High & Low Cortisol Effects on Thyroid Hormone, and Dispelling an Internet Myth

Higher cortisol levels reduce TSH (through the effect on the hypothalamic-pituitary system). A reduced TSH does two things:
  1. For those patients with working thyroid tissue, lower TSH will decrease thyroid hormone production.
  2. Lower TSH also reduces T4 to T3 conversion, resulting in even less T3 (active thyroid hormone)

Lower cortisol levels increase TSH. An increased TSH does two things:
  1. For those patients with working thyroid tissue, higher TSH will increase thyroid hormone production.
  2. Higher TSH also increases T4 to T3 conversion, resulting in even more T3. Studies have found that in severe cortisol deficiency, the TSH and FT3 are often high. Cortisol (HC) supplementation in these people, normalises the TSH and FT3.

We do know that cortisol and T3 are both required to increase mitochondrial energy production. So a lack of cortisol is likely to reduce T3’s effectiveness in the mitochondria. But there are probably other mechanisms at work, yet to be discovered 

T3 and cortisol both need to be at good levels. Cortisol increases T3-effect, and T3 increases cortisol-effect – they are in a partnership within the cells. T3 helps to keep cortisol levels up as it stimulates the hypothalamic-pituitary system more than T4. This latter point is why thyroid medications that contain T3 help to keep cortisol levels higher. Low cortisol is very common in thyroid patients, as many have lower T3 levels during the day and in the night than they had when they were well (for reasons I have discussed in my books and in various other blog posts). For all the reasons above, it should be clear that with low cortisol, T3 does not work as effectively as it should do. High cortisol also causes problems and can reduce the effectiveness of T3 within the cells, hence thyroid patients with high cortisol often complain of feeling hypothyroid even when they appear to have reasonable FT3 levels.
However, I know of no evidence whatsoever that T3 is less able to enter cells when cortisol is low. That would require low cortisol to affect the T3 transporter molecules in the cells’ membranes. There is no evidence that low cortisol does this, and I do not know of any researchers who believe this (and I have spoken to some about this specifically). I stated this clearly in the first edition of Recovering with T3. I said that cortisol was not required to allow FT3 to enter the cells. 

If someone comes to me and says that their FT3 level seems quite good or high, but they are not feeling well still, it simply means that there is another problem that needs to be resolved. If low cortisol is behind the problems, it can result in slightly higher FT3 due to the mechanism that I have described above.

The bottom line is that a high FT3 on a T3 combination treatment is not unusual, and this does not mean the T3 is not getting into the cells (‘pooling’ as some people call it). The T3 is still getting into the cells. The issue is simply that having enough FT3 is no guarantee that metabolism is going to work correctly. Many other things need to also be right, including the cortisol level, as T3 and cortisol work synergistically.

As I stated in the opening paragraph here, low cortisol does have some effect on raising TSH, which also increases T4 to T3 conversion. So, people whose cortisol is low may get a small amount of extra FT3 from this. But their cellular FT3 will also be a little higher too. It isn’t that the T3 cannot reach the cells.

When someone is still not feeling well, and their FT3 levels look good to high, it could be assumed that T3 is not getting into the cells. There are many reasons for metabolic rate not to be correct, including low cortisol.

However, low cortisol does not prevent the entry of T3 into the cells. The ‘pooling’ concept may scare some patients to rush into cortisol supplementation because many thyroid patients have come to associate this word with low cortisol issues. Yet, there can be many reasons for thyroid treatment not working. Yes, these need to be resolved. However, the reason for thyroid treatment not working is not always low cortisol.

A good FT3 level does not mean the T3 is not available in the cells – it will be. I have written about many reasons for thyroid treatment not working properly in my latest book, The Thyroid Patients Manual. Please see:
https://paulrobinsonthyroid.com/the-thyroid-patients-manual/ for more information on the book.

If someone is on too much thyroid hormone treatment, but it is not working well, and the issue that fixes the problem is resolved, they can feel hyper-thyroid. This is not a sudden rush of FT3 into the cells – the FT3 is already there!  It is simply that they were taking too much thyroid hormone, to begin with. The issue that has been resolved has simply allowed the biologically active T3 to do its job.

It is important to not rush into using synthetic cortisol (HC, Cortef, adrenal glandulars), as soon as thyroid treatment is not working and FT3 appears to be good-high. Cortisol needs to be tested fully (cortisol saliva testing and an 8:00 am morning cortisol test), and other vitamins and minerals need testing – see The Thyroid Patient’s Manual for details on these.

