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Thyroid UK
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Test result help please?: Low TSH and Low T3

Morning all

I'm on T3 only (80mcg) and was feeling a bit tired of late and it has been 4 months since my last period. I seen the doctor on Monday (after a 45 minute wait) followed by blood test. GP was a bit dismissive because she was running late and said the period thing most likely to do with thyroid. Anyhoo, a myriad of tests came back last night including thyroid:

Results that came back normal:

Serum testosterone level 1.99 nmol/L [0.5 - 2.6]

Haemoglobin A1c level - IFCC standardised 28 mmol/mol [20.0 - 41.0]

Free Androgen Index

Serum testosterone level 1.99 nmol/L [0.5 - 2.6]

Serum sex hormone binding globulin level 57.9 nmol/L [18.0 - 144.0]

Free androgen index 3 [2.0 - 10.0]

Serum follicle stimulating hormone level 4.4 iu/L

Result Checked

FSH & LH guide: Follicular FSH 3-10,LH 2-13, Mid-cycle FSH 3-33,

LH 9-76, Luteal FSH 2-9, LH <17, - Menopausal FSH >22 U/L

Serum LH level 9.5 iu/L

Result Checked

Thyroid Results:

Serum TSH level 0.01 mu/L [0.55 - 4.8]

Below low reference limit

Result Checked

Serum free T4 level NA

Serum free triiodothyronine level 2.8 pmol/L [3.5 - 6.5]


Below low reference limit

Patient on T3 but TSH now completely suppressed with apparently

low FT3 levels. ?T3 formulation. This assay may not be able to

detect exogenous T3 in the formulation given. TSH may be the

best indication of adequate / over-treatment for this patient.

However, this sample has been sent to another laboratory for

TFT measurement by an alternative analytical platform to

investigate possible assay-issues.

I had a similar result in March/April where my TSH was low/suppressed (which is good, I have Hashi’s) but T3 was also low. As per above they are sending the sample to get tested again so maybe a false reading? Any reason why my T3 could be this low? I just don’t want to get fobbed off by the GP’s but this is unfamiliar territory.

Thanks all

11 Replies

The fT3 figure is almost certainly low because you are leaving too long a period between your last L-T3 tablet and having the blood taken. It's possible that you are not absorbing the L-T3 well but in that case your TSH wouldn't be low.

Ideally I'd leave about six hours between taking my L-T3 and having the blood taken, assuming you take the L-T3 twice a day.

1 like

Thanks jimh111

I take 80mcg every morning. My blood test was at 10:15am on Monday morning so I didn't take a dose before then. Is that what has scuppered it?


Yes. It is very difficult, especially as you take it once daily. In any event as you need a supra-physiological dose it is important to pay far more attention to your signs and symptoms. Presumably you have some form of peripheral (non-pituitary) hormone resistance. The slight problem is that this can affect different organs to different extents, so they need to keep an eye on your heart, or possibly prescribe a low dose of beta blocker such as sotolol as a precautionary measure. (If you have your blood taken around mid-night you will get a reasonable fT3 figure. Ha Ha! ).


I can't find anywhere where you say how much T3 you're taking. Perhaps it's just not enough.


80mcg :)

As per Jim111 I didn't take my dose on Monday morning until after the test. Is that the reason? I take it all in one go as I've tried split dosing and never fit me well.


You should leave about 12 hours between your last dose and the test. I take mine all in one go, too. I take 75.

If you take it too close to the test, all you are measuring is what you've just taken. If you leave longer, as T3 leaves the blood quite quickly, you will get a false low. If you usually take your T3 at 8.0 am, say, and you got tested at 10.30 the following day, that's 26.5 hours, which is too much. The T3 has all left your blood, and you get a reading something like you would have had before you started taking T3, which doesn't help in telling you if you're on the right dose. You need a happy medium to get an idea of what the average circulating T3 is.

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Thanks eljii. I'll do some reading up on that , I'm not so clued up on the role of cortisol.


I'm sure the low reading is due to the >24 hour gap between taking your previous dose and taking the blood. The problem is, if you had taken your T3 in the morning as usual your T3 reading would have been so sky high that any doctor seeing it would have thrown a fit and promptly taken you off the T3.

I'm not sure what the answer is. If you wanted to get a result which pleased your doctor you'd probably have to change the time you took your T3 in preparation for the test, but when that would be? I have no idea.

How do you feel?


Tired and sore but not ridiculously so (not like I used to be on T4). Would the T3 really plummet that low? Thanks so much!


I take NDT and T3 in three doses per day. I stopped taking my meds for 24 hours and my last Free T3 was 3.1 pmol/L (3.1 - 6.8), so yes it can drop a huge amount. If I'd been on T3-only it probably would have dropped even more.

For someone on T3-only there are people on far more than 80mcg. It isn't really that high. Dr Lowe used to take 150mcg T3 during the night, and I think I read he did that for about 25 years (don't quote me on that).

1 like

T3 in the blood has a half life of about 24 hours. The manufacturers of liothyronine tend to say it has a half life of up to 48 hours. As you took your L-T3 a little over 24 hours earlier my best guess is that your 'real' fT3 would be closer to 5.0 rather than 2.8.

If you take your L-T3 within an hour or two of having the blood taken you get a silly result. Due to the short half-life of T3 it is difficult to get a realistic fT3 figure, the tT3 and fT3 levels fluctuate in a sort of sawtooth pattern. I think the best we can do is have the blood taken about half way between two doses, about five or six hours after a dose if you take L-T3 twice daily or about 10 hours after if you take it once a day. (half live decay curves are not linear, so the mean level occurs less than half way between doses).

It's very difficult to make sense of blood tests when taking T3 medication. The TSH is the best indicator as it reflects the action of T3 on the receptors in the pitutary. TSH is often suppressed when patients are taking L-T3, probably because they are taking a TSH suppressive dose.

A typical L-T4 dose of 120 mcg would correspond to 40 mcg L-T3. As you are taking 80 mcg L-T3 this is about twice as much hormone as a normal healthy person would have. Unfortunately some patients need this amount. It is then a question of recognising you are on a supra-physiologic dose and taking care to closely monitor your signs and symptoms, especially making sure that you do not develop a rapid heart beat or atrial fibrillation. There are patients who require high dose T3 medication just to be relatively normal (I'm one) but we should also recognise we need to take care. The endocrinologists kick up a fuss but these sorts of decisions are made all the time in other branches of medicine.


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