What is sick euthyroid syndrome (Non-Thyroidal Illness Syndrome), a condition where thyroid function tests are abnormal in the absence of thyroid disease, often seen in critically ill patients?

From the Research

Sick euthyroid syndrome, also known as non-thyroidal illness syndrome, is a condition where patients with normal thyroid function develop abnormal thyroid hormone levels during severe illness, and no specific treatment is needed for the thyroid abnormalities themselves, as they typically resolve once the underlying illness improves. This condition is characterized by low serum levels of triiodothyronine (T3) and high levels of reverse T3, with normal or low levels of thyroxine (T4) and normal or low levels of thyroid-stimulating hormone (TSH) 1. The focus should be on treating the primary illness causing these changes, as the thyroid hormone alterations are thought to be an adaptive response to conserve energy during illness rather than true thyroid dysfunction 2.

Some key points to consider in the management of sick euthyroid syndrome include:

• The severity of thyroid hormone alterations often correlates with illness severity and can have prognostic value, with more pronounced changes indicating more severe illness 3.
• Thyroid hormone replacement is generally not recommended unless there is clear evidence of primary thyroid disease, as studies have not shown benefit and may potentially cause harm 4.
• If thyroid function tests are abnormal during acute illness, it's best to repeat testing after recovery before considering thyroid-specific interventions 5.
• The changes in serum thyroid hormone levels in the critically ill patient seem to result from alterations in the peripheral metabolism of the thyroid hormones, in TSH regulation, in the binding of thyroid hormone to transport-protein and in receptor binding and intracellular uptake 1.

In terms of prognosis, the severity of thyroid hormone alterations can have prognostic value, with more pronounced changes indicating more severe illness 3. Additionally, some studies have suggested that certain thyroid indicators, such as reverse T3, may have prognostic value in critically ill patients 3. However, more research is needed to fully understand the relationship between thyroid hormone alterations and prognosis in sick euthyroid syndrome. The most important consideration in the management of sick euthyroid syndrome is to prioritize treatment of the underlying illness, rather than the thyroid hormone abnormalities themselves.