Help please with meds before blood test. - Thyroid UK

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Help please with meds before blood test.


I am on 150mcg Levo and 12.5mcg Lio, which I both take together at night. The Lio is self medicated and the GP does not know about it and it would be good if my results were not too out of what he expects from Levo only. I need to do a blood test and most likely ft4 and ft3 will be tested (they seem to be when my tsh is below or above range).

Not taking them until after the bloods is not a problem but will this work for accurate ft3? If I skip my dose the bloods will be c.28 hours after the previous dose. Will I be able to interpret for my dosing from this?

Any thoughts on how best to go about it?

14 Replies

If you leave more than 12 hours between your last dose of T3 and the blood draw, you will get a false low. You could get a rough estimate of your level by adding on 20%, but it will be very rough. You won't really know your FT3 level.



Is this a blood test at your surgery that your GP will see and act upon or is this a private test for your use only?

I am in the same position as you - self medicating T3 which my GP doesn't know about, along with prescribed Levo. I do private tests for my own use so that I can dose accurately, but make sure that any GP tests are acceptable for them (other than TSH of course which is always suppressed and I have that dreaded conversation every time).

Hi SeasideSusie. It is an NHS test, asked for d3 and ferritin test and the gp offered tsh too. What do you do to manage your bloods for the NHS? My gp has no Lio experience and his understanding of blood results is basic but he listens to me and says 'my thyroid is "a little different" as it goes up and down'. He also knows I am careful as I've increased Levo on my own too. He was okay with my tsh at 0.08 before. Just worry about a result that falls within unacceptable for NHS and is marked on the result as such and he needs to react, probably by reducing my Levo dose.

In Jan on 150mcg Levo my tsh was a bit under the range, ft4 was high in range but ft3 was only 4.5 (3.2-5.9) hence I decided to add in some Lio. Possibly the ft3 lifted and ft4 dropped a bit as a result.

Would it be more prudent to do a private test instead to see where things are?

SeasideSusieAdministrator in reply to Sybilla14


When I have a GP test, I know that the TSH will be suppressed - it always has been even on Levo alone - and the lab always tests FT4 but will add FT3 when TSH is suppressed. As I know my GP will give me the suppressed TSH lecture, I make sure I cam come back with the fact that my free Ts are well in range - somewhere usually between half way to 3/4s way through range. GP still doesn't like it so starts the yo-yo argument with her saying reduce dose and me refusing. I can ensure my free Ts are that level by altering my doses for a couple of days before the test.

But I do test regularly with a private test, with my doses as normal, to get my true levels so that I can keep an eye on things.

If your GP is not insisting on Thyroid tests at the moment, I'd just do a private one for your own guidance where doses are concerned. You probably know best where you need your levels to feel well.

Hi SeasideSusie, thank you for your posts. I have followed your advice and decided to wait with the NHS test and ordered medichecks kit. I take my Levo with Lio around 3-4am what would you recommend I do to get the ft3 accurate? Shall I take my dose early at 4pm before testing? Would this work?

SeasideSusieAdministrator in reply to Sybilla14


Last dose of T3 should be 8-12 hours before blood draw. Levo requires last dose 24 hours before blood draw.

Hi SeasideSusie. Would you mind sharing your strategy for NHS tests please? My last ft3 with medichecks was 8 (3.3-6.8), which was taken 11 hours after 19mcg of lio - this dose was only after 14 hours from my usual dose of 19mcg (as I was accelerating it for the blood test). Would you know what I would need to do to get my ft3 to c.6 for the test - is it a case of not taking any lio for a while? If so how long would you think this at be for? I had a call from the surgery this morning that my blood test is overdue (I was asked to do it by yesterday) and they are withholding the prescription now.

Thank you in advance!

SeasideSusieAdministrator in reply to Sybilla14

Apparently, the dose of T3 that you take is no longer detectable in the blood after 12 hours. I can't tell you how to get your FT3 down to around 6, maybe take your last dose 24 hours before blood draw as it was so high last time. If your FT3 comes back high with that, you will know your are overmedicated.

I normally take my T3 in one dose early morning with my Levo (between 3 and 6 am). When I know I am having a blood test, I alter the times the day before to suit the time of the test. So if I'm having a test at 9am, on the day before I take Levo and half of my T3 at 9am, then the other half of my Levo between 9pm and bedtime.

Thank you SeasideSusie. I will leave a longer period between my doses and the NHS blood test, may go beyond 24 hours to be sure. I’ve done 3 medichecks tests so far, which work great, so will continue using these for myself and ignore the nhs results.

It is interesting that you split your dose for the test, which I’ve not done but have wondered if it affects my ft3 results being over-range. I think I read that the half life of t3 is 12 hours hence the testing is recommended before then to establish the average level but I don’t know what this means for the other 50% of the t3 and how long that may linger in the blood. Not sure if there is an answer on this.

Your response is very helpful! Thank you very much for taking the time again 🙂

SeasideSusieAdministrator in reply to Sybilla14

My reading has shown that T3 has a half-life of 24 hours but the biological half-life is 2.5 days, that it is not detectable in the blood after 12 hours (it's moved from the blood to the cells and a blood test tests what's in the blood, not what's in the cells), and that the "withdrawal period" is 2 weeks (so if you stopped it it would take 2 weeks to be completely gone from your body).

