(a) Simplified overview of the major changes occurring within the thyroid axis during the acute and chronic phase of critical illness. The normal regulation of the thyroid axis is shown in black, whereas the alterations induced by critical illness are indicated in grey. As discussed in the text, for the acute phase of critical illness, thyrotropin and T4 levels are elevated briefly and subsequently return to normal. T2, diiodothyronine. (Reproduced from Van den Berghe G. Novel insights into the neuroendocrinology of critical illness. Eur J Endocrinol, 2000; 143: 1–13, with permission.) (b) Relationship between the different iodothyronines and the outer ring (D1, D2) and inner ring (D3) deiodination by the three deiodinases are shown on the left. Like D3, D1 has some inner ring deiodination capacity in vitro, but there is currently no evidence that this is of any significance in vivo (see also Chapter 3.1.2). This is therefore omitted from the figure. Observed changes in deiodinase activities and iodothyronine levels during critical illness, both in the acute and chronic phase of critical illness, are shown on the right. (Reproduced from Peeters RP, Debaveye Y, Fliers E, Visser TJ. Changes within the thyroid axis during critical illness. Crit Care Clinics, 2006; 22: 41–55; with permission.)
