Why is fasting important before a blood test? - Thyroid UK

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Why is fasting important before a blood test?


Hello, my GP has agreed to increase my levothyroxine back to 150mcg from 125mcg. I am so happy as I have had lots of hypo symptoms ( total thyroidectomy 35 years ago) since he reduced it a year ago. I think he was a bit reluctant but I know that's where I need to be.

I am having a blood test in 6 weeks and will follow the advice on this forum and take levothyroxine 24 hours before blood test and fast for 12 hours. If the blood test is drawn a little over 24 hours is that ok as it's difficult to know how long I will have to wait in clinic as depends on how many people want a test that day as to how long you wait. Also why is it necessary to fast before hand? What results would fasting alter?

One last question, I started taking the extra levothyroxine today but wondered how long before I should start feeling an improvement in my symptoms which are mainly aching legs, dizziness, lack of motivation, memory loss, insomnia and constipation?

Thank you in advance.

13 Replies

It takes 6-8 weeks to start to feel improvements

If you have been under treated for a year, low vitamin levels are possible/likely

Have you had vitamin D, folate, ferritin and B12 tested recently?

What vitamin supplements do you currently take?

TSH drops after eating or tea or coffee

Perfectly ok to eat as normal the night before blood test and plenty of fluids in day before

Just have water on morning of test and take delayed dose of levothyroxine immediately after blood test

Ask GP to test vitamins or test via Medichecks or Blue Horizon, especially if GP only tests TSH and FT4

I think fasting is a bit of myth, I never fast, wouldn't dream of it. Fasting is believed by some to raise the TSH and so make it easier to persuade a doctor to give you a higher dose of hormone.

This idea arose from an Indian study that took blood before breakfast, sent the subjects away for a few hours to have breakfast and took blood later in the day. The results from the early morning blood draw had higher TSH results than from later in the day. Of course this was due to the diurnal fluctuation of TSH, TSH is at its lowest around midday. The same team did a follow up study a few years later comparing TSH in subjects who continued to fast and subjects who had breakfast. There was no difference in TSH. Unfortunately, the damage had been done, word had got around that fasting raises TSH.

I've only seen one study which showed a slight difference in TSH due to fasting but this was a study that looked at Ramadan fasting which is quite different in that it also excludes drinking and usually the main meal is taken in the early hours of the morning. This disrupts the body clock and the subjects would still have been digesting their meal during the morning blood draw.

There's nothing to suggest fasting is necessary for a thyroid blood test (or even has any noticable effect). Of course, if you were concurrently having another blood test that required fasting you would need to fast

MaisieGray in reply to jimh111

I've never seen anyone suggest that fasting raises TSH, rather, that eating might lower it, which are of course, different. However, whether or not it makes any discernable difference, it is evidence of the sad and desperate state of the patients' fight against the dreaded 'rule by TSH' and how we'll jump through hoops to 'improve' levels in order to avoid the resultant dreaded dose reduction. I was recently backed into a corner whereby I had to have a GP-ordered blood test, which I normally avoid as far as possible. As I'd not very recently had my last private test carried out, I had no up to date benchmark of where my NHS results might sit and therefore what dire consequences may or may not befall me when a GP got hold of them. So I not only fasted for the TSH but went without my T4/T3 for 2 days in the hopes of slightly lowering and thereby 'improving' my FT4/FT3 ie safeguarding my current dose. This of course, makes a nonsense of the results, and unsurprisingly, even though the FT4 was very low in range, no GP has contacted me to suggest my Levo be increased .... For an accurate and useful monitoring of my levels, I'm waiting for the next appropriate Thursday price reduction; but back to the fasting/eating question, I agree with diogenes about the benefit of the 'controlled state' as the presentation norm.

I think that the benefit of a short (ie no breakfast) fast before early TSH measurement is that you are in what I term a "control state" on presentation. Breakfasts with sugary food of course lead to immediate body responses which may have some minor effects on parameters. So different breakfasts might have different effects an hour or so later. I'd view short fasting as a baseline concept to ensure you are always in the same state each time a measurement is taken,

helvellaAdministrator in reply to diogenes

I'd also suggest that there is a possibility that next time you have a blood test, some test that actually requires fasting might be included. By always not having breakfast you ensure that any possible difference between fasting and non-fasting on the thyroid-related tests is avoided, even if that impact is modest.


