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Thyroid UK
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Science behind no meds for 24 hours prior to blood test

As I expected when I shared my medichecks thyroid results with my GP all I got was ‘well different labs give different results’ so the upshot is I’ve got to get another test before I can get an increase in Levo. You would think they would only be too happy that I have paid for the test.

But here’s my question. What exactly is the science behind the fasting test with 24 hours with no Levo, as I’ve never done the no meds’ for a GP test?

I can see the fasting aspect, but how does the no meds affect the TSH or other results given the T4 is active in the bloodstream for 7 days?

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The idea of the blood test is to measure the circulating FT4 not the peak of levels of Levothyroxine in your blood shortly after a dose is ingested. Levothyroxine can peak in the blood stream for up to 8 hours after taking a dose and if your practice tests FT4 may show a high FT4 if you've taken it before your blood test. It doesn't affect TSH. You don't have to leave 24 hours, 8-9 hours is sufficient, but as most people take their Levothyroxine in the morning it will usually work out to be 24+ hours.


Thanks for that, I don’t think the surgery usually test for T4, although I might get them to add it.

I’ve had to work hard on them before to increase the dose to get TSH to the bottom end of the range and that particular GP has now left the oractise so starting again with a new GP.

It usually works out that an increase of 25mg moves me down 3 units on the TSH range. TSH seems to have crept up over the last year from 0.5 to now be 6. Not far below where I started originally at 8.5.

Maybe a flare up of the thyroid?



If you have autoimmune thyroiditis (Hashimoto's) that progressively destroys the thyroid gland. Thyroid function declines with age too, whether or not you have Hashimoto's.

The goal of Levothyroxine is to restore the patient to euthyroid status. For most patients that will be when TSH is 0.3 - 1.0 with FT4 in the upper range. FT4 needs to be in the upper range in order that sufficient T3 is converted. Read Treatment Options in thyroiduk.org.uk/tuk/about_... Email louise.roberts@thyroiduk.org if you would like a copy of the Pulse article to show your GP.


Thanks for the help. Have emailed Louise for the Pulse article.

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