Thyroid Hormones and Illness

Abstract

To review illness-associated changes in thyroid hormone levels with respect to the alterations in metabolism, regulation, and transport of thyroid hormones present in such a setting. We summarize normal pituitary-thyroid function and categorize the types of changes that have been noted during illness. In addition, we examine studies that have addressed the metabolic status of patients with low thyroid hormone levels and the potential utility of thyroid hormone treatment in severely ill patients. The finding of altered thyroid hormone levels in hospitalized patients confirms the participation of these factors in the adaptive response to illness. Thyroid hormone alterations that occur during illness can be classified into three categories: (1) decreased production of triiodothyronine, (2) decreased serum binding of thyroid hormones, and (3) decreased secretion of thyrotropin. Study of these hormonal changes has provided insight into normal thyroid hormone regulation and metabolism as well as into neuroendocrine adaptations to illness. It seems reasonable to conclude that illness-associated changes in thyroid hormone levels are generally beneficial to the patient as a whole; however, the possibility of concomitant regional or individual tissue hypothyroidism exists. Conclusions about the potential utility of pharmacologic administration of triiodothyronine in selected diseases and clinical settings must await additional clinical trials. Finally, the predictable occurrence of altered thyroid hormone levels in hospitalized patients necessitates caution in both the selection and the interpretation of thyroid hormone tests in this patient population.