The word ‘Pooling’ has been used a fair amount on the Internet. However, there is no evidence to support this idea. It is just another Internet myth that has been spread for too long. If I could be given $1 or £1 for every person that spoke to me and said, “I have been told I am pooling, and need to begin using hydrocortisone”, I would be a rich man. What is always important to focus on is what is stopping a treatment working, fixing it, and getting the treatment to be successful. 

To make matters even clearer, there is new research as of 2021 that has actually tested whether hydrocortisone has an inhibitory effect on T3 transport into cells. They discovered that LOW doses of hydrocortisone do not inhibit T3 transport into cells via the MCT8 transporter, but HIGH levels do inhibit T3 transport into the cells. Also, Dexamethasone, a steroid that also activates cortisol’s receptors, is more potent than hydrocortisone as an MCT8 transport inhibitor.

These recent research findings confirm that the ‘pooling’ idea is incorrect. The research is saying that high cortisol levels actually prevent some level of T3 transport into the cells and that low levels of cortisol do not do this. See Cosmo et al, 2021: “Dexamethasone and Some Commonly Used Drugs Inhibit MCT8-mediated T3 Transport in Vitro.” This might go some way to explaining why some patients with low cortisol do not respond well to hydrocortisone, as it might inhibit T3 entry into the cells.

I hope you found this helpful.

Best wishes,


Paul Robinson

Paul Robinson is a British author and thyroid patient advocate. The focus of his books and work is on helping patients recover from hypothyroidism. Paul has accumulated a wealth of knowledge on thyroid and adrenal dysfunction and their treatment. His three books cover all of this.

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  1. Barb on 30th June 2020 at 10:36 pm

    I am on NDT (30 in am and 30 in pm) I am also on Cortef. I have tested my cortisol via saliva twice and both times my cortisol showed I was very low. I am presenting as Hyperthyroid. Every time I try to lower the ndt (I am not taking much) because I am presenting high my blood pressure goes up. When I got off of Cortef for several months, that too raised my blood pressure. I am back on both again and I know it will show me as hyperthyroid but my blood pressure is back to normal. So how is this phenomenon possible.

    • Paul Robinson on 1st July 2020 at 10:12 am

      Hi Barb,
      That is a very specific question and I would need a lot more information to answer it.
      This website is not the best way to go about discussing such specific details.
      For example ‘presenting as hyperthyroid’ – if this means your labs suggest it, well I don’t think lab test results are always the best indicators, e.g. low TSH should not be used to suggest ‘hyperthyroid’ and even high FT3 can be misleading. If you mean hyperthyroid because you have high heart rate, this could be because your cortisol is low and your body is compensating with adrenaline.
      Basically, there isn’t enough information above to tell and I can’t discuss more details on the site via comments to blog posts. I do offer 1-1 coaching and this is the only way in which I focus on individual people and try to assess what might be going on.
      I do suspect it is all perfectly explainable though.
      Best wishes, Paul

  2. Suzanne on 25th July 2020 at 12:37 am

    Hi Paul, I have been taking 5mcg of t3 for a few months now. I live in the tropical rainforest and considering moving because I now have mould illness which has caused inflammation full stop to cut a long story short I am 10 kilos overweight. I see an integrative Doctor Who’s tried to manage my levels however there’s really been no significant changes over the last 12 months. We have tested my reverse T3 many times and it’s been up and then it’s been a good level full stop the last 2 months I have quickly put on another 2 kilos and so have decided to increase my medication 15mcg. I have been on this amount for one week and it is not making any difference to my weight I just can’t lose any. I eat gluten free and a clean whole foods diet. I must have missed I don’t do much exercise as I have suffered chronic fatigue for many years and of course it made me feel worse to do too much. Having said that I am just able to tolerate more thyroid hormone because I am now on iron tablets 3 days per week. We finally found the right amount to sustained me. For as long as I can remember my iron levels are always low and the symptoms from that have been not good as you would know. So surprisingly I’m now able to tolerate more than 5mcg. It’s been a week now that I it’s been a week now and I have had no adverse effects from increasing the T3 dose. However I I’m still not losing any weight . I am not on any other medication except for iron and zinc and some other supplements. I am postmenopausal and was on all the bioidentical hormones for many years. I was given hydrocortisone for 12 months about 5 years ago and it ruined my life. I went from being a size 8 to a size 16 in a few months. That drug absolutely terrifies me!! What I am asking is do you think I should wait another week or two and see if I can push the T3 by another 5mcg, considering I am now tolerating it with good iron levels. Thank you so much for your page.

