Table 2.

Summary of alterations in plasma concentrations of THs during critical illnesses

Condition	THs Alterations	Main Mechanisms
Acute illness	↑rT3, ↓T3, ↓fT3, normal T4, normal fT4, normal TSH	Inhibition of 5′‐MDI, which deiodinates T4 to T3 and rT3 to T2
Severe chronic illness	↑rT3, ↓T3, ↓fT3, normal fT4, ↓T4, ↓/normal TSH	1. Inhibition of 5′‐MDI
		2. Low binding of THs to serum proteins (especially low TBG binding)
		3. Reduction of TRH release by the hypothalamus, causing a decrease in TSH secretion

Abbreviations: 5′‐MDI, type I iodothyronine 5′‐monodeiodinase; fT3, free triiodothyronine; fT4, free thyroxine; rT3, reverse triiodothyronine; T2, diiodothyronine; T3, triiodothyronine; T4, thyroxine; TBG, thyroxine‐binding globulin; THs, thyroid hormones; TRH, thyrotropin‐releasing hormone; TSH, thyroid‐stimulating hormone.