Thyroxine Absorption Test: I just had a telephone... - Thyroid UK

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Thyroxine Absorption Test


I just had a telephone consultation with a new endo (UK). He asked me if they could do a Thyroxine Absorption Test on me.

The test involves me taking 750 mg of thyroxine, and then having blood drawn every couple of hours for six hours.

I asked him what would happen if the test showed that I didn't absorb thyroxine. Would I be prescribed Liothyronine? No he said, that wouldn't be possible, so I asked him what was the point of the test.

I have searched for anything on here regarding the test, but found no mention of it. So I wondered if anybody has been offered the test, or taken it?

30 Replies

It's done in research and if they suspect you are not absorbing levothyroxine. Are you on levothyroxine? If you are taking T3 also it may confuse your blood test results lowering fT4.

Hypopotamus in reply to jimh111

I'm on NDT. My TSH has gone up to over 30 since changing to a different brand, and I am going to try adding some thyroxine.

The guy that I spoke to is new, young, and obviously keen to understand why I want/take anything but thyroxine.

Hey there

Sorry to read about the duff NDT more like an UNTRU - thyroid hormone replacement.

I was offered a similar test by an Endo 8 years ago. He told me that its purpose was to gauge cellular uptake. I declined.

Unless they promised me something else if the test showed that I couldn't absorb thyroxine, it doesn't appeal to me either.

They could possibly try you with liquid Thyroxin? if you dont absorb well through the gut. Ive heard another HU colleague is having some success with it 😊

Thanks, that may be an option. My other thought is that although I was definitely not responding to thyroxine 20 years ago, if could be possible that 20 years of Liothyronine, and then NDT, has kickstarted by receptors. I will know when they do another blood test in the new year.

Hi Scrumbler

why did you decline?

helvellaAdministrator in reply to Scrumbler

If I am reading that right, that was an inappropriate test.

A thyroxine absorption test gives an idea as to how much gets from gut to bloodstream. But says effectively nothing about how much gets from bloodstream to cells. (Leaving aside the issue that the thyroxine (T4) needs to be converted to T3 before it is available to be taken up by cells and and having the required effect on the T3 receptors.)

On the basis you were given, I too would have declined it.

Hypopotamus in reply to helvella

That also occurred to me. I'll try and remember to mention it next time that I speak with them.

DippyDame in reply to helvella

Hypopotamus I agree with helvella.

As someone with RTH my understanding is that I can have a high level of T3 sloshing around in my serum with no symptoms of overmedication - it does nothing there unless and until it reaches tissue level where it gets to work. This point seems to be missed.

With any form of RTH a high level of serum T3 is needed to act as a force against the hormone resistance and if that force (dose,) is great enough it will "push" some of the T3 into the cells/tissues where the receptors, will respond ....only then does it get to work. The excess T3 will be excreted.

There is no test, I understand to check the tissue level of T3 and dosing has to be based on symptoms.

I suspect the two ( absorption) issues are often confused i.e gut to serum and serum to tissue.

I reached a dose of 200mcg with no symptoms of overmedication, that lasted for several months until I did become overmedicated with obvious symptoms. It appears that the high dose eventually kick started dormant receptors,...and I felt the consequences!

I stopped all T3 for 5 days and started to increase from 25mcg. 62.5mcg which is getting close to optimal. I suspect 75mcg might be enough this time around.

I too would have declined the test!

Background is on my profile. I was on levo for 20 years.

Sorry , bit of a ramble, brain on go slow at this hour!

Good luck.

helvellaAdministrator in reply to DippyDame

Always looks like there is an assumption that sufficient of something in the bloodstream always ends up with sufficient in the cells. We do hear of insulin resistance - but that is about it.

I do accept that the thyroxine absorption test can help to identify problems absorbing from the gut. Even then, understanding is required.

