Log in
Thyroid UK
94,987 members108,991 posts

Hypo, NDT and blood test timing

Hi all

Newbie here, I found you looking for RT3 information.

Quick question here. Many years ago, when I was on T4, I was told by a doctor to have my blood test first thing on an empty stomach and then do blood tests. I still do it now that I'm on NDT (once a day; that's changed now, but it was once a day when the blood test I refer to below was taken).

I keep reading wildly conflicting suggestions about timings of meds and blood draws. One FB group says to skip meds the day *before* and until just after the test (so, take Sunday, skip Monday, test Tuesday, take pills Tuesday after test); another group says minimum 12-15 hours after dose. Others say just do it first thing and take pills immediately after.

Wikipedia is letting me down with regard to half life of the T3 in NDT but says the elimination half life of liothyronine is 2.5 days. If it is similar, that suggests that a decent chunk should still have been there at the 24 hour mark.

Any thoughts? I had some confusing results and maybe the timing contributed. It was probably 24-25 hours after the dose at the time of the blood draw.

TSH - low, below range but detectable

FT3 - mid range

FT4 - a quarter of the way up the range

RT3 - high, out of range

Been feeling really hypo and awful lately. Doc changed meds a little after those results and I'm feeling a bit better.

Any thoughts?



21 Replies

Ergh I can’t type.

Doc recommended blood draw first thing, followed by meds.


Welcome to our forum and am sorry you have hypothyroidism. If you wish to edit anything in your posts, click on the down arrow next to More and select the one you want, this enables you to amend anything :)

First, your doctor was correct in how he told you to get the best results i.e. a gap of 24 hours between last dose and it test and take afterwards.

I know some people split doses of NDT but one of our Advisers always stated it should be taken once daily. The Adviser was a scientist as well as a doctor and would never, ever prescribe levothyroxine. He only prescribed NDT or T3 for thyroid hormone resistant patients. The reason he gave was that T3 has to 'saturate' the T3 receptor cells and its work lasts between one to three days.

Doctor Lowe took a blood test for the initial diagnose only. Thereafter the concentration was all on the relief of clinical symptoms and making patient well.

When you give results you have to state the numbers and you must also give the ranges. Ranges are important so that members can respond.

When blood tests are taken, they were introduce along with T4 only (levothyroxine) therefore when we take T4/T3 or NDT, they should only be used as guides as they contain more than T4.

The aim is a TSH of 1 or lower with FT4 and FT3 in the upper part of the ranges.

Read all the questions/answers on the following link and January 30, 2002 in particular.



Thanks Shaws. I will have to wait to read that until I’m on a slightly bigger screen. In the meantime, these are the results as best as I can recall them.

TSH - 0.05 (.5-5)low, below range but detectable

FT3 - 4.8 (3.1-6) mid range

FT4 - 12 (9-19) a quarter of the way up the range

RT3 - 607 (?? - 540) high, out of range

Will double check when back at desk a bit later, esp that lower range figure I’ve forgotten from the Rt3.




When we take NDT, blood tests could be looked at separately and patient considering 'How they Feel' on patircular doses. The reason being the blood tests are for the use of T4 alone. This is an excerpt from an expert on hormones:-

IF you continue to suffer from the symptoms of hypothyroidism, you have the right to demand that your physician instead give you T4/T3 (inactive/active) thyroid replacement therapy. Your physician can either add sufficient T3 (10 to 20mcg) to your T4 dose, or lower your T4 dose while adding the T3. The most convenient form of T4/T3 therapy, with a 4:1 ratio, is natural desiccated thyroid (NDT-- Armour, NP Thyroid, Nature-Throid). If you have persistent symptoms, you can demand that your physician change you to NDT and adjust the dose to keep the TSH at the bottom of its range-- when you have the blood drawn in the morning prior to your daily dose. This may be sufficient treatment, but IF you continue to have hypothyroid symptoms, and no hyperthyroid symptoms, demand that your physician to increase the dose to see if your symptoms will improve, even if the TSH becomes low or suppressed. You can prove to your physician that you're not hyperthyroid by the facts that you have no symptoms of hyperthyroidism and your free T4 and free T3 levels are normal in the morning, prior to your daily dose.

They may even be below the middle of their ranges. Your free T3 will be high for several hours after your morning T4/T3 dose, but this is normal with this therapy and produces no problems. You should insist that testing be done prior to your daily dose, as recommended by professional guidelines. It's simple common sense. TSH is not a thyroid hormone and is not an appropriate guide to thyroid replacement therapy. The hypothalamic-pituitary secretion of TSH did not evolve to tell physicians what dose

of levothyroxine a person should swallow every day. A low or suppressed TSH on replacement therapy is not the same thing as a low TSH in primary hyperthyroidism. If you have central hypothyroidism, the TSH will necessarily be low or completely suppressed on T4/T3 therapy; your physician must treat you according to symptoms and the free T4/free T3 levels."



