Blood testing when on T3 only: I recently wrote... - Thyroid UK

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Blood testing when on T3 only


I recently wrote on here about whether to have a private blood test when on T3 only, and I know some felt it wasn't needed, and up until now I haven't been too bothered either, as my GP lets me get on with it, but something has now happened that makes me ask what in fact happens to the FT3 if I do decide to go privately and have it tested?

Unfortunately I was caught on the hop a couple of weeks ago by the the hospital checking me when I was there for something entirely different, so I assume my FT3 was pretty high as I had just taken my T3 medication. TSH was suppressed of course, FT4 was 0.3 (range 11-24) and FT3 was 10.2 (range 4 - 6.8).

Am I right in thinking that if I had not taken the T3, say overnight, that the FT3 might have been within range? I know we are told on here never to take meds before testing, but would it be likely to drop quite significantly?

The specialist at the hospital, who is a Gastro, has now written to the GP suggesting that I am overdosed on T3. I am thinking of doing a Blue Horizon test privately on just the FT3 to demonstrate the difference and explain to my GP in case she contacts me. If I do this, how long should I go without T3 before testing? Would overnight be enough? Obviously being T3 only I feel the lack of T3 pretty quickly.

Thank you in advance to anyone who knows.....

10 Replies

Agapanthus, it is important to have thyroid bloods tested when on any thyroid replacement including T3 only. If a GP practice will only test TSH & FT4 I would encourage someone on NDT or T3 only to have private FT3 tests.

T4 and T3 can peak in the serum for up to 6 hours so advice is to leave 24 hours between last dose and blood draw to avoid peaks of recently ingested T4 and T3 being measured. That can be partiularly important to people trying get a dose increase. If you really can't manage 24 try 12 hours. Most T3 should be out of the serum in 6/12 hours.

I very much doubt your FT3 would be within range if you had left 12 or 24 hours between last dose and blood draw. Another member has been experimenting and taking T3 9, 6 and 3 hours before blood draw. She has found FT3 only varies by a few decimal points ie 5.2, 5.5, 5.8 not in whole numbers.

It was explained to me that leaving 24 hours and extrapolating the FT3 result by +20% gives a reasonable estimate of normal circulating FT3 ie FT3 4.5 24 hours after last dose would be 5.4. If I do that in reverse and reduce your 10.2 by -20% the result is 8.16 which is still over range. I'm really not sure of the validity of doing that though :)

My FT3 was over range 8.4 (3.2-6.2) 24 hours after last dose. T3 was halved to 20mcg for 2 weeks to allow levels to drop quickly and I then resumed 30mcg. 3 months later FT3 was 4.5 24 hours after last dose. I'm fairly confident that FT3 dropped into range 7 weeks after reducing dose as hair shedding stopped.

Agapanthus in reply to Clutter

Thank you Clutter for your detailed reply. So you were on 40mcg T3 only out of range and are now on 30 mcg T3 and are now in range?

How are you feeling on that dose? I guess I am worried that if I reduce the dose that I will feel hypothyroid, but it would be interesting to try that I suppose, though I think I would prefer to do it slowly and not cut it so drastically. I was on 55mcg until a few months ago and I did drop to 50mcg because I knew my adrenals were struggling too.

Also interesting what you say about the hair shedding situation. I gather that can happen on both too much and not enough thyroid meds. I do have hair shedding at the moment, but it does vary. One thing that changes is that when I added in T4 last year and reduced my T3, my hairs on my legs stopped growing, and under my arms. When I went T3 only the hairs began to grow again. At the moment though they are not so good so maybe it's a delicate balance getting it just right.

However my BP is on the low side, and my temp and pulse are OK.

Clutter in reply to Agapanthus

Agapanthus, I'm on T4+T3. Because FT3 was high I reduced T3. I have to juggle a bit because TSH needs to be suppressed. 75+20 didn't. 75+40 TSH suppressed but FT3 over range. 75+30 bloods ok, and I didn't feel hypothyroid. I became a little more tired but really not sure whether it was the dose reduction or the darker, shorter days late September. Raised T4 to 100 end Dec as tiredness and cold were annoying but again may have been due to winter. Had to reduce T4 from 100 to 75 last summer as felt a bit too hot on some of the very hot days but with hindsight I should have reduced T3 instead. Will see what bloods say in June on 100+30.

