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What Is Euthyroid Sick Syndrome?

Learn how abnormal thyroid test levels along with an acute illness can point to euthyroid sick syndrome.
What Is Euthyroid Sick Syndrome?
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Euthyroid sick syndrome (ESS) is not a thyroid disorder. Instead, it’s a term used to describe changes in thyroid levels that occur as a result of other acute illnesses or health conditions. Ahead, a look at ESS and its symptoms, and how it’s diagnosed and treated.

What is euthyroid sick syndrome (ESS)?

Euthyroid sick syndrome (ESS) refers to abnormal thyroid levels – resulting from an acute illness – in a patient with no preexisting or current dysfunction in the thyroid gland. ESS is also known as nonthyroidal illness syndrome (NTIS), sick euthyroid syndrome (SES), low T3 low T4 syndrome, and thyroid allostasis in critical illness, tumors, uremia, and starvation, or TACITUS.

While the mechanism behind ESS is not well understood, it’s theorized that in the face of an acute illness – known as a nonthyroidal illness, or NTI – the body goes into metabolic overdrive, and speeds up the process of muscle loss and more rapidly burns calories. As a way to help throttle back the speed of the metabolism, cytokines and other inflammatory mediators in the body reduce levels of thyroid hormones. Specifically, triiodothyronine (T3) and thyroxine (T4) can be reduced, and the conversion of T4 into T3 – the active thyroid hormone – is also slowed. In this way, it’s theorized that ESS may be the body’s “adaptive” way to conserve energy in response to acute illness.

Who is at risk of ESS?

ESS can develop when you have a nonthyroidal illness (NTI), which is defined as any severe illness that causes decreases in thyroid hormone levels. An NTI can develop during any acute health condition, but most commonly develops when you’re hospitalized for burns, malignancy, sepsis (widespread infection), major surgery, pneumonia, heart attack, heart bypass, bone marrow transplantation, hypothermia, renal failure, diabetic ketoacidosis, and cirrhosis of the liver. Chronic starvation, anorexia, or calorie deprivation are also known triggers of both NTI and ESS.

It’s estimated that around 75% of hospitalized patients have ESS. The rate may be even higher in patients who are hospitalized in an intensive care unit (ICU).

ESS can affect males and females of any age.

Newborns in neonatal intensive care units (NICUs) also face a higher risk of developing ESS.

What are the different types of euthyroid sick syndrome?

The most common – and less dangerous -- pattern in ESS is a low T3 level, with low or normal T4 and TSH levels.

A less common but more severe form of ESS is referred to as “low T3 and low T4 syndrome.” Low T3 and low T4 are more common in patients who are hospitalized with severe illness in the ICU. The mortality rate of ESS increases significantly with increasing severity and length of the underlying health problem, and a drop in the T4 level.

What are the symptoms of euthyroid sick syndrome?

The symptoms of ESS correlate with common symptoms of hypothyroidism and include fatigue and tiredness, body and muscle weakness, weight gain, puffiness in the face and extremities, intolerance to cold, constipation, and a slowed heart rate. Other common symptoms include:

How is euthyroid sick syndrome diagnosed?

Typically, to diagnose ESS, thyroid function tests are needed -- specifically, the Thyroid Stimulating Hormone (TSH), free T4, and free T3 levels. In some cases, reverse T3 may be tested as well.

According to experts, the biggest diagnostic challenge is in differentiating between actual hypothyroidism and ESS. Typically, if the TSH level is elevated, and Free T3 and Free T4 levels are low, hypothyroidism is the likelier diagnosis. To diagnose ESS, practitioners look for:

  • Normal thyroid antibodies, indicating the absence of Hashimoto’s or Graves’ disease
  • Absence of goiter or other indicators of structural thyroid disease
  • Low Free T3 levels
  • Low Free T4 levels
  • Elevated Reverse T3 levels

Note: During recovery from the NTI, the TSH level can sometimes become elevated for a short period, before returning to normal.

How is euthyroid sick syndrome treated?

According to experts, there is no treatment for ESS other than to treat the underlying condition, which resolves abnormal thyroid test levels. Thyroid function is not affected after recovery.

The issue of thyroid treatment in ESS is controversial, however. Noted endocrinologist Dr. Leslie DeGroot argued in a journal article that “non-thyroidal illness syndrome is a manifestation of hypothalamic-pituitary dysfunction, and in view of current evidence, should be treated with appropriate hormone replacement therapies,” including thyroid hormone.

Several studies have shown the benefits of treatment with T3 in selected cases of ESS, but more research is needed.

How is euthyroid sick syndrome connected to COVID-19?

ESS has been in the news lately, with the discovery that low T3 levels in COVID-19 patients are, according to researchers, a "predictive marker for poor prognosis, even in the early stages of the disease."

Also, people who have COVID-19 and develop ESS have longer hospitalizations, higher rates of intubation and other interventions, and a higher mortality rate.


A note from Paloma

Unless you are hospitalized with a critical illness or are in a situation where you are severely restricting calories, you’re not at a high risk of developing ESS. That said, it’s also important to understand that if you have undiagnosed and untreated hypothyroidism, you’re at an increased risk of developing ESS if you become ill with an acute health condition. That means that it’s important to detect – and treat – underlying hypothyroidism as early as possible.

The Paloma Complete Thyroid Blood Test kit can help you stay on top of your thyroid health and test levels. This kit – which measures TSH, Free T4, Free T3, and Thyroid Peroxidase antibodies (TPOAB) – makes it easy to get accurate and painless thyroid testing from the convenience of home.

Dealing with Hypothyroidism?  Video chat with a thyroid doctor

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McIver B, Gorman CA. Euthyroid Sick Syndrome: An Overview. Thyroid. 1997;7(1):125-132. doi:10.1089/thy.1997.7.125,

Ganesan K, Ashorobi D, Wadud K. Euthyroid Sick Syndrome. PubMed. Published 2020.

‌What Is Euthyroid Sick Syndrome (ESS)? Published August 22, 2022. Accessed January 21, 2023.

Chopra IJ. Euthyroid Sick Syndrome: Is It a Misnomer? The Journal of Clinical Endocrinology & Metabolism. 1997;82(2):329-334. doi:10.1210/jcem.82.2.3745

De Groot LJ. Non-thyroidal illness syndrome is a manifestation of hypothalamic-pituitary dysfunction, and in view of current evidence, should be treated with appropriate replacement therapies. Critical Care Clinics. 2006;22(1):57-86, vi. doi:10.1016/j.ccc.2005.10.001

Yair S, Ruth P, Bernice O, Yaffe D, Eyal Z, Amir T. Sick euthyroid syndrome on presentation of COVID-19 patients, a potential marker for disease severity. Endocrine Practice. Published online January 2021. doi:10.1016/j.eprac.2021.01.001

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Mary Shomon

Patient Advocate

Mary Shomon is an internationally-recognized writer, award-winning patient advocate, health coach, and activist, and the New York Times bestselling author of 15 books on health and wellness, including the Thyroid Diet Revolution and Living Well With Hypothyroidism. On social media, Mary empowers and informs a community of more than a quarter million patients who have thyroid and hormonal health challenges.

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