Changes within the thyroid axis during the course of critical illness

Area of Science:

• Endocrinology
• Critical Care Medicine
• Neuroendocrinology

Background:

• Critical illness causes significant alterations in plasma thyroid hormone levels.
• These changes occur in both acute and prolonged phases of critical illness.
• The low triiodothyronine syndrome is a hallmark of critical illness, impacting thyroid hormone homeostasis.

Purpose of the Study:

• To review the mechanisms of thyroid hormone level changes during critical illness.
• To elucidate the neuroendocrinology of the low triiodothyronine syndrome.
• To explore thyroid hormone metabolism, including deiodination and transport, in critical illness.

Main Methods:

• Literature review of studies on thyroid hormone levels in critical illness.
• Analysis of neuroendocrine regulation of thyroid hormones.
• Examination of thyroid hormone deiodination and transport pathways.

Main Results:

• Critical illness disrupts the hypothalamic-pituitary-thyroid axis.
• Deiodinase enzyme activity and thyroid hormone transporter function are altered.
• These metabolic changes contribute to the characteristic low triiodothyronine state.

Conclusions:

• Thyroid hormone level dysregulation in critical illness is multifactorial.
• Neuroendocrine and metabolic factors play key roles.
• Further research is needed to optimize thyroid hormone management in critically ill patients.