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Low T3 / Euthyroid Sick Syndrome Causes and Treatment

Written by Mathew Eng, PharmD | Last updated:
Puya Yazdi
Medically reviewed by
Puya Yazdi, MD | Written by Mathew Eng, PharmD | Last updated:

Triiodothyronine (T3) is the active thyroid hormone that affects many processes in the body–from metabolism to digestion to energy levels to cognition. When your T3 levels are low, you may feel tired and cold, have slow digestion and constipation, and experience brain fog.

What is Low T3 or Euthyroid Sick Syndrome?

Probably half of the clients I get have thyroid hormones on the low side. They’re usually hovering either slightly below or slightly above the lower end of the reference range.

In the literature, this is called ‘Euthyroid Sick Syndrome’ or ‘low T3 syndrome’ – when T3 and/or T4 are at unusual levels, but the thyroid gland does not appear to be dysfunctional [1].

Make Sure You Get The Right Tests

You want to get some basics out of the way before you figure out why your thyroid hormones might be on the low side.

Many doctors only test for TSH, which is not an adequate marker to assess your thyroid health. You also need to check your total and free T3.

You can request that your doctor test your thyroid hormone levels. Conventional doctors will look at high or low thyroid hormone levels and not mention anything. Sometimes, a lab result may be in the reference range, but not actually be in the optimal range. Reference ranges are not the same as optimal ranges. This is why thyroid hormones even in the ‘normal’ range can be unhealthy and indicate that certain processes in the body aren’t optimal.

There are also indirect ways to check for low T3.

Causes & Symptoms of Low T3

1) Autoimmune Thyroid Issues

Assuming you’ve taken the recommended tests, you’ll see if you have high thyroid antibodies. If you do then this indicates an autoimmune condition and that’s the most likely reason your thyroid hormones are lower. I’d recommend inducing tolerance if this is the case and going on a lectin avoidance diet for a few months.

2) Chronic Inflammation and Oxidative Stress

If you don’t have thyroid antibodies, then the next most likely scenario is inflammation and oxidative stress.

Most people who feel fatigue and also have a low level of thyroid hormone think that this is the cause of them to be tired.

However, the likely truth is that low thyroid hormones are not the cause of their problems, but a side effect of a larger issue.

The real cause has to do with chronic inflammation more often than not. IL-6, TNF, and other cytokines decrease thyroid hormones [2, 3, 4].

Cytokines like TNF and IL-1b also cause fatigue by suppressing orexin.

Therefore, inflammation is causing you to have a sluggish thyroid AND also feel fatigued. Your thyroid isn’t actually the main cause of fatigue itself.

IL-6 also causes an increase in reverse T3 (an inactive form of T3) [5].

IL-1b and Interferon-gamma/Th1 dominance also decrease the enzymes needed to make T3 (IL-1b is more significant) [6].

IL-1b can make the thyroid hormones you do have become less effective because it decreases the receptors it binds to [7].

What this means is your thyroid hormone levels can seem normal, but you’re still not getting the energy that you should from a given level of thyroid hormone.

This is possibly why taking thyroid hormones may make some people feel a bit better even if their levels are normal – they require more thyroid hormones to function at the same level as someone who has normal receptors.

Another study found that the reason IL-6 decreased thyroid hormones was because of oxidative stress [8].

The same study also found that the decrease in thyroid function could be reversed by giving NAC, which increases glutathione [8].

So what we see is that inflammation and oxidative stress are the root causes of thyroid problems, in the absence of antibodies.

People with thyroid antibodies usually have elevated inflammation in general, so it’s likely that these people experience a triple whammy of antibodies, inflammation, and oxidative stress.

It’s my position that it would be wise to get at the root of the issue before introducing thyroid hormones in as a confounding variable and which may also worsen your condition.

Once you get on, it’s not easy to get off. However, having low T3 itself is unhealthy, so I’m not advocating that you stay in a low T3 state.

If you want to know what to do about the inflammation then click on the links.