Link about research into fasting testing


As you would want to take Levo immediately after test, this should be on empty stomach...so even if fasting makes no difference to results you would want to fast

Whoops, I shouldn't have written 'raised the TSH', I meant give a higher TSH result. SlowDragon gives a reference is to the offending study, these doctors have done a lot of harm with their sloppy research. I take levothyroxine either at bedtime or just before a breakfast of (usually) porridge oats and fruit, I take my levothyroxine the night before a blood test. I normally have my blood taken late morning if private, my NSH surgery does blood tests the afternoon. Being a creature of habit this gives a fairly constant state not that I think it makes much difference as TSH is a pulsatile hormone and my TSH is pretty low.

I appreciate some patients have to get a higher TSH result because it is the only way they can get treatment. This is frustrating, TSH is a useful marker, nothing more. I feel it’s important that those of us who are already getting prescriptions do not try to fiddle our blood test results so that doctors can see that a very low TSH is sometimes needed. If we fiddle the results to show a ‘normal’ TSH all the time doctors will believe that all patients get well when their TSH is ‘normal’. It’s important that doctors see that TSH does fluctuate in patients, that this is perfectly normal, and that signs and symptoms are paramount. Presenting perfect TSH levels at each consultation just reinforces doctors’ reliance on blood tests.

humanbean in reply to jimh111

I think you have no understanding at all of how badly women are treated by doctors in connection with the thyroid. You are someone who once admitted in a forum post that your thyroid treatment started when your TSH was between 1 and 2 (if I remember correctly).

I was first told that my thyroid was "borderline underactive" in the early 1990s. I didn't get my first prescription for Levo until 2013. I had copies of my blood test results going back 4 years at that time and my TSH was over the range every year. But I'm female, and therefore not to be believed when it comes to saying how bad I felt.

You may be right about showing doctors that TSH fluctuates. But what it would mean for women is that their treatment would fluctuate alarmingly too. Some women would no doubt have their treatment taken off them altogether. And the older the patient the more likely this would become.

MaisieGray in reply to jimh111

I absolutely agree with you, in theory, re the principle of presenting our condition, our symptoms, and our bloods levels "as is". But unfortunately, due to circumstances not of our making, we've ended up having to tie ourselves in knots both to access treatment and to be given it at the correct dose. And partly for that reason, I don't agree that you can put the onus on established patients to freely present challenging results without taking steps to 'adjust' them, if they feel the true results put them at risk of dubious decisions being reached by their GP, because none of us are safe from the axe falling at any time. There was I, happily pootling along with 125 mg T4 and 20 mcg T3, until one day without warning and certainly with no valid clinical reason, my T3 was withdrawn and my T4 cut to 75 mcg. Having - for now - got my T3 back, I'm not therefore willing to give my GPs reason to reduce or withdraw my meds again; so I'll continue to resist requests to have their TSH and FT4 tests, and if I can't avoid them, I'll put my effort into manipulating the results in my favour. I'd love not to have to but we're no longer in the time of presenting symptoms and how we feel, and being given treatment accordingly. It's all cookie cutter, or even totally random, decision-making now, and pretty small cookies at that.

greygoose in reply to jimh111

Jim, do you ever actually read any of the posts on here? Do you understand what they're saying? Does it sink in? You appear to be in a privileged position, for some reason. You had no trouble getting diagnosed, you appear to have no problem getting the right dose for you. That is far from the case with most people. Most of us are fighting for our lives - literally. And, you are not helping anybody with your devil-may-care attitude. Oh, I have my tests in the afternoon, my doctor understands about the TSH… Well, you should know by now that most of them do not understand about the TSH. And they will happily - and it really appears to give them great pleasure - drop a persons dose at the slightest little excuse. They know next to nothing about thyroid, and are basically all misogynists at heart, meaning that it is very, very hard for the average woman to get the treatment she needs. How many thousands go undiagnosed? That is something we shall never know. I'm I'm guessing quite a lot. But you had the extraordinary good luck of getting diagnosed with a very low TSH, and therefore seem to think it's the same for everyone. It isn't. And I do wish you would show more appreciation of the luck you have, and please take that into account before telling people to take their levo with porridge and get their TSH tested after breakfast. The majority of us need all the real help we can get. And that means getting realistic.

jimh111 in reply to jimh111

humanbeing MaisieGray greygoose

I was eventually diagnosed by Dr Gordon Skinner and at the time my TSH was 1.0. I was incredibly ill and failed to get a diagnosis from GPs and endocrinologists. It was extreemly diffiicult to persuade my GP to prescribe, requiring a lot of preperation and tact. Tact is not easy when you are mentally distured from years of hypothyroidism. You can see the physical effects in the photographs here ibshypo.com/index.php/hypot... . I put on a great deal of weight, had severe IBS and a complete inability to concentrate. I used to be able to assimulate a 1,000 line computer program in my head, restructure it and rewrite it in new code incorporating upgrades, I was reduced to writing a shopping list when I went for a newspaper and a loaf of bread. The consequences were that my personality changed, I was irritable and asocial. I was forced to take early retirement which I calculate cost me over £250,000 in terms of net lost salary and pension rights. The personal effects were far worse. Above all twenty years of my life were wasted.

