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Thyroid Testing: What is Reverse T3 and Why is it Elevated?

Thyroid lab testing for reverse T3

Testing for thyroid function with TSH, free T3 (fT3), and free T4 (fT4) are routine labs for thyroid disease and many doctors don't order testing beyond these three. I've talked about other tests to consider if you have hypothyroidism and one of them I touched on was reverse T3 (rT3). The clinical use of this thyroid hormone is controversial, but some naturopathic doctors test for it and find it useful.

In my practice I'll order it along with a thyroid panel if someone isn't improving with treatment or if their fT3 or fT4 are low. Both of these hormones can be converted to rT3 and it's a way to see if active hormone is being converted into an inactive form.

There are many symptoms of hypothyroidism including: fatigue, brain fog, dry skin, weight gain, hair loss, constipation, irregular menstrual cycles, coldness, high cholesterol, depression and enlarged thyroid gland (goiter).

If you have hypothyroidism and are curious about this test keep reading to learn more about the function of rT3 and what might cause it to become elevated.

What is reverse T3 (rT3)?

Most doctors will test thyroid function with thyroid stimulating hormone (TSH) and thyroxine (T4) but there's another thyroid hormone called triiodothyronine (T3) that is secreted by the thyroid gland or converted from T4, and controls many body processes in our brain, heart, immune, skin, and reproductive systems. Too much T3 can be overstimulating so the body has a way to prevent this by creating rT3. Reverse T3 is like a safeguard against too much thyroid hormone activity which can result in elevated heart rate, heart palpitations, weight loss, insomnia, and anxiety. When the body wants to conserve rather than burn energy it will produce rT3.

It's easy to test for rT3 with a blood test. If your fT3 or fT4 are low they may be converting to rT3 depending on your body's metabolic needs. I usually add it onto a thyroid panel and if it's elevated and we are working on underlying health issues I'll check it again a few months later.

Why would rT3 be elevated on lab results?

There are two ways that I determine if rT3 is elevated: by looking at the ratio of fT3:rT3 and the actual lab value of rT3. Each lab's range may be different but my local lab uses Labcorp.

Labcorp's upper limit for normal is around 24 ng/dL, but according to some naturopathic colleagues, if rT3 is more than 15 ng/dL it's considered to be elevated. If the ratio of fT3:rT3 is less than 0.20 ng/dL, meaning there is a lot of rT3 compared to fT3, it's considered to be elevated.

I use both values to give me an idea of where rT3 is at while also taking other thyroid labs and patient symptoms into consideration.

There are quite a few reasons why rT3 might be elevated. In general, the body will convert thyroid hormones to rT3 when you're sick, stressed or undernourished. The enzymes that convert T4 to rT3 are stimulated in these circumstances so more rT3 is produced.

Reasons rT3 may be elevated:

  • blood sugar issues or insulin resistance - high blood sugar impairs immunity and creates a lot of inflammation in the body

  • chronic illness - such as viral or digestive infections

  • stress - cortisol that's produced during the stress response puts your body into "storage" mode and can push both T4 and T3 into rT3.

  • low caloric intake or dieting - the body may be stressed and undernourished

  • liver or kidney issues - hormone conversion occurs in these organs

If any of these issues are going on along with hypothyroidism it's important to address them with naturopathic care to decrease rT3 production and increase T3 and T4 hormones. A prescription for T3 (Cytomel) might also be helpful to reduce rT3 production and help alleviate symptoms as someone is working on these health issues.


Routine thyroid testing includes TSH, fT3, and fT4, but rT3 is not often ordered due to controversial clinical usefulness. Reverse T3 acts like a "storage" form of thyroid hormone and can be converted from T3 or T4. It may be considered elevated if it's above 15 ng/dL or the ratio of fT3:rT3 is less than 0.20 ng/dL but other factors like labs and symptoms should be taken into account. Reasons that rT3 might be elevated include high blood sugar or insulin resistance, chronic illness, stress, low caloric intake and liver or kidney issues. Addressing underlying health issues that increase the production of rT3 is important in the management of hypothyroidism. This can be done with naturopathic medicine, which might include prescribing T3 hormone in the form of Cytomel.

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In health,

Dr. Jamie


Bove, M. (2018, December 4). 10 Botanicals to Support a Healthy Thyroid. Lecture presented at Gaia Herbs Professional Solutions Web Seminar.

Groves, Margaret. “Clearing up Confusion about Reverse T3: Part 1. The Deiodinases & Thyroid Hormone Bioavailability.” ZRT Laboratory, 2015,

Groves, Margaret. “Clearing up the Confusion about Reverse T3: Part 2. The Role of Reverse T3 in Thyroid Assessment.” ZRT Laboratory, 2015,

Romm, Aviva. “Hypothyroid Testing: 6 Labs To Ask For.” Aviva Romm MD, 17 Dec. 2018,

Thyroid lab testing for reverse T3

This website is not intended for the purpose of providing medical advice. All information, content, and material of this website is for informational purposes only and are not intended to serve as a substitute for the consultation, diagnosis, and/or medical treatment of a qualified physician or healthcare provider.

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