Factors that interfere with thyroid tests

[helvella]

Have you had test results that don't seem to make sense?

helvella - Factors that interfere with thyroid tests

A summary of factors known to interfere with thyroid tests.

helvella.blogspot.com/p/hel...

[Gymbuni]

Thanks helvella I have had a quick read a note that furosemide is on the list that can interfere with thyroid tests, I don't take it the morning before a blood draw as always worried that the phlebotomist may have difficulty getting a blood draw (as I am not great with blood tests and sometimes it can be rather slow), should I stop taking these for a longer period before a blood draw, like we are advised to with biotin? I am only on a small dose 20mg for fluid retention and I am hoping once I finally get my therapeutic dose of thyroid meds I can finally come off it altogether.

Incidentally I have just had blood test this morning, so I have been taking the furosemide up to yesterday morning.

[helvella Administrator Thyroid UK in reply to Gymbuni]

I'm afraid I can see that the paper says regarding furosemide:

As in patients known to have a genetic variant in TH transport proteins, some drugs also affect the equilibrium between T4 or T3 and their binding proteins, thus resulting in altered free TH concentrations.

But I cannot see further detail anywhere. I only get the abstract here:

degruyter.com/document/doi/...

That means, I cannot give a good and helpful answer, I'm afraid.

[Gymbuni in reply to helvella]

Thanks helvella I will play it safe in future and stop taking it 3-4 days before a blood test.

[helvella Administrator Thyroid UK in reply to Gymbuni]

So long as not taking it doesn't cause its own issues?

[Gymbuni in reply to helvella]

No, just terrible fluid retention in my legs, rather unsightly 🙄.

[Star13]

Thank you for this. I keep reading about the interaction between heparin and thyroid meds/tests, however I cannot find anything that suggests the amount of time between the two meds or tests. I even asked my Heamatologist after it was in a paper about Covid and Thyroid but never got a reply!

Can anyone else shed any light on it?

[helvella Administrator Thyroid UK in reply to Star13]

The paper I linked to includes this:

The artifactual hyperthyroxinemia due to heparin has been widely studied (8, 150, 151). The administration of heparin to healthy volunteers and subjects with hypothyroidism showed a rapid increase (2 to 15 minutes) in FT4 concentrations (up to fivefold) (151) due to the generation of nonesterified fatty acids.

That is, it can produce a high FT4 result. The paper includes at least three onward links - as in the (8, 150, 151) above.

[Star13 in reply to helvella]

Thanks but I can get no advice as to when to distance the two medications in order to mitigate the effects of heparin on the T4.

I have to inject heparin twice a day so my last dose before a thyroid test would be 12 hours previous but the last T4 dose would be at almost the same time as a heparin dose. I don’t know if any of these factors affect results and that’s not counting T3!!

[helvella Administrator Thyroid UK in reply to Star13]

I'm not sure whether any time gap is sufficient to avoid the issue entirely. But the paper does say:

Taking a blood sample at least 10 hours after the last heparin administration and analyzing the sample immediately thereafter can reduce this artifact. The assessment of total THs along with TSH and TBG appeared to be a valuable alternative to confirm the euthyroid status of such patients.

Afraid it will be up to the lab whether that can and will do TT4 and TT3 as well as free tests. You need to discuss with them - directly or indirectly via a doctor.

[Star13 in reply to helvella]

10 hours after last heparin administration for a test - but that does not tell you the time between heparin and last DOSE of Thyroxin.

Additionally, can you imagine getting your sample tested “immediately” let alone getting the additional tests done just to see what your results are. I think I have to accept that I can’t rely on my results! I guess no one has actually worked these things out in practise.