# Reverse T3

Category: Hormones

Also known as: rT3

Reverse T3 (rT3) is an inactive thyroid hormone metabolite produced when the body perceives an excess of thyroid hormone, primarily formed by removing the wrong iodine atom from thyroxine (T4). Peat argued that rT3 is mostly caused by high cortisol and stress hormones, and that…

11 passages · 2 authors · 2012–2022 · Most-cited: [Ray Peat](https://bioenergeticoracle.com/md/voices/ray-peat/index.md)

Canonical page: https://bioenergeticoracle.com/concepts/reverse-t3

## Synthesis

**Reverse T3 (rT3)** is an inactive thyroid hormone metabolite produced when the body perceives an excess of thyroid hormone, primarily formed by removing the wrong iodine atom from thyroxine (T4). [Source 1] Peat argued that rT3 is mostly caused by **high cortisol** and stress hormones, and that a supportive diet of fruit, orange juice, and milk can resolve the issue as effectively as taking small amounts of T3. [Source 1] The molecule acts as a competitive antagonist, partly occupying the same response site that active T3 would, thereby blocking its metabolic effects. [Source 3] Crucially, *T3 itself cannot be converted into rT3*; the inactive metabolite is made exclusively from T4. [Source 6, 8]

The primary driver of rT3 accumulation is the stress-induced activation of specific deiodinase enzymes. Peat explained that high cortisol causes the liver to turn T4 into rT3, which interferes with active T3 function even more than T4 itself does. [Source 5] Dinkov elaborated that any condition inhibiting liver function—such as increased fatty acid oxidation during fasting—leads to a buildup of T4, which the body then converts into rT3. [Source 10] Estrogen also plays a key role by blocking the conversion of T4 to T3 in the liver, shunting it toward rT3 production instead; this explains why women under stress are particularly susceptible to this pathway. [Source 6, 7] Local enzymes can eliminate rT3, but when those are blocked by stress substances, rT3 accumulates and exacerbates a hypothyroid state. [Source 3]

Clinically, Peat observed that administering pure T3 can rapidly reverse the anti-metabolic effects of rT3. He described a patient in **myxedema coma** whose condition worsened as her T4 dose was escalated to 500 micrograms daily; she recovered immediately when switched to 25 micrograms of T3. [Source 6] Small doses of T3 usually tend to normalize rT3 if it is high, because rT3 is made from T4 and T3 supplementation bypasses the problematic conversion step. [Source 2] However, Peat warned that taking large boluses of T3 on an empty stomach creates a *toxic excess* perceived by the liver, which then upregulates deactivating enzymes and can lead to extreme hypothyroidism hours later. [Source 4] He recommended nibbling small amounts of a T3 tablet throughout the day to avoid this counterproductive enzyme induction. [Source 4]

The broader metabolic context reveals that rT3 elevation reflects a **catabolic state** driven by low sugar availability and chronic stress. Peat noted that a low-sugar diet can cause chronically high cortisol, which induces the deiodinase that makes inactive rT3. [Source 11] Fasting exacerbates this by breaking down muscle tissue, flooding the bloodstream with antithyroid amino acids like tryptophan, cysteine, and methionine that suppress thyroid output. [Source 10] Even when supplementing T3, ongoing stress can still generate rT3 because cortisol release and estrogen production independently drive the inactivating enzyme pathway. [Source 4, 6] The condition reflects a *vicious circle* where stress, endotoxin, and estrogen lower carbon dioxide production and thyroid function, further impairing liver clearance and allowing more rT3 to accumulate. [Source 9]

## People also ask

### What causes the body to produce reverse T3?

Peat argued that high cortisol and stress hormones are the primary drivers, causing the liver to convert T4 into rT3 instead of active T3. Estrogen also shunts T4 toward rT3 production by blocking the T4-to-T3 conversion in the liver.

### How does taking T3 help lower reverse T3 levels?

Small doses of T3 can normalize high rT3 because rT3 is made exclusively from T4, and T3 supplementation bypasses the problematic conversion step. Peat described a patient in myxedema coma who recovered immediately when switched from a high T4 dose to a small amount of T3.

### Why does fasting or a low-sugar diet increase reverse T3?

The corpus describes that fasting increases fatty acid oxidation and inhibits liver function, leading to a buildup of T4 that is then converted into rT3. A low-sugar diet can cause chronically high cortisol, which induces the deiodinase enzyme that makes inactive rT3.