DippyDame in reply to helvella

I'm sure the absorption test has some value by revealing any gut to serum problem....but that won't help reveal any serum to tissue problem and I guess that"s a problem that's more common than is understood....or a possibility that isn't considered by many medics unless they have positive results for the alpha and beta receptor genetic test. ......overlooking the possibility that as yet undiscovered genetic variants may be at a geneticist suggested to me.Down to trial and error again which is how I eventually found recovery with the help of Dr Lowe's work....and his followers..... Low tissue T3.....possibly the cause of more problems than is research has now revealed this in a number of CFS patients

Sorry .....back on my soap box!!!!

helvellaAdministrator in reply to DippyDame

One of my concerns about gene tests is that unrecognised variants will be totally missed as you so rightly point out.

Yes, of course gene tests can identify the known variants and that alone can be very helpful. But if they miss unknown variants and combinations of variants, they could well serve to exclude people from diagnosis. The test for the DIO2 variant is an example - if you don't have that, any chance of getting T3 reduces, probably to zero.

DippyDame in reply to helvella

I tested positive for the Dio2 polymorphism....homozygous, so a greater impact on conversion...because of RTH I need a high dose of T3 I buy my own....endo maintained thyroxine would made me ill in the first place!Dio2/homozygous + RTH = trouble!!!

I'm not sure that some variants are missed so much as that they have not yet been discovered and fully understood.....too complicated for me!!

helvellaAdministrator in reply to DippyDame

I did mean missed because we don't know about them rather than known ones being ignored - though that is also a possibility.

DippyDame in reply to helvella

We're on the same page!

Hillwoman in reply to helvella

That was my thought too. :-)

DippyDame in reply to Hillwoman

Just re- read the sentence.....brain not yet in gear this morning!Use of "missed" threw me!

Apologies to helvella

helvellaAdministrator in reply to DippyDame

Not needed! :-)

DippyDame in reply to helvella

Note to self. "I must not be an old fuss pot"....write 100 times on the blackboard!!

My first thought was, how can we miss something if we don't know it's there!


This is a paper published in 2019 - well, abstract. Be aware that pseudomalabsorption is a term meaning the patient didn't take the levothyroxine as they were supposed to.

The Levothyroxine Absorption Test: A Four-Year Experience (2015-2018) at The Mayo Clinic

Kristen M Gonzales 1 , Marius N Stan 1 , John C Morris 3rd 1 , Victor Bernet 2 , M Regina Castro 1


• PMID: 31680654

• DOI: 10.1089/thy.2019.0256


Background: Levothyroxine (LT4) is the mainstay of therapy for hypothyroidism. Yet, despite physician efforts at dose titration, some patients remain hypothyroid on LT4 doses in excess of weight-based calculations, a condition known as refractory hypothyroidism. The LT4 absorption test (LT4AT) has been proposed to have utility in these patients by enabling distinction of LT4 malabsorption from pseudomalabsorption, a condition of intentional nonadherence. Given its rare use in clinical practice, we reviewed our institution's experience with the LT4AT to assess its impact on management of refractory hypothyroidism.

Methods: We reviewed the charts of 16 patients diagnosed with refractory hypothyroidism and who had completed the LT4AT between January 2015 to January 2019. The primary aim was to determine the utility of this test in distinguishing LT4 malabsorption from pseudomalabsorption. Secondary aims were to determine whether the results of this test impacted physicians' management decisions, as well as to report on clinical outcomes at follow-up. Our LT4AT is a six-hour test wherein patients receive a weight-based dose of LT4 followed by serial measurements of total thyroxine (TT4) and thyrotropin (TSH). Percentage absorption is calculated using the following formula, with normal absorption being ≥60%: [Formula: see text]

Results: Percentage absorption was calculated in 13 of 16 patients due to lack of TT4 data for 3 patients. Absorption was impaired in one patient (% absorbed = 0), who had known causes of malabsorption. The remaining 12 patients had normal absorption by hour 4 of the test (% absorption 60-158) in conjunction with upward TT4 trends. Clinical follow-up ranged from 1 to 32 months (median 6.5 months), with 11 patients having follow-up data. Six of these had normal or suppressed TSH values at most recent follow-up, and four had improved but persistent TSH elevations. The one said patient with malabsorption improved with intravenous LT4.

