The hypothalamus-pituitary-thyroid axis in critical illness

Area of Science:

• Endocrinology
• Critical Care Medicine
• Molecular Biology

Background:

• The hypothalamus-pituitary-thyroid (HPT) axis regulates thyroid hormone (TH) production and metabolism.
• Critically ill patients frequently develop the low T3 syndrome (non-thyroidal illness), characterized by reduced circulating and tissue TH levels.
• The clinical significance of the low T3 syndrome, whether protective or detrimental, remains debated.

Purpose of the Study:

• To explore recent insights into the pathophysiology of the low T3 syndrome in critical illness.
• To investigate potential compensatory mechanisms involving TH transport and deiodinase activity.
• To elucidate the role of hypothalamic thyrotropin-releasing hormone (TRH) in prolonged critical illness.

Main Methods:

• Review of recent studies in animal models and human patients.
• Analysis of alterations in thyroid hormone (TH) transport mechanisms, including monocarboxylate transporter 8 (MCT8).
• Examination of changes in intracellular iodothyronine deiodinase activity.
• Investigation of hypothalamic TRH expression levels.

Main Results:

• Evidence suggests alterations in TH transport and deiodinase activity in critical illness.
• These alterations may represent compensatory responses by peripheral tissues and the hypothalamus to low circulating TH.
• Reduced hypothalamic TRH expression is implicated in the prolonged phase of critical illness.

Conclusions:

• The low T3 syndrome in critical illness involves complex alterations in TH metabolism and transport.
• Peripheral and central compensatory mechanisms may be activated.
• The upstream triggers for hypothalamic TRH downregulation in critical illness require further investigation.