## Related concepts

- [T4 (Thyroxine)](https://bioenergeticoracle.com/md/concepts/t4-thyroxine/index.md)
- [Acidosis](https://bioenergeticoracle.com/md/concepts/acidosis/index.md)
- [Arthritis](https://bioenergeticoracle.com/md/concepts/arthritis/index.md)
- [Beta oxidation](https://bioenergeticoracle.com/md/concepts/beta-oxidation/index.md)
- [Carbon Dioxide (CO2)](https://bioenergeticoracle.com/md/concepts/carbon-dioxide-co2/index.md)
- [Eczema](https://bioenergeticoracle.com/md/concepts/eczema/index.md)

## Cited passages

Passage numbers match the `[Source N]` markers in the synthesis above.

### Source 1 — Politics & Science: Two Hour Fundraiser II

Ray Peat · Interview · Feb 22, 2012 · http://l-i-g-h-t.com/files/politics-science-two-hour-fundraiser-2.mp4

> ## Reverse T3
>
> **John Barkhausen:** Wade asks: "I would like to ask Ray's thoughts on Reverse T3. Is it really necessary to use T3 treatment only to clear out the accumulated Reverse T3, or will a combination thyroid product and proper nutrition resolve the issue?"
>
> **Ray Peat:** I think it's mostly caused by high cortisol and stress hormones. Eating a good supportive diet—plenty of fruit, orange juice, and milk—will help keep the stress down. I think it works just as effectively as taking small amounts of T3.
>
> **John Barkhausen:** What causes Reverse T3? It is basically where your whole thyroid system shuts down, right?
>
> **Ray Peat:** Yeah. If your body thinks that you're having an overexposure to thyroid, it gets rid of the excess by turning thyroxine into an inactive kind of T3. It takes off the wrong iodine. If you're fasting, for example, part of the thyroid is being excreted the way estrogen or toxins are being excreted, but part of it is being inactivated in a way that has an anti-thyroid effect. It interferes with the active T3. It's an extra fast way to turn off your thyroid function.

### Source 2 — Ray Peat Email Advice Depository — Post 508

Ray Peat · Email · Jul 28, 2018

> **Question:** Hi Ray, What are your thoughts on the safety of shining redlight in the eyes? There is evidence it could help with eye health and I find it has nice systemic effects.\* Do you think it's risky to use small doses of t3 throughout the day without any supplemental t4? How to avoid excess rt3? [references]
>
> **Ray Peat:** Reverse T3 is made from T4, so small doses of T3 usually tend to normalize rT3 if it’s high. Ordinary red light has an antiinflammatory effect, helps to neutralize the effects of too much u.v. and blue light. If it’s too bright, it can overheat the eye internally.

### Source 3 — Ask the Herb Doctor: Rheumatoid Arthritis

Ray Peat · Interview · Oct 21, 2016

> ## Thyroid Hormones: T3, T4, and Reverse T3
>
> **Caller:** Does T3 affect TSH?
>
> **Ray Peat:** Yes. Normally T4 suppresses TSH because the pituitary converts T4 to T3 locally. But if you get enough T3 into your brain and pituitary, it is at least as effective as T4.
>
> **Caller:** What about Reverse T3?
>
> **Ray Peat:** Reverse T3 partly occupies the same response site that active T3 would, simply competing against it. Local enzymes can eliminate Reverse T3, but if those are blocked by stress substances, Reverse T3 can accumulate.
>
> **Sarah Murray:** That's why taking temperatures and pulses will tell you if you're getting enough T3 or converting T4 properly.

### Source 4 — Reverse T3, HCG, EMF & Viruses, Boosters, Kid's Vaccines, Lung Failure & More! Q&A

Ray Peat · Interview · Dec 15, 2021 · https://www.youtube.com/watch?v=0oLpC6F-qIY

