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What is Thyroid Pooling?

Learn about thyroid pooling and the role free T3 plays in it.
What is Thyroid Pooling?
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As you search for information on hypothyroidism and thyroid disease on the web, you may encounter a theory known as “thyroid pooling.” The basic idea is that thyroid hormone builds up -- or “pools” -- and cannot move into cells to do its job. In this article, we take a look at this controversial concept and whether it could be having an impact on your thyroid health

The basics of thyroid dysfunction

Thyroid impairment results when your thyroid produces too much or little thyroid hormone.

In hyperthyroidism, your thyroid is overactive and produces too much thyroid hormone. Too much thyroid hormone speeds up your metabolism, resulting in symptoms that can include anxiety, excessive sweating, trouble sleeping, and a fast heartbeat.

In contrast, hypothyroidism is an underactive thyroid, and your gland produces too little thyroid hormone. Because of this, your metabolism slows down, causing symptoms that can include:

  • Constipation
  • Cold intolerance
  • Fatigue
  • Memory impairment
  • Weight gain


T3 and T4: understanding the difference

Your thyroid gland produces and releases two types of thyroid hormone. T3 (triiodothyronine) is the active hormone, and T4 (thyroxine) is the inactive hormone. About 80% of thyroid hormone released from your thyroid is in the inactive form of T4.

Once in your bloodstream, T4 travels to different cells and is converted to T3. Both T4 and T3 enter your cells using specific transporters. The majority of T3 is inside your cells, while the majority of T4 remains in your bloodstream. This makes sense as T3 is the active form of thyroid hormone and is responsible for how your cells function. And to do that, T3 has to be inside your cells.

Your body balances the amount of thyroid hormone in your cells with the amount in your bloodstream. A change in this balance, even a small one, can alter how your body functions. These changes could be related to stress, medications, or changes in other hormones.

What is thyroid pooling?

Thyroid pooling is a controversial theory that free T3 isn’t reaching cells to facilitate the delivery of oxygen and energy. Instead of entering your cells, T3 remains -- or “pools” -- in your bloodstream. 

Thyroid pooling theory then claims that elevated levels of free T3 can cause common symptoms of hyperthyroidism, similar to those with hyperthyroidism or Graves’ disease.

There are three ways you can develop high free T3 levels:

  • Increased T3 synthesis
  • Faster rates of T3 secretion from the thyroid gland
  • Increased levels of enzymes that convert T4 to T3 speed up the rate of conversion

What causes thyroid pooling?

Since thyroid pooling is a theory and isn’t a topic researchers have studied, there are several unproven ideas about the cause.  

Cortisol link: One theory is that pooling results from low cortisol (stress hormone) or iron levels. But the data seems to suggest otherwise. A 2021 study showed that low doses of hydrocortisone did not affect how much T3 your cells can receive. But, higher doses of hydrocortisone did inhibit T3 uptake. Hydrocortisone is a lab-made version of cortisol. This contradicts the theory that low doses of cortisol prevent T3 uptake.

Thyroid hormone transporters: Another theory relates to the role of thyroid hormone transporters. Free T3 needs specific transporters to move into the cells. This has led some to speculate that a lack of -- or dysfunction in -- thyroid hormone transporters might affect blood levels of T3. This hypothesis is not backed up by research, however. A 2019 study published in Endocrine Review reveals that as long as your cell transporters are available, T3 will enter those cells. There is a medical condition characterized by a lack of functional thyroid hormone transporters that would affect T3 uptake by your brain cells, but it is very rare.

Is thyroid pooling a proven condition?

At this point, it’s too early to say whether thyroid pooling is more than a theory. Researchers haven’t explored the topic specifically, so we have no data or published studies to refer to for reliable information. 

Whether or not thyroid pooling is eventually proven to be a genuine condition,  we do know that imbalances in T3 levels can play a role in hypothyroidism symptoms. Ultimately, a thorough thyroid evaluation will be the most important next step if you are hypothyroid and still have troublesome symptoms.  

How to evaluate your thyroid function

The only way to know how well your thyroid functions is by measuring blood levels of specific thyroid hormones.

Thyroid-stimulating hormone (TSH) and free thyroxine (free T4) are the most common hormones tested as they give the best overall view of your thyroid function.

  • High TSH with a low T4 indicates hypothyroidism is likely
  • Low TSH with a high T4 indicates hyperthyroidism is likely

Free Triiodothyronine (free T3) is also a useful test, showing how much active hormone is available to enter the cells.

Reverse T3 (rT3) levels may also be a helpful value in thyroid function. Reverse T3 is an inactive form of T3 produced when the body is under stress or when T4 to T3 conversion is impaired somehow. Some believe that high rT3 levels may be associated with hypothyroid symptoms.

A note from Paloma Health

Testing your thyroid panel is the only way to know if your free T3 levels are too high. In the past, you would have to go to the lab to have your blood drawn. But, with Paloma Health’s at-home thyroid testing kit, you can skip the lab visit.

Paloma’s thyroid testing kit requires a painless fingerprick blood sample that you mail in and measures levels of:

  • TSH
  • Free T4
  • Free T3
  • Thyroid peroxidase antibodies (TPO), a marker of autoimmune thyroid disease

In addition, you can also have your reverse T3 and vitamin D levels added to your lab panel. You can expect your results within a week with an interpretation of your results. Paloma Health’s thyroid experts can further help you understand your results and help determine the next steps.

Dealing with Hypothyroidism?  Video chat with a thyroid doctor

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Braunstein GD. Overview of the Thyroid Gland. Merck Manuals Consumer Version.Full review/revision September 2022. Accessed April 24 2023.

Pirahanchi Y, Toro F, Jialal I. Physiology, Thyroid Stimulating Hormone (TSH). Published April 25, 2019. Last updated May 8, 2022. Accessed April 24, 2023.

Salas-Lucia F, Bianco AC. T3 Levels and thyroid hormone signaling. Front. Endocrinol. 2022;13. doi:10.3389/fendo.2022.1044691

Bianco AC, Dumitrescu A, Gereben B, Ribeiro MO, Fonseca TL, Fernandes GW, et al. Paradigms of Dynamic Control of Thyroid Hormone Signaling. Endocrine Rev. 2019;40(4):1000-1047. doi: 10.1210/er.2018-00275

Di Cosmo C, De Marco G, Agretti P, Ferrarini E, Dimida A, Falcetta P, et al. Dexamethasone and some commonly used drugs inhibit MCT8-mediated T3 transport in vitro. Research Square. 2021. doi:10.21203/

Allan-Herndon-Dudley syndrome: MedlinePlus Genetics. Accessed April 24, 2023.

American Thyroid Association. Thyroid Function Tests | American Thyroid Association. American Thyroid Association. Published 2016. Accessed April 24, 2023.

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Emilie White, PharmD

Clinical Pharmacist and Medical Blogger

Emilie White, PharmD is a clinical pharmacist with over a decade of providing direct patient care to those hospitalized. She received her Doctor of Pharmacy degree from Massachusetts College of Pharmacy and Health Sciences. After graduation, Emilie completed a postgraduate pharmacy residency at Bon Secours Memorial Regional Medical Center in Virginia. Her background includes caring for critical care, internal medicine, and surgical patients.

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