It sounds as though my doctor is on the right track, and I've always done the blood tests like that, well, for many years.

Those figures above are okay, except I can't currently put my hands on the range for the RT3. It's maybe 140-540? Ish, anyway.

I read a thought recently that the single dosing of NDT might contribute to the RT3 being high - that the body sees the influx of T3 as "too much" and pulls back and turns some of the T4 into RT3 so as not to make it "too much more". In that case, splitting the dose might help that to come down a bit.

Thanks again



There are lots of misconceptions and rumours about RT3. It shouldn't cause any problems and this is an excerpt which I hope to reassure you. We get caught up sometimes in worrying when we're not improving as we think we should:

Excerpt from statement below:

Under normal conditions, cells continually convert about 40% of T4 to T3. They convert about 60% of T4 to reverse-T3. Hour-by-hour, conversion of T4 continues with slight shifts in the percentage of T4 converted to T3 and reverse-T3. Under normal conditions, the body eliminates reverse-T3 rapidly.

This is from Dr. Lowe:

"Dr. Lowe: Some readers will not be familiar with reverse-T3, and I know from experience that many others harbor misconceptions about the molecule. Because of this, I have summarized in the box below what we know about reverse-T3. I've answered your question below the summary.

Conversion of T4 to T3 and Reverse-T3: A Summary

The thyroid gland secretes mostly T4 and very little T3. Most of the T3 that drives cell metabolism is produced by action of the enzyme named 5'-deiodinase, which converts T4 to T3. (We pronounce the "5'-" as "five-prime.")

Without this conversion of T4 to T3, cells have too little T3 to maintain normal metabolism; metabolism then slows down. T3, therefore, is the metabolically active thyroid hormone. For the most part, T4 is metabolically inactive. T4 "drives" metabolism only after the deiodinase enzyme converts it to T3.

Another enzyme called 5-deiodinase continually converts some T4 to reverse-T3. Reverse-T3 does not stimulate metabolism. It is produced as a way to help clear some T4 from the body.

Under normal conditions, cells continually convert about 40% of T4 to T3. They convert about 60% of T4 to reverse-T3. Hour-by-hour, conversion of T4 continues with slight shifts in the percentage of T4 converted to T3 and reverse-T3. Under normal conditions, the body eliminates reverse-T3 rapidly. Other enzymes quickly convert reverse-T3 to T2 and T2 to T1, and the body eliminates these molecules within roughly 24-hours. (The process of deiodination in the body is a bit more complicated than I can explain in this short summary.) The point is that the process of deiodination is dynamic and constantly changing, depending on the body's needs."


I was feeling quite awful and it stood out as abnormal on the blood test. The Ft3 and Ft4 are actually higher than they have been in the past, and I didn't feel so awful then. If it shouldn't be as high as it is, then there must be a cause and is the cause the same thing that was making me feel so awful? I seem to be making a lot of the T4 I'm taking into RT3 where I'd prefer to be making more T3. :) Also, my resting metabolic rate is really low - well below the bottom of the normal range when measured, and that seems to be associated with high RT3.

I have had a change of meds and am feeling a bit better now.


Sometimes, if we've been on the same thyroid hormones for a while, we either need a small increase and sometimes there's been a change in the fillers/binders in the product of which we are unaware which can also affect us.

If you try taking one anti-histamine tablet one hour before your next dose and your symptoms are relieved, you may be sensitive to something within the fillers/binders. (I have read this). This would mean you'd have to try another make.

I am not in the least qualified - just a hypothyroid person. This is a guess:-


There is no other make as such, because it comes from a compounding pharmacy, but I think they'd be open to using a different filler. I did call them and ask if there was anything different about this batch, but they said no, there wasn't. These are hypo symptoms - cold, tired, achy, gaining weight. They don't feel like allergic/sensitive type symptoms.


You know your body better than anyone so I'd go on your instinct.


This is an excerpt from another post which I will link you to. I thought it may be helpful as coincidental it was about RT3 too:

"The rT3 ratio is psuedo-scientific mumbo-jumbo. It doesn't mean anything, it doesn't show anything, it doesn't prove anything. It was low because your FT3 was low and rT3 top of the range. It certainly wasn't responsible for making you feel bad. And, the reason you felt better when you started taking T3 is because it raised your FT3 - it's low T3 that causes symptoms. Your rT3 then dropped because your FT4 dropped - it usually does when you start taking T3. And your rT3 ratio then rose because your FT3 had risen and your rT3 had dropped. It's just a numbers game. Doesn't mean anything.




My T4 has always been low on just NDT. My doctor added some T4 to it a few years ago which got it into range, but didn't affect how I felt. The result above (12) is high for me, but really is not high at all.