> The T4 excess turns into Reverse T3, [28:20] and turns off your metabolism. The higher the stress acting on the siroxin in the absence of T3 turns fairly massive amounts of the T4 into blocking Reverse T3. So if they're supplementing T3, they don't have anything to worry about that converting to Reverse T3, but they still need to dial in their stress, because just that act of always releasing cortisol and making new estrogen from stress could cause more Reverse T3, even in the midst of like a T3 medication? Yeah, and lots of pharmacists and doctors advise if a person is prescribed T3, it's common to advise them to take their pill 25 or 50 micrograms, [29:22] whatever is prescribed for them to take that pill on an empty stomach in the morning. And what happens then is that your body gets approximately 25 times higher T3 at that moment than is physiological, because you might produce 50 micrograms in the whole day. But if you take that breakfast on an empty stomach, it almost all goes immediately into your bloodstream. It is perceived as a toxic excess, and so your liver increases enzymes and your liver begins excreting, inactivating the high level of T3 in the morning. And after getting your liver accustomed to doing that, [30:30] nightfall 12 to 15 hours later, your body is experiencing extreme hypothyroidism because you have trained the liver to excrete it 25 times faster than it should. So they really need to, how would you recommend dosing? It sounds like they need to split up the dose, maybe twice a day, or maybe they shouldn't take a huge dose first thing in the morning. Definitely, yeah. I experienced that myself taking my pill only in the morning, but I developed an extremely erratic heart rhythm in the middle of this interrupted heartbeat. I chewed up a 10 microgram tablet of T3, [31:32] and within about 15 seconds of chewing the pill, my heart was perfectly rhythmic. And after that, I recommended that people just take a little bite of T3 tablets all during the day, so they can get the approximately right amount during the day, but never anything like 20 or 25 micrograms at one gulp.

### Source 5 — Current Trends in Nitric Oxide KMUD 2015

Ray Peat · Interview · 2015 · https://www.youtube.com/watch?v=6TquRcpR8RY

> **Herb Doctor:** Yea, that would be outside of the range according to the lowered advised figures that have been issued.
>
> **Herb Doctor:** And isn't it correct, Dr. Peat, that while someone might be taking T4 and not really converting it much to T3, their TSH would still lower?
>
> **Ray Peat:** Yea, and the stress hormones will lower it too. So you can't really diagnose anything by TSH alone, even though that’s how some doctors do it.
>
> **Herb Doctor:** Especially when someone who is low thyroid. They are usually running on excessive adrenaline. And excessive adrenaline will lower the TSH and give you a false positive, rather than a false negative.
>
> **Ray Peat:** And high cortisol lowers it. And also at the same time causes the liver to turn thyroxin into rT3, which interferes even more than thyroxin with the active T3 function.
>
> **Herb Doctor:** It blocks the real T3 from actually doing anything.
>
> **Caller:** That's interesting.
>
> **Herb Doctor:** I thank you... Do you have anything else to say. I am sorry, I don't want to cut you off.
>
> **Caller:** Are his books still available?
>
> **Herb Doctor:** Dr. Peat?

### Source 6 — Reverse T3, HCG, EMF & Viruses, Boosters, Kid's Vaccines, Lung Failure & More! Q&A

Ray Peat · Interview · Dec 15, 2021 · https://www.youtube.com/watch?v=0oLpC6F-qIY

> And I mentioned that to other people, especially people who were slightly taller than average, told me that they hadn't finished growth until they were in their mid 40s. You know, I would believe that because I was always 5'9 when I was in my teens and 20s, and now when I tell people I'm 5'9, they go, I don't think you're 5'9, I think you're taller than that, but I haven't ever measured myself, but I just wonder if maybe I have grown over the years, because I don't seem to be, I don't seem to fit that mold of the 5'9 or whatever. Yeah, but my experience with the marked door frames, I was about 50 at the end of that period, [25:10] and I just haven't noticed. Okay, so Rhonda asks, can you discuss Reverse T3 and what to do about it? Can you get Reverse T3 from using too much T3 medication? So what about Reverse T3? Yeah, T3, the active hormone can't be changed in Reverse T3. And one of my first eye-opening observations in the thyroid function was [26:16] at the University of Oregon Medical School, a patient who was brought in completely unconscious in a mixadema coma, and her doctor had been giving her 100 micrograms of T4 synthroid, and she got worse, more hypothyroid, progressively increased it to 200 and 300, when he reached 500 micrograms of T4 per day, she entered the coma, and a doctor in the hospital had the insight that women in particular, because of the effect of estrogen on the liver, [27:17] estrogen blocks the conversion of T4 to T3 and turns it into Reverse T3. So he put her on pure T3 around, I think it was 25 micrograms per day, zero siroxin, and immediately she recovered, all she needed was a more balanced ratio between T4 and T3. And when your stress hormones rise, happens because you get enough T3, and the sugar and carbon dioxide metabolism are suppressed. The T4 excess turns into Reverse T3, [28:20] and turns off your metabolism. The higher the stress acting on the siroxin in the absence of T3 turns fairly massive amounts of the T4 into blocking Reverse T3.