Some patients have to fiddle their results but many do not, it's just an easy option. This has two disadvantages. 1. It perpetuates the impression that TSH is definitive. 2. Many patients remain hypothyroid because their TSH is too low, reducing T4 to T3 conversion. Thus by attempting to obtain a higher TSH they are shooting themselves in the foot. I will publish some details on this later this month. If doctors never see real results we can't blame them for thinking TSH dependent therapy works.

I'm sorry if it upsets people but it is cruical that we stick to and push for good science in thyroidology. I know some patients, especially new patients, need to trick their way into treatment. Bad practice and attitudes that are not evidence based have gone on for far too long. (For example, there is no evidence whatsoever that TSH reflects thyroid hormone status in all cases). We can either carry on forever with the current mess or challenge it and demand better research and treatment.

Whilst I agree there is misogynism it would be easy to assume it always comes from male doctors, my impression is that female doctors are at least as bad. Unfortunately, I obviously do not experience it but I would encourage you to challenge it whenever it occurs, always having someone with you during consultations will help.

greygoose in reply to jimh111

Do you think you're telling us something we don't know? I'm sorry to hear how badly you suffered, but thousands of women go through the same thing and are sent to psychiatrists! No-one is suggesting that you weren't deserving of your diagnosis, but even with all those symptoms, a lot of people still don't get diagnosed because of the ignorance and prejudice of doctors.

And, here, I would like to put my hand up and say that I was one of the lucky ones. I had a TSH of something like 11 when I was diagnosed, and I didn't seek diagnosis, because i wasn't ill, just 'fat'! But, I am more than conscious of the struggles some people have from reading on here every day. Where I was unlucky was that I'd more than likely been hypo since the age of 8, but no doctor thought to test me until I was 55. They by far preferred to tell me I was greedy, lazy, hysterical and a bit thick, and refer me to a psy.

In an ideal world you would be perfectly right in what you say about not 'fiddling' results - although I can't help thinking that 'fiddling' is too strong a word, I would say just turning nature to our advantage. And, I totally agree with you that female doctors are just as just as misogynistic as male doctors - maybe even more so. I would never seek out a doctor just because she was female. But, where I think you are wrong is in your belief that if we all behave like perfect 'gentlemen' and play by the rules, doctors will change their ideas and attitudes. They won't even notice! And, lets face it, as you've said, most of us are far too ill to be able to fight our corner. Besides, a fighting female just gets labelled 'hysterical' and sent to a psychiatrist. It just isn't going to happen. Their ideas are far too entrenched, and change is going to have to come from within, not from a bunch of sick, fat females, who all doctors know lie, exaggerate and dramatise. So, what are you, as a representative of the male hypos, doing to change the status quoi and educate doctors?

Oh, and by the way, most important of all: just because no-one has proved that eating lowers TSH, equally, no-one has proved that it doesn't.

jimh111 in reply to greygoose

I wrote that those of us that are diagnosed and capable should get accurate results and challenge doctors, as opposed to giving them TSH numbers that 'confirm' their attitudes. We should challenge them not let them continue to believe they are correct. This is best done with politeness, assertiveness and tact. You can imagine I had to work hard on the last one! Bringing someone with you to the consultation will help.

As a male hypo my efforts to change the status quo and educate doctors consists of lobbying them at meetings (this must be done in a constructive manner - to be successful), participating where I can (e.g. treatment guidelines, NICE and responses to journal articles). I also carry out fundamental research. I’ve tried to get this published but as a patient come up against even more prejudice than our good thyroid specialists. Peer review is little more than an old boys (and girls) act with censorship. Consequently, I’ve set up a website to put across my research in the hope patients will produce ‘bottom up’ pressure for change. I feel that most research is not addressing the severe symptoms patients experience. It may be high quality and well intentioned but only addresses minor deficiencies in circulating hormone levels. I’m hopeless at administration, organising and interacting with people but good at logical thinking so feel I can best help if I stick to my strengths.

The few studies there have been show that eating a moderate meal has little effect on TSH. It’s a reasonable question to ask based on the hypothesis that increased somatostatin after eating might lower TSH. The studies show that any effect is very small. We have to follow the evidence, even if we don’t like it youtube.com/watch?v=5v8habY... .

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