Hi everyone, Just wanted to let you know that I was asked by my NHS surgery to come in and have a blood test. Firstly I had to ask what it was for and secondly no mention was made of my having to fast before the test and not to take my thyroid meds which is something I've always had to do when having a private blood test. Can someone please tell me what is the official protocol on this?
Many thanks
Roz
I disagree with the need to fast before a thyroid blood test but most patients believe you should.
If you take your levothyroxine before the blood test it will skew the results a little and you may end up under medicated. Leave at least four hours between your last levothyroxine dose and having the blood taken.
There is little proof that fasting before thyroid tests makes a significant difference.
Nonetheless, we know that there are many blood tests for which fasting is important.
We might go for years not fasting when having thyroid tests. Then, because a test which requires fasting is to be done alongside a thyroid test, the next time we fast. At that point we have made the conditions for the thyroid test different.
If there really is no impact of fasting, then it makes no difference. But if there is even a modest impact, that effect is now in play.
And, just in case they do something more than TSH, we need to be sure there is no impact on FT4 and FT3 tests.
All in all, my view is that we might as well fast and be able to ignore any possible impact of not fasting. Even if that is itself minor.
It is beyond question that you need to stop any meds containing biotin a week before the test, or the biotin will distort the results.
Beyond that, the GP surgery has very little knowledge about matters thyroid, so I would go by the advice here: have an early morning test (because your TSH is highest then, and so many GPs are TSH-obsessed) and leave 24 hours from taking your levo, because this gives the most representative level of thyroid hormone - taking your levo sooner gives a slight "false high" and a bigger gap a slight "false low". The blood test is necessarily a snap-shot of thyroid levels at that particular moment - so it would be a shame to get medication reduced, possibly for a year - on the basis of a false high result.
I don't have much of a view either way about fasting (although it obv makes sense if you are planning to take your meds after the blood draw, as you should have a minimum 2 hour gap after eating/drinking. I think Jim's 4 hour gap is something of a minority view
My four hour gap was based on the research referenced in Fig. 1 of this paper thyroidresearchjournal.biom... . (The original paper is behind a paywall). Having looked at it again I would recommend a six hour gap between levothyroxine and having the blood taken. This will avoid an inaccurate fT4 result.
I think we might be over-stating the biotin issue.
The earliest research was from people taking huge doses - often for Multiple Sclerosis.
Ordinary dietary intake, as in the amount found in most diets, is not believed to have any impact.
The amount in some supplements is usually far closer to dietary levels than MS megadoses. And is likely not to require a full week to wash out sufficiently to have no impact.
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