### Source 7 — Iodine, Supplement Reactions, Hormones and More KMUD - Ask Your Herb Doctor - 2016

Ray Peat · Interview · Feb 19, 2016 · https://www.youtube.com/watch?v=huW5IRBJKiY

> **Ray Peat:** Yeah. When people have suffered for a long time with a low thyroid they are likely to have extremely high adrenalin and cortisol levels and that causes them to turn T4 into reverse T3 blocking the actual active T3 hormone and then if they accumulate more and more T4 that will interfere competitively with the little bit of T3 that they do have, so they can exaggerate the state of their hypothyroidism if they’re in that extreme stressed state.
>
> **Andrew Murray:** Would you say that T4 perhaps is maybe only 10% as active as the active T3 hormone?
>
> **Ray Peat:** It really varies. In the 1940s, when they first synthesized it, they tested it on male medical students and it was exactly as effective as Armour natural thyroid and that’s because young men, 20-22 years old, have very good livers that can perfectly convert it, but even at the same age women are more likely to have problems with plain thyroxine.
>
> **Andrew Murray:** So when you talk about T4 being weakly active, how do you interpret that or how do you see T3 versus T4 in terms of orchestrating metabolic events?
>
> **Ray Peat:** Well the standard textbook idea is that T3 is 4 times more powerful than T4 but really if your liver is good you can get 100% of the benefit out of T4 and if you are a woman under stress with high oestrogen your liver isn’t going to convert any of it to the right active hormone, and the more you take – I’ve known of one woman who was hospitalized and got more and more hypothyroid the higher they raised her thyroxine dose - and as soon as they gave her T3 she came right out of the myxedema coma, but I have seen people in less extreme states who got more and more depressed or psychotic or whatever when they increased their thyroxine dose.

### Source 8 — #14: Aging, Mitochondria, Hypoxia, DHEA, Fertility, Serotonin, and Progesterone with Georgi Dinkov

Georgi Dinkov · Interview · Dec 13, 2019 · https://open.spotify.com/episode/6KylLfMv3kOY5gPEL8PAV3

> **Danny Roddy:** So yeah, not only T4.
>
> **Georgi Dinkov:** I know this is like a myth propagated by the doctors, but actually T3 cannot directly get converted to reverse T3 despite the unfortunate naming convention. But it's metabolized T1 and T2 care.
>
> **Danny Roddy:** I didn't know that. Van Den Beld from a paper from 2005. They say high serum reverse T3 may result from a decreased peripheral metabolism of thyroid hormone due to aging process itself and or disease and may reflect a catabolic state. So just what we said. Okay. So thank you so much for that, son of Rome. Sincerely appreciate it. There's one message redacted, so I can't see who actually did this one, but it was for $5. Thank you so much. Son of Rome for another $5. Thank you so much. For lingering Lyme-related knee issues, what kind of topicals would be safe and effective to experiment with?

### Source 9 — Ray Peat and Bud Weiss - The Biology of Carbon Dioxide - 2010

Ray Peat · Interview · Sep 18, 2022 · https://www.youtube.com/watch?v=0tgKX-DkEm4

> **Ray Peat:** Besides the thing I mentioned about the rabbit bones being overdeveloped in the presence of either carbon dioxide or thyroid. There have been many experiments using just T3, the active thyroid hormone, in mouse skull bones, which you can grow in a culture dish. And they showed that T3 very quickly stimulates respiration. the deposit of calcium carbonate as the new bone. New bone is calcium carbonate rather than calcium phosphate. As I said, the cytochrome oxidase is what thyroid acts on primarily. So I think its main thing is to increase the production of CO2 and the affinity for oxygen. And that ends up suppressing lactic acid formation and doing the opposite of what estrogen and polyunsaturated fats do to your respiratory system. Oh, and one other whole system that I didn't mention that causes biochemical hyperventilation. In other words, the production of lactic acid instead of carbon dioxide is the endotoxin or lipopolysaccharide from bacterial activity in the intestine. Under stress, the circulation to the intestine is reduced. And it becomes more permeable. More endotoxin gets into the bloodstream. And once it gets past the liver, it releases the inflammatory cytokines, nitric oxide, and tumor necrosis factor and such that work with estrogen as an anti-respiratory adaptation. And anything that slows your liver function, such as low thyroid, will let more endotoxin get in your bloodstream and let more estrogen remain there. And both of these, in turn, lower your CO2 production and thyroid function. And so it can be a vicious circle that starts just with stress or eating something wrong. Go ahead.

