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The Importance of Reverse T3 in Thyroid Function + RT3 Ratio Calculator

The Importance of Reverse T3 in Thyroid Function

The thyroid is responsible for producing an impressive number of influential hormones that impact nearly every part of the body. Of these hormones, thyroxine (T4), triiodothyronine (T3), and reverse triiodothyronine (RT3) are particularly important regarding not only the thyroid but metabolic activity, cognitive function, and other essential processes as well. Although T4 and T3 are typically of primary concern, it is important to not overlook the critical role of RT3.

A Brief Introduction to Thyroid Hormones

T4, inactive thyroid hormone, T3, active thyroid hormone, and RT3, the mirrored and suppressive form of T3, are perhaps the most important hormones produced by the thyroid. When working as intended, each of them help maintain proper bodily function. However, an imbalance of these hormones, particularly RT3, can trigger significant dysfunction resulting in conditions such as hypothyroidism.

The Role of RT3

RT3 regulates thyroid activity and helps maintain healthy thyroid function. It accomplishes this by inhibiting conversion of T4 to T3 and blocking T3 receptor sites. Without the regulatory influence of RT3, thyroid activity is erratic. Thyroidal irregularity due to RT3 can cause sudden fluctuations in energy level, systemic dysfunction, and increase the risk of chronic disease.

When functioning properly, the thyroid converts about 40% of T4 into T3 while 20% is converted into RT3. However, factors such as stress, thyroid disease, and lifestyle can increase the amount of T4 converted to RT3 by upwards of 50%. Even though RT3 is a necessary component of proper thyroid function, too much of it can cause major problems. When in excess, RT3 overly inhibits T4 to T3 conversion and can completely blocks T3 receptor sites. This results in a decrease of cellular metabolism and promotes hypothyroid-like symptoms including system-wide sluggishness.

Recognizing an Imbalance

It is common practice for medical professionals to assess thyroid function by testing levels of thyroid stimulating hormone (TSH) or T4. Unfortunately, relying solely on TSH and T4 tests completely overlooks the importance of RT3 – learn more about the inadequacy of the TSH test here. An unrecognized imbalance of RT3 can cause a patient to experience thyroid symptoms even if levels of TSH, T4, and T3 are within normal ranges.

Due to a lack of thorough testing, many thyroid patients continue to suffer from thyroid-related symptoms even after their doctor states that their thyroid is functioning properly. In such situations, it is often the case that an imbalance of RT3 has been overlooked resulting in continued dysfunction. For a more accurate assessment of individual thyroid function it is important to have RT3 levels checked in addition to measuring the ratio of T3 to RT3. If this ratio is imbalanced, thyroid dysfunction is likely to occur if it has not already developed.

Click here to download a thyroid panel sample lab slip.

If you are not sure what your T3/RT3 ratio is, don’t worry! If you have the results of your most recent thyroid test, including T3 and RT3, you can check your ratio by using a simple calculator such as the one found here. A healthy Free T3/RT3 ratio is 20 or higher while a total T3/RT3 ratio should be 10 or higher. If your results are not within or near these ranges, you should consult a trusted physician to see if they can help you can identify the source of imbalance.

Even though RT3 is not given as much attention as T4 and T3, it is a critical component of thyroid function that should not be overlooked. Get the full picture of thyroid function by having the essential thyroid hormones including TSH, T4, T3, and of course Reverse T3 tested by a trusted physician.


1. Reverse T3: Another Thyroid Component. Holtorf Medical Group.

2. Rt3-ratio. Stop the Thyroid Madness.

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Nicolette Graunke
Nicolette Graunke
3 years ago

Good luck finding a doctor who agrees. My t3 to rt3 is ~5. I even went to a doctor from the thyroid chang . org list. I can not afford a doctor who does not take insurance. I had to pay for my own rt3 test too.

3 years ago

Quick question. My RT3 is always a percentage not a decimal measured in pg/ml or any other such form. How do suggest finding out ratio based on this?
RT3 17% (high end)
FT3 2.2 pg/ml (low)
FT4 0.9 ng/dL (low)
T3 uptake 28% (low)
TSH 2.28 mIU/L (high)
All of these are “off” based off my Drs optimal levels.
I have Hashimotos
Any suggestions, clarity or help would be much appreciated.

Cindy Matheson
Cindy Matheson
2 years ago

Excellent article.

My Free T3/RT3 is 15 – under the 20 minimum.

I have massive stress looking after 2 aging parents.

My naturopath says to use Ashawaganda to assist?


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