[Low T3 syndrome]

Many nonthyroidal illnesses are associated with reduced serum T3 and, in more severe conditions, T4 concentrations, without increased serum TSH secretion. The term euthyroid sick syndrome (ESS) identifies these low T3 and low T3-T4 states. It may be difficult to exclude central hypothyroidism in patients admitted to medical intensive care units. The mortality rate has been found to be inversely correlated with the serum T4 concentration. These abnormalities are a consequence of altered hepatic T3 production from T4 (through type 1 5'-desiodase inhibition), T4 serum protein binding and hepatic transport, and TSH secretion. Many mediators and drugs are involved in the pathogenesis of ESS. The role of cytokines has recently been emphasized. It remains unclear whether ESS represents a physiological protective mechanism, or a damaging maladaptative response and whether early thyroid hormones administration might improve survival in severe non thyroidal conditions.