Hair's been returning to lower legs and pits since I added T3 to T4 early 2014. Loss of body hair was the only good thing about T4 only but I was too unwell to care about things like that at the time, although I was annoyed by the hairs on my chin.

It is such a fine balance. I was shedding hair on T4 only but when FT3 was over range it was clumps of loss and nails, which had been good, became very weak and flaky. I didn't feel overmedicated but I did have very slight hand tremors. I know what to look out for now. Tremors and a full hairbrush are signs and cheaper than private blood tests :-D

Agapanthus in reply to Clutter

It really does sound like a fine balancing act. I haven't had such extreme symptoms as you describe (ie hair shedding but not in clumps, nails OK) and just have never recovered my previous stamina, but then I have a diagnosis of ME and think there are other things happening that maybe I don't understand.

Maybe I should try Thyroxine/T4 again, though at the moment I am thinking of possibly mixing some NDT with T3 (reducing the T3) as that would lower my T3, and also add in some T4, and it's the one thing I haven't tried so far! I don't really want to go fully NDT as the T3 is free on the NHS.

It is entirely likely that the GP will ignore the letter from the consultant - we will see.....

Clutter in reply to Agapanthus

Agapanthus, If Levothyroxine+T3 didn't suit, NDT+T3 is worth trialling. Some people don't tolerate any form of thyroxine though.

I've been on T3 only for up to 3 months at a time on two occasions but I like the security of having T4 in my system should I be unable to take T3 for a couple of days for any reason. I'm thyroidless so dependant on replacement.

I've still to recover my energy and stamina after being largely bedbound for 15 months. It is slow but I hope to get there.

Agapanthus in reply to Clutter

I am sorry to hear you are so ill, Clutter. I hope that your careful work with T3/T4 will bring the changes you need.

For myself, it's always seemed unlikely that I should suddenly be not OK on Thyroxine having been on it for 17 yrs but as I have developed multiple food intolerances it may well be the fillers rather than the the actual hormone.

At the time I moved onto T3 only I was certainly low on FT3, so there was something wonky there, and so I was deficient in some way.

However, like you, I would prefer to have SOME T4 in my system for security in the longer term.

Incidentally, my recent thyroid test only showed and FT4 of below 0.3, and I do still have my thyroid gland (though have been hypothyroid for 20yrs). If we take T3 only does this suppress T4 production entirely? What I mean to ask is - is this really all my thyroid gland will produce now on FT4 (as I am not taking any in of course), or is the intake of Liothyronine telling my thyroid gland not to bother?

Clutter in reply to Agapanthus

Agapanthus, I'm not ill now but I do need to regain strength and stamina. FT3 was below range before I added T3 but the best thing is that T3 calmed the adverse effects Levothyroxine caused.

Basically taking Liothyronine only means your thyroid doesn't have to bother. You don't need T4 to convert to T3 because you are taking T3 orally. My FT4 is mid-range on T4+T3 combi but lower than it was on T4 only.

T3 suppresses TSH. Suppressed TSH doesn't stimulate the thyroid to produce T4. If you stop taking T3 your thyroid will produce more T4, probably not much if you've been hypo 20 years. I hope the NDT+T3 works for you.

Agapanthus in reply to Clutter

Thank you very much for that explanation Clutter, that is very helpful.


Yes, if you'd just taken your T3, it would have skewed the results. A recommendation of 24 hours between taking thyroid hormones and blood tests is best. I did the same as you and my GP nearly had a heart attack himself but before he could say anything, I jumped in with 'ignore blood test please as I took hormones just before'. So he relaxed and a new blood test was given - I had alarms everywhere the day before and morning of test. Test was good and he was happy to prescribe.

Thank you Shaws. I think I will at least try the 24 hrs check and do the test privately and see what comes up as a result. Though as Clutter has said, my T3 wasn't just over the line, so I am guessing it will still be on the high side.

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