    • Paul Robinson on 25th July 2020 at 8:40 am

      Hi Suzanne, it sounds like you’ve had quite a struggle but might finally be on the right track. I obviously cannot work on detailed suggestions on this site but low estrogen and progesterone would not be helping with weight loss either. T3 takes time to slowly find the right dosage and 15 mcg is very little T3 if the person needs more thyroid hormone. So, patience is usually needed because it is important to go very slowly with thyroid hormone changes for safety reasons.
      I wish you the very best of luck!

    • Grace on 30th December 2020 at 2:33 am

      Suzanne, how did you get off the hydrocortisone?

  3. Tracey catchpole on 19th August 2020 at 1:50 pm

    I’ve been on T3 only treatment for ten years.. Feeling very unwell.. I go up and down it never makes a difference.. Dr’s have told me that it’s in my blood but they can’t help me get it into the cells and that I’m one of 10% of people that it doesn’t work for. My health is just declining as I stay hypo…would like to have call with Paul can someone please tell me how to arrange this.. Many thanks.

    • Paul Robinson on 19th August 2020 at 5:31 pm

      Hi Tracey,
      Things sound tough indeed.
      See the front homepage of this website there is a short section near the bottom of the page about coaching and a contact button.
      Best wishes,

  4. Jerry R on 12th January 2021 at 4:05 am

    Hi Paul, enjoyed your post. Quoting from your post, “Lower cortisol levels increase TSH. An increased TSH does two things: For those patients with working thyroid tissue, higher TSH will increase thyroid hormone production. Higher TSH also increases T4 to T3 conversion, resulting in even more T3. Studies have found that in severe cortisol deficiency, the TSH and FT3 are often high. Cortisol (HC) supplementation in these people, normalises the TSH and FT3.”

    It is possible that lower TSH can increase cortisol levels? Not sure if that would be an up or down stream effect? If something was suppressing thyroid hormone production, I’m wondering if might increase cortisol?

    • Paul Robinson on 12th January 2021 at 10:28 am

      Hi Jerry, I don’t believe that that is a given, no. If the person is hypothyroid in terms of their symptoms, their low cortisol might be linked to this. In this case, having more thyroid meds and a higher FT3 might help raise cortisol. But the connection isn’t as clear cut as the other way around as I describe in the post.
      If you have low cortisol I would read my posts on CT3M (my protocol for helping to resolve low cortisol).
      Best wishes, Paul

  5. Coral on 20th July 2021 at 3:25 pm

    I have been on Synthroid for nearly 50 years. This treatment has always worked well for me, until a few months ago. All of a sudden, I could not tolerate taking any of it for a few days. Over the last 10 years or so, I had been slowly lowering the dose as I would feel too hyper. I’m 69, and was down to taking 50 mcg daily. Occasionally, I would feel too hyper, and skip the next day or two, and then feel ok again.
    But this time, I could not go back without severe dizziness and chest pains. I gradually tried taking smaller doses, but nothing worked. I have no function in my gland, and my TSH went up to 34 after a few weeks. In desperation, I now break a 25 mcg tab into 12 pieces, as I can tolerate no more than 2 mcg at a time. I feel a surge from each dose (as if I’m taking T3). I take a dose every 2-3 hours. I don’t feel great, but at least can function somewhat. When I try to take any more, I am back to the terrible hyper symptoms.
    I’ve tried a compounded version but have the same issues with that too.
    Any suggestions you might have would be much appreciated. Thanks for all your research and efforts in this field.
    My latest test results were TSH around 10, with T4 and T3 both just under the low end of the normal range.

    • Paul Robinson on 20th July 2021 at 5:21 pm

      Hi Coral, it is hard to say without more lab test results on other things like cortisol.

      My The Thyroid Patient’s Manual book includes a list of some of the most important other tests.

      Sometimes though, things change temporarily due to other factors like stress or a virus. It is important not to knee jerk and start changing the thyroid medication dosage. Better to stick with the same dosage and just ride it out until you get lab tests that suggest it ought to be altered, or symptoms and signs that also show this (body temperature, heart rate, BP, EKG for instance).