3) Lifestyle, Diet & Supplements

Many supplements decrease thyroid hormones, but not too significantly usually. The exception is iodine.

Though the most important factors are inflammation and oxidative stress, it’s certainly possible other factors are at play and that’s why I’m bringing them down here.

Lifestyle/Dietary factors that decrease thyroid hormones

Numerous toxins, including BPA, pesticides, mercury, and flame retardants such as PBDE, are shown to block tissue thyroid receptors and reduce T4 to T3 conversion.

  • Calorie restriction [9, 10] – significant. If you consume less than 2,100 calories, then that’s a significant reason you’d have lower thyroid hormone levels. You have to measure your calories for a week.
  • Yoga (certain types, subjective). Significant, but only in the short term. I don’t think it will influence your blood test.
  • Stress [11]/Cortisol supposedly blocks conversion to T3 and increases rT3. A study in healthy young men and women could only found that higher cortisol was associated with higher TSH, but not T3 or T4 [12]. A different study could not either find that cortisol decreases T3, even though exercise decreases it. Actually, it found that a lack of cortisol spike decreased both T3 and T4 [13].
  • Depression [11]
  • Low leptin [14] – leptin increases thyroid hormones
  • Saturated fat
  • Low carb diets
  • Low Estrogen [15] – decreases T3 and T4 and increases TSH
  • BPA/Estrogen mimics [16]
  • Insulin Resistance [11]
  • Chronic pain [11] – suppresses Type 1 deiodinase, which lowers T3
  • Low Testosterone [11]
  • Low Growth Hormone [11] – decreases the conversion of T4 and T3
  • Alcohol
  • Toxins: Fluoride, BPA, lead, mercury, Pesticides, radiation
  • Tea, Cruciferous veggies, Soy, Millet, Fenugreek

Nutrient Deficiencies and T4 to T3 Conversion

A deficiency in any of the following can lead to lower thyroid hormone levels. T4 is converted to T3 by Type 1 deiodinase.

  • Iodine – Significant, but not common.
  • Selenium [17] – Needed to convert T4 to T3. Protects against autoimmune thyroid conditions [18]. Too much Selenium (300 mcg) lowered T3 in men in one study [19], but a larger study couldn’t replicate the results [20]. Selenium also increases chromium excretion, so make sure to take chromium as well. I recommend 150 – 250mcg from all sources daily (RDA is 55mcg).
  • Zinc [21] – higher levels were associated with lower free and total T4 in males [22].
  • Iron [21] – needed for TPO.
  • Chromium
  • Copper – higher blood levels were associated with free and total T4 in males and both total T4 and T3 in females [22].
  • Vitamin A [23] – in obese women, vitamin A increases T3 and lowered TSH. There’s a decent chance that this will work in others.
  • B Vitamins: B2B6B12

Supplements that decrease thyroid hormones

Health Risks of Low T3

Hypothyroidism is a very significant predictor of heart disease. People who are hypothyroid are 4X more likely to get heart disease – even after adjusting for traditional risk factors [32]. Since heart disease is the number one killer in the US, this is very significant.

Low free T3 levels even within the normal range are correlated with heart disease and low T3 people also have a worse prognosis, even after adjusting for traditional risk factors (T4 was not associated) [33].)

In a recent observational study, low T3 levels within the ‘normal’ range were correlated with heart disease. The level of free T3 was 4.0pmol/L (2.6pg/ml) in the higher risk group versus 4.6pmol/L (3.0pg/ml) in the lower risk group. Free T3 was also lower in patients with severe heart disease when compared to mild heart disease [34].

Hypothyroidism itself can increase heart disease risk by causing high LDL, excess homocysteine, blood vessel dysfunction, hypercoagulability, impaired fibrinolysis and platelet abnormalities [35]. It also increases LP(a), which is another risk factor for heart disease [36].

Not only does hypothyroidism increase LDL, but it also increases oxidized LDL, which is much worse [37].

However, inflammation and oxidative stress are perhaps the most significant drivers of heart disease, so I believe these are both increasing heart disease risk and also causing low T3.

