What euthyroid sick syndrome?

Updated: Aug 28, 2020
  • Author: Serhat Aytug, MD; Chief Editor: Romesh Khardori, MD, PhD, FACP  more...
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Euthyroid sick syndrome (also known as nonthyroidal illness syndrome) can be described as abnormal findings on thyroid function tests that occur in the setting of a nonthyroidal illness (NTI), without preexisting hypothalamic-pituitary and thyroid gland dysfunction. After recovery from an NTI, these thyroid function test result abnormalities should be completely reversible. [1, 2]

Multiple alterations in serum thyroid function test findings have been recognized in patients with a wide variety of NTIs without evidence of preexisting thyroid or hypothalamic-pituitary disease. The most prominent alterations are low serum triiodothyronine (T3) and elevated reverse T3 (rT3), leading to the general term "low T3 syndrome." Thyroid-stimulating hormone (TSH), thyroxine (T4), free T4 (FT4), and free T4 index (FTI) also are affected in variable degrees based on the severity and duration of the NTI. As the severity of the NTI increases, both serum T3 and T4 levels drop and gradually normalize as the patient recovers, as shown in the image below.

Euthyroid sick syndrome. Relationship between seru Euthyroid sick syndrome. Relationship between serum thyroid hormone concentrations and severity of nonthyroidal illness (NTI). Abbreviations: reverse triiodothyronine (rT3), thyroid-stimulating hormone (TSH), free thyroxine (FT4), thyroxine (T4), triiodothyronine (T3).

TSH is affected in variable degrees, but, in the overwhelming majority of patients, TSH is above 0.05 μ IU/mL. In severe, critical illness, most patients have reduced T4 levels. In patients hospitalized for NTI, about 10% have abnormally low TSH values; the highest incidence occurs in the most severely ill group. In the sickest patients who manifest low T4, TSH elevates to hypothyroid levels at the recovery phase, returning to reference range levels with complete recovery, as shown in the image below.

Euthyroid sick syndrome. Relationship between seve Euthyroid sick syndrome. Relationship between severity and duration of nonthyroidal illness (NTI) and thyroid hormone levels. Abbreviations: reverse triiodothyronine (rT3), free thyroxine (free T4), thyroxine (T4), triiodothyronine (T3).

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