      I’m tempted to say go back to your original dosing and just stick with it until it settles as the situation has now been put into some imbalance with much higher TSH.

      But without a lot of tests and symptoms and signs it is hard for me to know.

      Best wishes, Paul

      • Furkan on 6th October 2021 at 8:16 pm

        I feel not energetic but my problem is my cold hands and feet especially against cold weathers and overstress.. i have got a microprolactinoma with a prolactin level of 21. My labs are two weeks ago after a month of testogel.

        Testosterone;800 ng/dl
        Tsh:1.66 (0.35-4.94)
        Ft3:3.30 (1.71-3.71)

        Exogenous t replacement somehow seemingly increased tyroid levels a bit but i feel no change. I have got slow metabolism indicated with shbg is only 16 🙁 i have got normal sodium(142) and potassium(4,72) levels. I have tried almost everything my b12 is in 700’s my vitamin D is in 60’s. I have been taking iodine, selenium, complex b vitamins, magnezium, vitamin k2, vitamin E and A and omega3, probiotics i don’t eat junk foods… i don’t feel any difference that my testosterone is in 200’s or 800’s.

        Dealing with this for 20 years and feel that stress drained mt adrenals. Until 5 years ago, my morning serum cortisol levels were always at the high normal range. Specifically for last 2 years morning cortisol is at lower mid range. Insulin tolerance test was performed to assess growth hormone levels it was 27 way above 5 which is cut off value for deficiency. At the same time my cortisol at base was 9.1 (3.7-19.4 ug/dL) after 30 minutes during ITT went up 17.3 and 17.0 after an hour… i don’t know, no-one interpreted the cortisol for me my endo gave a blind eye to it.

        I read about pooling last week and 4 days ago began taking hydrocortisol (5mg,5mg,2.5mg) total of 10 mg. I am measuring my body temperature orally and it shows between 36.0-36.7. That may indicate an adrenal issue, i am not sure though. When my hands feel very cold today and i measured it and it is 36.1. After 4 hours i felt relatively warming hands and measured it once again and it was 36.5… hydrocortisone seems not very effective or am i under dosing it? I don’t know how to act and overcome all of this mess.

        • Paul Robinson on 7th October 2021 at 10:47 am

          Hi Furkan,
          As I say in my blog post I do not believe in the idea of pooling at all. It is a made-up idea that has no basis in science.
          I am also not sure what to say to your message. It is too detailed for me to respond to. I would not have enough information for me to make any useful or safe suggestions. I don’t do detailed advice on the website, only quite general advice which is safe to offer.
          I hope you find some answers.
          Best wishes, Paul

  6. DD on 17th December 2021 at 6:55 pm

    Hi Paul! Wow great information! Thank you for being a light in this topic exactly what the world needs! 💛

    My question is this, was on thyroid t4 med only for 11 years…finally got placed on t3 cytomel & t4 about 4 months ago.

    Bad history of autoimmune inflammation for years, nothing has worked to rise my cortisol levels using a natural approach. My rhythm is good but low throughout 24 he period.

    Because my ft3 has been low 2.4 left me with insulin resistance as well.

    My doctor wants to try me on 5mg of hydrocortisone in the am & afternoon to “kick start” my adrenals. Not sure if this is the right choice. She thinks maybe HPA shut down for so long adrenals are “asleep” I guess for me I just don’t want to jeopardize all the healing I have done to fix my body but it’s been 3 years & no improvement in cortisol levels.

    From what I understand I will need cortisol along with raising the t3 levels to get thyroid working optimally. So without more cortisol I may never get the thyroid working better even with t3?

    I guess I am just on the fence about using HC not exactly what I wanted to have to do but I am not thriving in life either & I am tired of being tired. Thank you for your time.

    • Paul Robinson on 17th December 2021 at 9:48 pm

      I would get the FT3 up to high levels first before venturing with HC.

      Reading my The Thyroid Patient’s Manual book and/or Recovering with T3 book will let you know.

      I would leave HC or other cortisol supplements until you have your thyroid hormone levels at a good level.

      If you are on T4 meds and FT3 is low then I would ask for T3/T4 or natural desiccated thyroid.

      Getting FT3 much higher often raises cortisol.

      Best wishes, Paul

      Good luck

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