One Reason Why TSH can be Low or Suppressed on Treatment with Higher Levels of T3 (as Liothyronine or NDT)

This article was written by Dr Tania S. Smith of Thyroid Patients Canada. It explains one reason why TSH can often be very low or suppressed when taking reasonably high doses of T3 thyroid hormone.

I have explained many times that TSH being low on thyroid treatment is not necessarily a problem if the person does not have extremely high levels of FT3 and has no hyperthyroid symptoms or signs. Being on thyroid treatment is an entirely different situation from not being on any thyroid treatment. There is already plenty of research available that shows that suppressed TSH on thyroid treatment does not have to be an issue – search my site using the Spyglass (magnifying glass) for ‘suppressed TSH’ and articles will be listed.

However, this article explains why using T3 can raise levels of the thyroid hormone metabolite Triac for a short while. Triac is known to powerfully suppress TSH. So, it should be no surprise to see people on NDT or on reasonable levels of T3 have low or suppressed TSH.

Here is the article:

I hope you enjoy the article and find it useful. Many thanks to Dr Tania S. Smith for writing it.

Here is another research article that talks about T3 suppressing TSH more powerfully than T4. The essence of this article is that when T4 enters the circulation it gets converted to T3 through the process of deiodination. T4 and T3 can then exert negative feedback on TSH levels (high levels of T3/T4 decrease TSH release from the anterior pituitary, while low levels of T3/T4 increase TSH release). T3 is the predominant inhibitor of TSH secretion.

Here is the article:

Best wishes, Paul

p.s. Here is more support for not caring about suppressed TSH, when the thyroid patient is not showing any signs or symptoms of hyperthyroidism and FT3 is not clearly high. It is a quote by Dr G Foresman, MD in the USA:

“Studies from decades and decades ago indicated that a suppressed TSH might be an issue. What they used to do more than twenty-five years ago was to give high doses of Synthroid (T4) to patients with the intention of suppressing their TSHs in order to shrink thyroid gland nodules. A very bad, outdated protocol that doctors no longer practice. At the time, they used such high doses of T4 that they were making some people chronically hyperthyroid and of course, they were seeing bone loss, arrhythmias, etc. The medical community stopped implementing that practice because of those outcomes. But unfortunately, as a result of that antiquated practice, there are still doctors who think that a suppressed TSH is dangerous to the patient. 

All of the studies in the last few decades indicate that TSH suppression has no association with some of those feared results, like osteoporosis. Interestingly enough, my patients with the lowest TSH values have the best bone density scores. I have had patients move to another state, and their new doctor refuses to prescribe desiccated or compounded. And as soon as the doctor sees a suppressed TSH, the doctor freaks out and lowers the patient’s thyroid medication.

It’s an antiquated belief system based upon the decades-old history of using suppressive thyroid hormone to shrink thyroid nodules. As a result, doctors are still afraid of suppressing TSH, even though the literature has shown for decades now that you can suppress TSH with no metabolic consequences whatsoever. A suppressed TSH does not lead to heart failure, it does not lead to arrhythmia, and it does not cause osteoporosis.” 

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. Beaux on 4th January 2024 at 9:21 pm

    I’m wondering if Triac is the missing link to understanding differences NDT preparations – why some work, then suddenly stop working or some work better than others. My understanding is that Triac is not commonly tested for in NDT preparations.

    • Paul Robinson on 5th January 2024 at 9:59 am

      Beaux, I wasn’t recommending TSH suppression as a go-to solution. I was simply giving people information because many patients do very well on many different types of thyroid treatment when TSH is suppressed but then their medication is reduced because doctors don’t want the TSH to be suppressed. Sometimes TSH has to be suppressed to get the FT3 level up high enough (on T4 only, on T4/T3 combination and on T3 therapy). In many cases, the pituitary is maladapted and tends to produce a lower TSH than it ought to and it can be impossible to get the FT3 level high enough without TSH being very low. Doctors seem to think that the pituitary gland is always perfect and its TSH level is always to be absolutely trusted. That is definitely not the case.
      However, for someone with sub-clinical hypothyroidism (not usually the type of thyroid patients that speak to me) then keeping TSH above suppression can be important if they are on mainly T4 meds.
      Best wishes, Paul

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