Nonthyroidal illness

Abstract

A few hours after the onset of acute illness, marked changes in serum thyroid hormone levels occur. This is referred to as nonthyroidal illness (NTI). The most characteristic and persistent abnormality is a low level of serum triiodothyronine (T3). Despite these low levels of serum T3, patients usually have no clinical signs of thyroid disease. Other terms for this disease state have been used, e.g. the low T3 syndrome and the euthyroid sick syndrome. In addition to nonthyroidal illness, a low T3 in euthyroid patients is seen during caloric deprivation and after the use of certain types of medication (see Chapter 3.1.4). Low levels of thyroid hormone in hypothyroidism are associated with a decreased metabolic rate. Both in nonthyroidal illness and in fasting there is a negative energy balance in the majority of cases. Therefore the low levels of T3 during nonthyroidal illness and starvation have been interpreted as an attempt to save energy expenditure, and intervention is not required. However, this remains controversial and has been a debate for many years. In this chapter, the changes in thyroid hormone levels, the pathophysiology behind these changes, the diagnosis of intrinsic thyroid disease, and the currently available evidence whether these changes should or should not be corrected will be discussed (Box 3.1.5.1).