What is euthyroid sick syndrome (also known as non-thyroidal illness syndrome or NTIS), a condition where thyroid function tests are abnormal in the absence of primary thyroid disease, often occurring in patients with systemic illnesses?

From the Research

Euthyroid sick syndrome, also called nonthyroidal illness syndrome, is a condition where patients with acute or chronic non-thyroidal illnesses have abnormal thyroid function tests despite not having thyroid disease, and treatment should focus on addressing the underlying illness rather than thyroid hormone replacement, unless true hypothyroidism is confirmed, as evidenced by the most recent study in 2021 1.

Definition and Pathophysiology

Euthyroid sick syndrome typically presents with low T3 levels, normal or low TSH, and normal or low T4 levels depending on severity, as a result of adaptive mechanisms that alter thyroid hormone metabolism in response to serious illnesses, such as severe infections, trauma, heart failure, cancer, or critical illness 2.

Clinical Presentation and Diagnosis

The syndrome can be challenging to diagnose, particularly in patients without prior thyroid function test results, as the changes in serum thyroid hormone levels can be seen within the first hours of critical illness and correlate with final outcome 2.

Treatment and Management

Thyroid hormone supplementation is generally not recommended unless true hypothyroidism is confirmed, as the changes represent a physiological adaptation to illness, and the thyroid abnormalities usually resolve when the patient recovers from the primary condition, although some studies suggest that thyroid hormone supplementation may be beneficial in selected pediatric sub-populations 1.

Key Considerations

The underlying cause of euthyroid sick syndrome has not yet been fully elucidated, and multiple mechanisms, including alterations in iodothyronine deiodinases, thyroid-stimulating hormone secretion, and thyroid hormone receptor activity, contribute to its development 3.

• The syndrome is commonly seen in patients with severe infections, trauma, heart failure, cancer, or those who are critically ill.
• Treatment focuses on addressing the underlying illness rather than thyroid hormone replacement.
• Thyroid hormone supplementation is generally not recommended unless true hypothyroidism is confirmed.
• The changes in serum thyroid hormone levels can be seen within the first hours of critical illness and correlate with final outcome.
• Further studies are required to assess the beneficial effect of thyroid hormone on patients with end-stage heart failure supported by ventricular assist devices 1.