My Experience With Low T3 Syndrome

I’ve experienced low T3 syndrome for years until I lowered my inflammation levels. Most critical to this was my lectin avoidance diet.

Are You Tired Because of Low Thyroid Hormones?

Naturally, people who have health issues/fatigue will make the connection that since they feel tired a lot and their thyroid hormones are on the low side then it must be their thyroid is what’s causing their issues!

This is not an illogical connection because thyroid hormones are involved in metabolism/energy production. Low thyroid hormones are associated with fatigue, but thyroid hormones themselves aren’t the principal system for wakefulness.

As an aside, the adrenal glands certainly aren’t involved in wakefulness. See my post where I debunk the myth of adrenal fatigue.

Wakefulness is mainly controlled by the hypothalamus and it works by releasing the neurotransmitter orexin. Most of the wakefulness we experience is controlled by orexin.

Thyroid hormones can certainly contribute to wakefulness, but fatigue is more often at its core a hypothalamic/inflammatory issue than a thyroidal issue.

Many people with Hashimoto’s/hypothyroidism are fatigued, but find out after taking thyroid hormones that they still experience fatigue. If you bring your thyroid hormones up and you’re still fatigued, this means that chronic inflammation is your main issue, not your thyroid hormones.

If you want to fight fatigue, you need to activate orexin. I discuss many ways to do this. One way to increase Orexin is by increasing TRH, the hypothalamic precursor to TSH. You can, therefore, indirectly check TRH by looking at your TSH levels. If you have low TSH levels, that will indicate lowish TRH levels.

Don’t get me wrong. If you have low thyroid hormones, you’ll feel fatigued from that. The point I’m trying to bring home is that it makes perfect sense to still feel tired even after you’ve normalized your thyroid hormones.

Be Wary of a Thyroid Diagnoses if You’ve Got Low T3 Syndrome

In my consulting practice, I often see people making a big mistake by taking thyroid hormones in response to fatigue.

People are influenced by what they read online, where others describe having gotten better from thyroid hormones even though their levels were normal.

Thyroid hormones are stimulating, so it’s not too big of a surprise to get a bit of a jolt from it, especially if you have slightly low levels.

But just like I wouldn’t prescribe cocaine to someone who is depressed (it will help in the short term), I wouldn’t suggest taking thyroid hormones if that’s not the root issue.

In the alternative health sphere, thyroid problems are one of the big over-diagnosed conditions. Alternative health practitioners who specialize in thyroid disorders (who can prescribe drugs) sometimes tell people they have a thyroid problem even when their hormones came back normal.

As the saying goes, when all you’ve got is a hammer then everything seems like a nail. The people who focus on thyroid function will dig up a thyroid issue when there is nothing actually wrong with your thyroid gland.

By Taking Thyroid Hormones You Might Be Making Your Condition Worse

What people don’t realize is that by taking thyroid hormones, you can actually be actually making your condition worse in the long term.

You see, inflammation (and T Cell proliferation) is closely tied to metabolism (the production of energy).

This means the higher our metabolism, the more inflammation we experience.

Our immune system requires massive amounts of energy to function and if it doesn’t have it, it will be idle.

The body is smart to become fatigued when we have inflammation. This allows us to conserve energy for our immune system!

This is why mTOR, which increases metabolism, also increases our immune system and inflammation. In the linked article, I listed a bunch of diseases that come from mTOR activation.

Guess what? Thyroid hormones are activators of mTOR [3839].

They can also elevate cytokines like IL-6 (dose dependent) [40].

Perhaps the most detrimental effect is that they increase the worst kind of free radical: superoxide. They do this by increasing energy production in immune cells (and others) [41].

Thyroid hormones also decrease the enzyme that breaks down superoxide (SOD)! [42]

That’s probably the reason why lower thyroid hormone levels are associated with longevity [43].

So, you feel a boost of energy, but your chronic inflammation also goes up.

If you’re fighting a pathogen then revving up your immune system is probably a good idea (if you also use other means to kill it), but if your inflammation is from lectins or some other inflammatory agent, then it’s not a good idea to stimulate your immune system even more.