### Source 10 — #04: Endotoxin, Otto Warburg, Cancer, Ketosis, and PUFA with Georgi Dinkov

Georgi Dinkov · Interview · Jun 14, 2019 · https://open.spotify.com/episode/1JcrW0U6ajwgFaDVe04f0m

> **Georgi Dinkov:** Well, maybe somebody can send him the article on metformin cancer. So he keeps taking it afterwards. Um, But, you know, to anybody who has learned just a little bit of physiology would immediately become obvious considering that study about increased fatty acid oxidation damaging the liver. Most of the production of T3 in the body happens in the liver. That's where T4 is converting to T3. Anytime you inhibit liver function, which is what fatty acids do very, very well, as that study showed, right, then you're going to have a buildup of T4. And that buildup, the body doesn't like a buildup of T4. And then it starts converting into reverse T3. That right there is a very simple and most likely explanation of why his reverse T3 showed up. But there are other things, as just as we discussed, like fasting starts breaking down muscle. All of these amino acids like tryptophan, cysteine, methionine, they start flooding your bloodstream. They're all goitrogens. They're all antithyroid. They all suppress the thyroid eventually putting out even T4. So all fasting does, aside from the initial and minor benefit of reducing endotoxin in the first 24 to 48 hours, everything after that is really detrimental. There is just no way a signal, an environmental signal that things are not going well, which is what fasting is for any animal, there is no way that this can be helped in the long run. I don't know of an organism that starves itself to health. I mean, I know animals that stop eating for a few days because they're injured or they, like, basically the food is really not, you know, is of bad quality. There are some reasons, but they never starve themselves on purpose and reverse sickness into health.

### Source 11 — Email Exchanges — Thyroid

Ray Peat · Email · 2015

> # Thyroid
>
> General Questions
>
> [IMPROVING T4->T3 CONVERSION] If you were deficient in selenium, the correcting effect would be quick, but if there was a problem with intestinal flora, that would have to be taken care of before conversion was good. Other nutritional deficiencies could be involved. Daily raw carrot, weekly seafood and liver, enough sunlight and vitamin D, a good ratio of calcium to phosphate, are often helpful.
>
> Calcium (two liters of milk), vitamin D and plenty of orange juice sometimes help to regulate things by balancing the minerals. A daily carrot salad should keep the small intestine fairly sterile.
>
> [Reverse T3] Yes, it's probably induced by stress, with cortisol inducing the type of deiodinase that makes the inactive rT3. A low sugar diet can cause chronically high cortisol. If you are eating enough fruit and protein, I think the T3 of natural thyroid will help to correct the stress/inflammatory metabolism that is connected with the reverse T3.
>
> There isn't any natural T3 product, in the sense of biologically created, but the activity of T3 is so great that the effective dose, of a few micrograms, couldn't introduce a significant amount of industrial junk; the excipients are the main concern, and whether the people making the tablets understand what they are doing. Cytomel and Cynomel, so far, have been very well made, and there isn't any other T3 product that I trust.
>
> Starting Thyroid Supplementation
>
> Sometimes it takes many months to get the metabolic rate stable at a higher level, and it's often necessary to use a thyroid supplement.
>
> Thyroid is the only thing that safely lowers cholesterol, but when your stress hormones are very high, you shouldn't take more than about one microgram of Cytomel at a time, and should accompany it with things like milk and orange juice.
>
> With your TSH so high, you should probably add a thyroid supplement, until you get it down to about 1.0, or less. (The normal range, according to the American Association of Clinical Endocrinologists, is from 0.3 to 3.0.)
>
> Thyroid Supplementation Doesn't Help / Causes Issues
>
> T3, by lowering stress, sometimes reveals a low basal metabolic rate, that was hidden by high stress hormones.

_Generated 2026-07-20 from the Bioenergetic Oracle corpus._