This is an interesting study

which showed these results

"Triiodothyronine concentrations peaked at 2.5 hours following liothyronine administration. Heart rate increased by 5 hours after liothyronine administration, subsequently reaching a value higher than baseline (p value 0.009). Suppression of thyroid stimulating hormone concentrations began at 2 hours. The nadir thyroid stimulating hormone value at 12 hours was significantly different from baseline (p <0.001), and remained lower than baseline for 2–3 days."

I have read that T3 peaks in the blood 2-4 hours after ingestion so that would seem to fit, allowing for how each individual differs.

Thank you!

Hi SeasideSusie. Since your last post (thank you again) I decided to reduce my t3 by 6.25mcg (with a lot of undesirable side effects!) in preparation for the nhs test, which I did last Friday. The blood draw was at 7am and 27 hours after my last doses of t4 and t3. The results were:

Tsh <0.05 (0.25-5.5)

Ft4 14.8 (12-22)

Ft3 4.2 (3.1-6.8)

I ca see online that the gp wants to see me and I’ve an appt on Friday. I’m expecting he’ll want to reduce the 150mcg levo he thinks I’m taking. I don’t mind the prescription change as I’m not taking that dose any more but I’m sure he’ll want to retest, which will lead to no different results...

Did you tell your gp that you’re self medicating t3 or did you manage to convince them to ignore your low tsh? Would you have any tips or studies to present about the suppressed tsh being ok in treated patients? I’m not entirely sure how to play this!

SeasideSusieAdministrator in reply to Sybilla14

No, I haven't told my GP I'm self medicating T3. My GP would totally freak, she only understands TSH and won't discuss anything else.

I was very fortunate to have an appointment with the Advanced Nurse Practitioner (ANP) at the surgery for something else and he noticed I hadn't had thyroid tests for 3 years (I had been dodging them!). When the results came back he mentioned my suppressed TSH and ended up saying perhaps I didn't need Levo after all! I replied that after being on it for 43 years then I think if I didn't need it then it would have been discovered a long time ago, and look at the FT4 and FT3 levels as they were well within range. Cue a long discussion (which is something the GP won't do - discuss!) about thyroid tests, the fact that I knew which were the thyroid hormones and which was the pituitary hormone and what it does, and lots of other thyroid related things. He acknowledged that I knew what I was talking about and agreed that my FT4 and FT3 were nicely in range and that I didn't need to alter my Levo. He said it had been nice to have a sensible conversation with someone who understood it!

I said to him that I appreciated the fact that he accepted my TSH being suppressed as long as hormone levels were in range, but what do I do if the GP sees my results and bangs on again about suppressed TSH. His advice was to point out that my TSH has been suppressed for years and that I'm just one of those people who have a suppressed TSH. I don't actually intend to have that conversation with the GP. I fully intend to see the ANP if thyroid test results need discussing in the future, and if the GP does ever mention reducing my dose again I will repeat what I told her last time - an endo reduced my Levo in the past to bring my TSH into range, in doing so this reduced my FT4/FT3 to such a low level that I was turned into a zombie, I had to give up work and be looked after for 2 years and that I don't intend repeating the experience for anyone.

What the ANP didn't know was that I was taking less than the dose on my prescription and adding some T3. Fortunately for me, to feel well thyroid wise I need both FT4 and FT3 around 75% through range so my results don't look out of balance and don't pose any questions about a high FT3 with a low FT4.

I have saved some old threads about suppressed TSH which I intend to plough through sometime, I haven't yet studied them to extract any useful information I'm afraid.

Your GP probably wants to reduce your dose because of your TSH, but point out your FT4 and FT3 results, say if you reduce your dose they will go under range.

By the way, you left T3 off for too long and you have a false low FT3. Levo is left off for 24 hours and T3 for 12 hours.

Thank you very much SeasideSusie. I’m thinking of telling the gp that I have reduced to 125mcg as I’m trying this out and taking at different times of day, etc. Last January when I was on 150mcg Levo only my ft4 was 22 and my tsh was ever so slightly u see range. The ft3 was 4.5 as opposed to 4.2. Insisting I’ve been on 150mcg wont stack up🙁 also, if he then thinks of reducing to 100mcg I can point him to previous tests on this dose when my tsh was not below 1, as requested by my endo.

I know my ft3 result was too low to use for dosing. I was intentionally manipulating it to make sure it does not look too high. I’ll do another medichecks test but will move my dosing to evening for a few days before testing so I can have 12 hrs reading without doubling the dose just before the test as I have done before. As I don’t feel overdosed and felt awful when I dropped 6.25mcg for this test. So my ft3 reading was either artificially high before due to the time of the dose or maybe I need my ft3 to be very high to feel well.

What I’m surprised by is that the ft4 dropped from 17.9 to 14.8 in just over 2 weeks. I didn’t ‘plan’ for it to be this low on this test. Not sure if this is a difference between the labs or my body needed to convert more when the t3 dose was reduced?

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