Thyroid hormone changes appear in patients with serious sickness such as sepsis and septic shock as a favorable adaptation response of metabolic functions to stress and critical sickness [21]. Forty patients were included in our study. NTIS was found in 19 patients (47.5%). Our results were comparable to those obtained by El-Ella et al. 2019 and Hu et al. 2015 who found that 63% and 57% respectively of critically ill children had NTIS [22, 23]. Similarly in neonates, NTIS was diagnosed in 60.7% of full term neonates with sepsis [24]. On the other hand, previous studies showed that all children with meningococcal sepsis and/or undergoing cardiac bypass surgery had NTIS [25, 26]. This wide variability could be explained by differences between the studies in terms of age of studied patients, underlying critical sickness, sample size, assay technique, or other factors like ethnicity or salt iodination. Moreover, medications widely used in PICU could be incriminated. We found that NTIS frequency in septic shock group was nearly double that of the sepsis (65% versus 30%) which was in accordance with results obtained by previous studies that reported thyroid hormone levels were significantly lower in children with septic shock, compared to sepsis [27,28,29]. Also, NTIS was more prevalent among non-survivors than survivors; however, the values did not reach level of significance. Moreover, literature showed that NTIS was significantly correlated with the severity of the disease, and the decline in FT3 levels is used to evaluate mortality [13].
As regards NTIS and FT3 levels’ correlations with stay in PICU, ventilation, catecholamines infusion, SOFA score, and CRP, we observed that
Our findings were consistent with the study by Marks et al. 2009 who reported that lower T3 levels were associated with delayed discharge from PICU, and also reported an association of T3 levels with duration of mechanical ventilation [26].
In 2012, Dilli and Dilmen reported that serum FT3 levels were negatively and strongly correlated with CRP in septic patients [30]. This could be explained by the fact that CRP is inducable by cytokines, especially by IL-6, which could also suppress the iodothyronine 5′-deiodinase which mediates the conversion of T4 into T3 resulting in low T3 levels [8]. These findings could be explained by the fact that the severity of NTIS has been associated with poor clinical outcomes of critical sicknesses [31]. Also, some studies indicated that decreased caloric intake during critical sickness is associated with more pronounced NTIS changes [4].
Specifically considering NTIS components, FT3 and rT3 follow-up data, in D-1 and D-5 of sickness, the following observations were obtained:
In our patients, we observed that NTIS extended with worsing of the condition in day 5 compared to day 1; where we observed that FT3 levels were significantly lower and rT3 levels were significantly higher in day 5 of sickness. Furthermore, in non-surviviors FT3 levels in day 5 were significantly lower, and rT3 levels in days 1 and 5 were significantly higher compared to survivors. Although the mechanisms for the elevation of rT3 levels remain uncertain, we could conceive that they are similar to those involved in NTIS pathogenesis [12]. One mechanism that could explain the isolated elevation of rT3 levels in D-1 is drop of its clearance in the liver that appears earlier, also leading to a fall in the FT3 levels. The low half-life of rT3 (around 3 h compared to 24 h for T3) makes rT3 a sensitive and earliest marker for acute changes in thyroid hormones’ metabolism [32]. Similarly, Hosny et al. 2015 found that only FT3 levels measured during follow up in day 5 was significantly decreased in non-survivors than survivors [33]. Also, Peeters et al. 2005 found significantly lower levels of T3, T4, and TSH in non-survivors only after 5 days from admission [34]. In contrast to our findings Hulst et al., 2005 found that in older children there was significant increase in T3 levels in day 4 and day 6 after admission when compared to admission levels, while levels of rT3 decreased significantly from admission to day 4 and from admission to day 6 [35]. The probable explanation of our findings could be that the decrease in serum thyroid hormone levels is a dynamic process, which develops over time. Hence, the adaptations of the thyroid axis could be delayed. Furthermore, it was probable that the stress response in our patients has not resolved yet, and the return to anabolism was not completed in day 5 of sickness. Besides, our patients showed sickness progression in day 5 confirmed by significance of SOFA score and CRP in day 5 of sickness in non-survivors than survivors.
As regards prediction of mortality
In our study, we found that FT3 levels in day 5 was the only independent predictor of mortality among the thyroid function tests, as indicated by the largest AUC of (0.847) followed by SOFA score with AUC (0.837). These findings were consistent with results in adults in the study by Hosny et al. 2015 who suggested that FT3 levels in day 5 were the only predictor of mortality among all components of thyroid hormones [33].
In conclusion, NTIS is common among critically ill children and significantly higher among septic shock group than sepsis, but it has variable presentations. Also, Thyroid hormone changes have a prognostic value in predicting mortality among critically ill children with sepsis and septic shock. Beside SOFA score, FT3 measured in day 5 of sickness and were the best predictors of PICU mortality. Further studies on larger numbers of cases are necessary to confirm these observations. More frequent assessments of NTIS are needed along course of sepsis and septic shock.
This study had some limitations, first, the small sample size, second, patients in PICU receive many drugs that might affect thyroid hormone levels. Third, some patients could have sub clinical abnormal thyroid functions before the onset of sickness, which could affect the results.