Conclusions: The LT4AT can provide valuable information for distinguishing malabsorption from pseudomalabsorption. Our findings support the combined use of calculated percentage absorptions with TT4 trends for at least a four-hour time frame when making determinations regarding absorption.

Keywords: LT4; levothyroxine; levothyroxine absorption test; malabsorption; pseudomalabsorption; refractory hypothyroidism.

Full paper is behind a paywall but the abstract is accessible here - as well as links to other papers on the same subject:

Hypopotamus in reply to helvella

Interesting. It makes me wonder if he wanted to help, or was doubting that I had ever taken the thyroxine.

Grasping at straws maybe!

re. 'doubting you'd taken the thyroxine'............I hesitated to mention that possibility when i read your post yesterday, because i didn't want to give you reason to feel angry with the doctor when this might not be the case. But whenever i have seen this test suggested in medical texts , reading between the lines , it has always seemed to me that the main reason is to rule out the possibility that the patient is simply not taking the thyroid hormones as prescribed, and this would explain the high TSH results, so they make the patient take it under supervision, and then check that taking it does effect the thyroid function tests. They won't make it obvious that it's 'under supervision'... but their instructions for how to perform the test will make certain that you are observed to take the medication

helvellaAdministrator in reply to tattybogle

This is a case report of someone with gastroparesis not adequately absorbing levothyroxine. It appears they only did a very brief version of the thyroxine absorption test - not testing every two hours but only once - probably perfectly adequate in the circumstances.

There are several broadly similar reports available.

Case Rep Endocrinol. 2016; 2016: 1316724.

Published online 2016 May 16. doi: 10.1155/2016/1316724

Levothyroxine Tablet Malabsorption Associated with Gastroparesis Corrected with Gelatin Capsule Formulation

David P. Reardon * and Peter S. Yoo


Treatment of hypothyroidism with levothyroxine sodium often requires multiple dose adjustments and can be complicated by patients with gastric and intestinal dysfunction that limits absorption. In these cases, doses are often titrated higher than commonly used in clinical practice. Multiple formulations of levothyroxine are currently available and some may be preferred in cases of malabsorption. We report a case of a 42-year-old female who presented with a living unrelated kidney transplant evaluation with myxedema while being treated with levothyroxine sodium tablets. She was noted to have gastroparesis secondary to Type I diabetes mellitus which may have contributed to levothyroxine malabsorption. Changing to a gelatin capsule formulation quickly corrected her thyroid function assays. This case suggests that gastroparesis may affect absorption of levothyroxine tablets and the gelatin capsules may be an effective alternative therapy.

Rest of paper freely accessible here:

tattybogle in reply to helvella

Yes , you're right ... i should try not to be so cynical. There's plenty of 'helpful' reasons why they might do this test.


"We report a case of a 42-year-old female who presented...//..... with myxedema while being treated with levothyroxine sodium tablets."

Does 'myxedema' mean they actually examined this woman, like proper doctors used to ?

Or is it just a different way of saying out of range blood tests

helvellaAdministrator in reply to tattybogle

Difficult to be sure!

When even the medical terms are used with such carelessness we always have to be aware that it could be either end of the spectrum of meanings.

Hypopotamus in reply to helvella

Gelatine capsules! At least they were investigating why she wasn't doing well on thyroxine.

Don't worry, I always try to give the other party the benefit of the doubt when it comes to their motive.

I have written a polite letter to the endo department pointing out the folly of doing the test in my case. Fortunately I do have a good relationship with the head endo, having seen him for 15 years now. It was one of his underlings who phoned yesterday.

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