To evaluate the course of hormonal parameters in relation to clinical parameters, illness severity and nutritional intake in children admitted to the pediatric ICU during the first week of admission.Prospective, observational study. Levels of triiodothyronine (T 3 ), reverse T 3 (rT 3 ), ratio T 3 /rT 3 and insulin-like growth factor I (IGF-1) were evaluated in 84 critically ill children (50 term neonates and 34 children aged 32d–15yr) admitted to our multidisciplinary tertiary pediatric intensive care unit within 24h after admission, on days 4 and 6 after admission. Changes in hormones levels over time were related to illness severity, C-reactive protein-levels and the adequacy of feeding.For both age groups IGF-1 levels remained low until day 4, but at day 6 IGF-1 levels were significantly higher than admission level. In 88% and 89% of the older children, T 3 levels remained below normal at days 4 and 6, respectively. In both age groups, rT 3 levels declined significantly over time, whereas the ratio T 3 /rT 3 increased. The increases in IGF-1, T 3 and ratio T 3 /rT 3 and decrease in rT 3 were significantly associated with decreases in CRP-levels. No significant relations were found between changes in IGF-1 and thyroid hormone levels during admission and the adequacy of energy and protein intake.During the first week of ICU-admission, abnormalities in levels of IGF-1, T 3 and rT 3 were frequently observed in critically ill children, especially in the children aged >1 month. The adequacy of feeding did not seem to affect the normalization of IGF-1 and thyroid hormone levels.