The ratio thing bothers me because it could look really good even if the numbers themselves were really bad - really low T3 and really low RT3 could give the "at least 20" result, but wouldn't necessarily tell you anything useful.

It mainly stood out as I've been so unwell and it was abnormally high. I've been googling for information about the pattern of my results and mine doesn't seem to come up as one of the usual patterns.

We dropped the extra T4 and upped the NDT a bit and I am starting to feel a bit better. I think the extra bit of T3 is helping.


The blood tests were introduced for levothyroxine (T4) alone.

So if we take NDT or add T3 to T4 the blood tests cannot correlate. So it is how 'we' feel on specific doses which is the best judge. We can start on a dose and every 2 weeks add 1/4 tablet always taking note of symptoms being resolved etc.

Levo is T4 only - NDT is, T4, T3, T2, T1 and calcitonin. So on NDT I would imagine T4 result would be lower (it converts to T3) but T3 of course is the Active hormone and the other Ts have some input but not sure of how much.

One of our Advisers had the right idea. He only took one test for the initital diagnosis. He diagnosed patients as hypo or Thyroid Hormone Resistance. For hypo they were given NDT which was increased by 1/4 tablet every two weeks until symptom-free.

For Resistant patients - these usually need high doses which would knock others off their feet - Took T3 only.


Thanks shaws

Upping dose is a bit complicated as NDT comes only in capsules here, albeit from a compounding pharmacy that can make any dose wanted. I'll discuss with the doc next time I see him.


Yes best to see the doctor.


I think I'm quite lucky with my doctor - he's approachable and knowledgeable about thyroid, it seems.


Hang on to him then as he seems to be in the 'rare' column. :)


We always say to leave a gap of 24 hours for levo only, and 8 to 12 hours for T3 or NDT. Have your test early in the morning and fast over-night. TSH is highest early in the morning, and drops throughout the day. It also drops after eating. The FT3 is also at its highest early morning, but about two hours after the peak in TSH. But, as most doctors only look at the TSH, and rarely test the FT3, that's not so important.

I don't think timing affects rT3 in any way. Nor does taking your hormone before the test. The trouble with the rT3 test is that many things can cause it to be high, but the test won't tell you which it is. Your FT4 is not high enough to be the cause of high rT3, so you'll have to search for what it could be. Did you have a virus at the time of the test? Were you on a low-calorie diet? Did you have low ferritin? Have you had your cortisol tested? Etc. etc. etc. Yes, it could be the cause of the high rT3 making you feel bad, rather than the high rT3 itself. RT3 only stays in the body for about two hours, before it's converted into T2. But, no-one really know what it does during those two hours. However, it is inert, so shouldn't be causing trouble. :)


Thanks greygoose

I have been trying to lose weight, so maybe that affected things. My ferritin is okay but maybe I could add some more liver to my diet. I've had a series of irritating colds lately, but nothing serious. Cortisol hasn't been tested in years, and that's something I could investigate.

With regard to rt3 breaking down to t2, I read somewhere - but neglected to pay much attention to where I found that - that tsh was needed for that conversion, and as mine is low at present,maybe that's a factor also. If t2 is helpful for metabolism and weight loss, I haven't seen any evidence that I have much of it. ;)

Doc always tests tsh, ft3 and ft4. I'm in Australia, where that is covered after diagnosis.

The gap of 12 hours - I'd need to delay meds until 8-9pm and go to the lab for 8am. It keeps me awake. I wonder if that's an issue?



I'm sorry, I'm having trouble following you, there. Maybe what affected trying to lose weight? Or do you mean you've been on a low-calorie diet, which could have affected your high rT3? If it's very low calorie, then yes, it could. How many calories have you been eating?

rT3 doesn't 'break down into T2' it converts into T2. TSH is necessary for conversion of some T4 to T3, but conversion isn't that simple. I believe it's a different deiodinase that converts rT3 to T2, although I couldn't give you the details off the top of my head.

It has been suggested that T2 is helpful for weight-loss, but then so is T3. So as your T3 isn't optimal, that will be affecting your weight-loss.

Does your doctor pay attention to your TSH? Or does he have the thyroid knowledge to go by the FT3? The only reason for having blood taken early in the morning is to catch the TSH at its highest. If that doesn't matter, then you could have your test in the afternoon, or whatever times suits you best. As long as you leave 8 - 12 hours so that the FT3 is accurate. :)


Hi again

Doc looks at all the numbers, never just tests tsh. I'm becoming aware that I might be lucky with that.

I haven't been on a super low calorie diet, but I've been trying to lose some weight. I'm not doing it now, I'm focusing on nutrition, not weight loss until I'm more well.

I feel like I'm having noticeable improvements almost daily.

1 like

You may also like...