The increased serum reverse T3 and decreased T3 during caloric deprivation and non-thyroidal illness is caused by decreased T3 production (with intact degradation) and reversed T3 degradation (with intact production) respectively. These changes can ensue from two mechanisms i.e. decreased 5'D of T4 and of reverse T3 (possibly caused by a decrease in naturally occurring reducing agents) or by decreased transport of T4 and reverse T3 into the liver (possibly caused by decreased ATP concentrations in the liver). The effects of the low T3 syndrome at the tissue level are in many instances comparable to those seen in hypothyroidism. The effects lead to conservation of energy and decrease of protein breakdown. These effects are considered to constitute a beneficial adaptative mechanism in situations in which the organism is endangered. There is no evidence that treatment of patients with the low T3 syndrome with thyroid hormones is of any benefit. Knowledge at the present moment suggests that administration of thyroid hormones during caloric deprivation or non-thyroidal illness should be avoided.