people on here who take the time to help

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Yup. You've got it.

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Yes, leave a 24 hr gap after your last Levo dose, and nothing to eat after dinner the night before the blood test.

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Shaws, there's probably some better research showing fasting affects TSH values, I have a vague recollection of seeing one where there was a small effect. Hence I wouldn't bother going hungry even if you want a higher TSH.

A lot of research makes me scream (internally). It's so frustrating that good research is blocked at peer review, for political reasons, it would upset the blood test only apple cart.

This study is dreadful. "Phlebotomy was performed after an 8-12 hour overnight fast between 7:30-8:30 am for free T4 and TSH measurements and the patients returned 2 hours after breakfast for their samples to be rechecked between 10:30-11:00 am on the same day. " Rather than taking all the blood at 10:30 with half the study group getting breakfast they take the fasting blood sample at 8 am, give the subjects breakfast and then take the blood around 10:45.

There is no way they can identify the effect of fasting as the major change in TSH will be due to the timing of the two blood takes. The doctors who carried out this study should have their blood taken - all of it.

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Everyone is entitled to their own opinion but those on this forum are here because they are not being cared for by their doctors as they should be.

I am not medically trained anything but I certainly wouldn't take the chance of doctors interfering and lowering my dose which they often do as proven on this forum and they are adjusted by the blood tests alone.

That's what they do and if you've struggled for years by not one person testing your thyroid hormones but diagnosing you with other things - maybe suggesting you are a hypochondriac as you've disabling symptoms that they do not know, you are not going to take that chance. of getting your dose reduced for any reason at all. This is yet another extract in the same vein:-

"How Fasting Affects Thyroid Tests

Most guidelines will suggest that fasting is not necessary when performing a thyroid blood test.

This can be a confusing issue since studies have regularly shown that early morning fasting translates to higher TSH levels—as much as 26 percent higher—compared to those taken later in the day with no fasting.

What this means is that as many as one in 15 people may be reclassified from subclinical hypothyroidism in the morning to normal in the afternoon.

How Timing and Medication Use Affects Thyroid Tests

The guidelines also suggest that it doesn't really matter when during the day you get your tests done, and, in large part, that's true. TSH tests, in particular, are meant to evaluate the impact of treatment over the course of weeks and, for the most part, hormone levels tend to remain relatively stable.

The same doesn't apply to T3 levels or the drugs use to treat them. While T4 medications such as Synthroid and Levoxyl have a half-life of five to nine days (meaning it takes five to nine days to clear the 50 percent of the drug from your body), T3 drugs like Cytomel (liothyronine sodium) have a half-life of as little as one day. This translates to greater fluctuation in T3 levels with peaks typically occurring within four hours of taking the drug.

What this means is any T3 test performed right after taking at T3 drug may reflect higher values than actually exist.

Even with T4 drugs, which maintain a far steadier state, a peak can occur within two to four hours and remain elevated for up to six hours after taking the dose.



Thanks for reply, I'm on T4 levothyroxine, I'm getting symptoms of real bad hair loss, head and body, anxiety, feeling scared and panicking all the time, heart is thumping, skin is dry, Dr tried to give me antidepressants but I refused to take them until my thyroid was checked, I'm totally stupid when it comes to understanding anything to do with my thyroid even though I've been diagnosed with underactive thyroid about 15 years ago, I don't get much time to try to learn about it as I'm full time carer to my autistic son


Although your symptoms are a bit mixed (anxiety can be hyperthyroidism) dry skin is very specific for hypothyroidism. On this basis you are still hypothyroid regardless of the blood test results, although having the results will give a clue as to what is going on.


That's my worry I don't want to take too much or not enough I want an accurate blood test


The blood test will help but your signs and symptoms should take precedence. Sometimes we can have perfect blood test results but be very hypothyroid. When you get your results post them along with a brief description of your symptoms.


The effect of fasting is small. The verywell.com link compares early morning fasting to later in the day non-fasting. The bulk of the effect is due to the time of day.

Unfortunately fiddling the results is counterproductive. It leads the doctor to belive the patient has mild primary hypothyroidism and that giving a little levothyroxine will sort everything. In reality the severity of the patient's condition is not due to a mildly failing thyroid gland. It is due to other more severe thyroid hormone issues that will be overlooked.

If you play the numbers game you can't complain if the doctor becomes number obsessive.


To clarify I take my levothyroxine at 7.30am then an hour later I take my blood pressure meds with a cup of tea, I never eat until mid afternoon I've always been like that, can't face food in a morning, my blood test is at 11.20am it was the only one available

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