Overview
Nonthyroidal illness (also known as sick euthyroid syndrome) is an adaptive response of the thyroid to acute of chronic illness.
Causes of Nonthyroidal Illness
- Starvation
- Sepsis
- Burns
- Trauma
- Surgery
- Myocardial infarction
- Malignancy
- Chronic liver disease
- Chronic kidney disease
Hormonal Changes
Look For
- Mild - normal / elevated TSH with elevated T4 and reduced T3
- Moderate - normal / elevated TSH with normal T4 and reduced T3
- Severe - reduced TSH, T4 and T3
- Recovery - elevated TSH with normal T4 and reduced T3
In mild nonthyroidal illness reduced deiodination of T4 to T3 results in elevated T4 and reduced T3. TSH may initially be normal or elevated due to lack of T3.
As nonthyroidal illness progresses and becomes more severe, T4 levels begin to fall; the mechanism of this is more complex. The total T4 level falls more significantly than the free T4, partly due to reduction in levels of thyroid binding globulin. TSH also becomes reduced, likely due to reduced hypothalamic function.
Severity of reduction in T3 and T4 in nonthyroidal illness is associated with risk of mortality.
During recovery from nonthyroidal illness, the T3 and T4 return to normal and there is often an increase in TSH as hypothalamic function returns.
Levels of reverse T3 (rT3) tend to rise during stress states, and as such are a marker of nonthyroidal illness. As this is mainly a clinical diagnosis, rT3 is rarely measured in clinical practice.
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