When we have chronic inflammation, the body is smart to lower our thyroid hormones a bit so that the inflammation is reduced. It’s called homeostasis…

Feedback from My Clients

One client yesterday sent me an email of all of the supplements he’s taking and said this: “T3/T4 cap 114 mcg/27 mcg – 1 cap AM (actually forgot to take this for two days, and think I actually felt better?)”

This client also said, “I did think it was unusual that the more my old doctor increased my thyroid meds, the worse my thyroid-related symptoms became”.

This client was duped into taking it by ‘progressive’ alternative doctor. He’s still taking it even though he didn’t experience any benefit from it because it’s very difficult to get off. And it made his condition worse.

Also, it would be wise to experiment with many other things first before turning to thyroid hormones.

If you’ve tried everything and you still have low thyroid hormones, then you can experiment taking it and only continue with it if you see a significant benefit (in the long run as well).

I would much rather people take TRH for a sluggish thyroid, although it is quite expensive.

My Own Experiments Verify This Theoretical Framework

Last year, I decided to take a high dosage of iodine supplements to see what it was like. There were people who swore by it.

Anyway, it induced a state of weakness and my pulse dropped to under 45 from 65. I didn’t even take that much…

After some research, I realized that a high iodine dosage induces a hypothyroid state and this is what happened to me.

What was magical about this was that all of my inflammation practically disappeared – even after eating gluten or whatever. No fatigue after meals. I did this many times for experimental purposes.

Whenever I do any activity that stimulates my thyroid hormones (like walking for a long time), then the opposite occurs and I get a spike of inflammation from eating something that normally would only cause moderate inflammation.

So my own experience validates that thyroid hormone levels correlate with the inflammation I experience.

I am not advocating to cure your inflammation by being hypothyroid. There are many downsides to being in a hypothyroid state. I ate less and slept much more. I generally felt weak and my libido took a nose-dive.

What I am arguing is to at least not increase it or do so as a last resort if you’ve got no antibodies.

Natural Treatments for Low T3 Syndrome

1) Inhibit mTOR

Since mTOR worsens inflammation, one of the best ways to inhibit chronic inflammation is by inhibiting mTOR.

This pathway is particularly important to people with Hashimoto’s and Grave’s because both of these diseases originate from elevated Th17 inflammation, which is caused by mTOR activation.

2) Be Careful About Taking Iodine

My iodine recommendations are to get a total of 400 mcg and make sure to take it with 50 mcg of selenium (or more).

Getting more than 400mcg can be problematic for many and some would argue even that’s way too much.

Excess iodine increases Th1 and Th17 immune responses [44].

Excess iodine increases IL-17, IL-23, IL-6, and TGF-b and it decreases Tregs–meaning it increases inflammation, especially the Th17 immune system.

It’s no wonder why high iodine intakes are associated with a higher likelihood of autoimmune thyroid disease [45].

4) Beneficial Supplements

Eating foods that are rich in selenium, including brazil nuts, sardines, and shrimp can help increase the conversion of T4 to T3, which increases free T3 levels [46, 47].

Supplements that can help:

  • Selenium [46]
  • Zinc [46]
  • Iron. Iron supplements should be taken with caution and only if iron deficient [48]
  • Taurine [49]
  • N-acetylcysteine (NAC) [50]
  • Vitamin A [51]
  • Vitamin D [52]

5) Lifestyle Changes

Avoid endurance exercise as it has been shown to decrease thyroid function including free T3 levels [53].

If you are consuming too much alcohol, reducing alcohol consumption can help increase free T3 levels [54].

About the Author

Mathew Eng

Mathew Eng

Mathew received his PharmD from the University of Hawaii and an undergraduate degree in Biology from the University of Washington.
Mathew is a licensed pharmacist with clinical experience in oncology, infectious disease, and diabetes management. He has a passion for personalized patient care and believes that education is essential to living a healthy life. His goal is to motivate individuals to find ways to manage their chronic conditions.


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