Blood test

[Shanghai-Suki]

Should I stop taking my Levo before I go for my annual review ie blood test & if so how long before would be recommended? Thanks in advance x

[SeasideSusie Remembering]

Shanghai-Suki

When doing thyroid tests, we advise:

* Book the first appointment of the morning, or with private tests at home no later than 9am. This is because TSH is highest early morning and lowers throughout the day. If we are looking for a diagnosis of hypothyroidism, or looking for an increase in dose or to avoid a reduction then we need TSH to be as high as possible.

* Fast overnight - have your evening meal/supper as normal the night before but delay breakfast on the day of the test and drink water only until after the blood draw. Eating may lower TSH, coffee affects TSH so it's possible that other caffeine containing drinks may also affect TSH.

* When taking thyroid hormone replacement, last dose of Levo should be 24 hours before blood draw, if taking NDT or T3 then last dose should be 8-12 hours before blood draw. Adjust timing the day before if necessary. This avoids measuring hormone levels at their peak after ingestion of hormone replacement. Take your thyroid meds after the blood draw. Taking your dose too close to the blood draw will give false high results, leaving any longer gap will give false low results.

* If you take Biotin or a B Complex containing Biotin (B7), leave this off for 7 days before any blood test. This is because if Biotin is used in the testing procedure it can give false results (most labs use Biotin in the assay).

These are patient to patient tips which we don't discuss with doctors or phlebotomists.

[Shanghai-Suki in reply to SeasideSusie]

Thanks so much for getting back to me so quickly I really appreciate the help!!

[Trill in reply to SeasideSusie]

Thanks for that. I have a blood test tomorrow but not until 2.30pm. In the UK at the moment we have to take what is offered, realistically. Could you tailor your advice please for that time of day? I did not take levothyroxine last night and I won't take it tonight, but what about eating, and that morning coffee? Many thanks igf you can get back to me. What I don't understand on a monitoring test is that presumably the medication effect is being measured, so if you artificially alter the picture by leaving out the medication, how can the GO judge the effectiveness of the existing regime, and might she merely up or lower the number in the false belief that the existing prescription is too low? Then you have to own up to not being on the medication just before the test.

[SeasideSusie Remembering in reply to Trill]

Trill

It all depends what you want from your blood test results and what medication you are taking. Also whether your GP is a slave to the TSH or whether she listens to you and takes into account how you feel and if you have any symptoms.

Are you optimally medicated and want to keep on your same dose? Then your GP, if typical of most GPs, will be looking at TSH only, if that is within range your dose probably wont be altered. If it is below range then you are likely to have your thyroid meds reduced.

However, TSH is not an indicator of thyroid status, it's not a thyroid hormone, it's a pituitary hormone. The thyroid hormones are FT4 and FT3 and these are the results that are important but unfortunately doctor's don't know this.

So to avoid a reduction in dose based on a too low TSH, we advise having the test done no later than 9am - reasons given above.

I have a blood test tomorrow but not until 2.30pm. In the UK at the moment we have to take what is offered, realistically. Could you tailor your advice please for that time of day?

No, I can't tailor my advice around a blood draw at that time I'm afraid. The time of your test is when TSH is at it's lowest point of the day, see the first graph in this post which shows the daily rhythms of thyroid hormones:

healthunlocked.com/thyroidu...

All I can suggest is that you cancel the appointment and rearrange it for another time when you can get an early morning one.

but what about eating, and that morning coffee?

Depending on what you eat it can lower TSH, and coffee affects TSH.

Thriva, one of the private testing companies recommended by ThyroidUK, say this on their website:

thriva.co/hub/thyroid/how-t...

Fasting

Generally, you don’t need to fast before doing a thyroid function test. However, not fasting is sometimes linked to a lower TSH level. This means your results might not pick up on mild (subclinical) hypothyroidism — where your TSH levels are only mildly elevated.

When our own thyroid works properly it's giving out small doses of T4 throughout the day as and when it's needed.

We don't want to measure our FT4 when it's at it's peak level after ingestion of exogenous thyroid hormone. When we take exogenous thyroid hormone then we're giving a massive hit of T4 once a day, we want to avoid measuring this because it will show our FT4 level to be falsely high for a period of time.

Nor do we want to measure it at it's very lowest which would be more than 24 hours after our thyroid hormone because it gives a false low level.

Leaving 24 hours between last dose and blood draw is said to give a measure of your normal circulating FT4 level.

Similarly, timing of T3 or NDT (if also taking this) is crucial. Because of the shorter half life of T3 then 8-12 hours is the recommended time of last dose.

so if you artificially alter the picture by leaving out the medication, how can the GO judge the effectiveness of the existing regime

You're not leaving out the medication, you're timing the test to get a more accurate picture of the normal amount of circulating hormone.

I did not take levothyroxine last night and I won't take it tonight,

So by the time you have your test at 2.30pm tomorrow, assuming you take your Levo around 10-11pm, it will be somewhere in the region of 38-40ish hours since last dose. Far, far too long a gap and this will give a false low FT4 result.

When you normally take your Levo at night then you'd alter timing for a day or two before the test. Say you were having your test on Monday morning at 9am, you'd delay Saturday night's dose until Sunday morning, do test Monday morning, take Sunday night's dose immediately after test, then take Monday night's dose as normal.

SlowDragon has a link to NICE guidelines about taking Levo after the blood test (SD, if you reply with the link, can you tag me so that I can take a copy of the link for future reference instead of keep asking you 🙂)

EDITED TO ADD:

SlowDragon - Don't worry, I've found it now and made a note of it 😊

Trill - Scroll down to Page 13 of this pdf from the Specialist Pharmacy Service NHS England

sps.nhs.uk/wp-content/uploa...

and where it says

Primary Care Monitoring

it says:

Monitoring is by measuring TSH levels from blood tests taken prior to the morning medication.

[Trill in reply to SeasideSusie]

Thanks very much for a lot of explanation and information. I will see what the results are and if wildly different from those last year I shall just have to get a private test done; the test will be TSH only. I have heavy, tired legs but that hasn't changed throughout a few years of meds and may equally be CKD3a also being tested tomorrow again, or, as combined with breathlessness, has cardiac implications, What I can't do is cancel just hours before, especially as a second test is being taken for the CKD. It's my fault accepting the wrong time and I shall have to sort it out if needs be.

[SlowDragon Administrator in reply to Trill]

Suggest you write new post of your own

With latest results and exactly how much levothyroxine are you currently taking

Plus vitamin results and supplements

Low GFR can be linked to being on inadequate dose levothyroxine

academic.oup.com/jcem/artic...

[SlowDragon Administrator]

As Seasidesusie says.....very important when you test TSH ....

researchgate.net/publicatio...

“According to the current TSH reference interval, hypothyroidism was not diagnosed in about 50% of the cases in the afternoon.”

“Further analysis demonstrated inadequate compensation of hypothyroidism, which was defined in 45.5% of the morning samples and in 9% of the afternoon samples”

ncbi.nlm.nih.gov/pubmed/252...

TSH levels showed a statistically significant decline postprandially in comparison to fasting values. This may have clinical implications in the diagnosis and management of hypothyroidism, especially SCH.

[Trill in reply to SlowDragon]

Thank you, Slow Dragon: can you please read the reply I gave above, and we will see what outcome there is pretty soon.