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Department of Internal Medicine (R.P.P., E.K., H.v.T., T.J.V.), Erasmus University Medical Center, 3015 GE Rotterdam, The Netherlands; and Department of Intensive Care Medicine (P.J.W., G.V.d.B.), Catholic University of Leuven, B-3000 Leuven, Belgium
Address all correspondence and requests for reprints to: Greet Van den Berghe, M.D., Ph.D., Department of Intensive Care Medicine, Catholic University of Leuven, B-3000 Leuven, Belgium. E-mail: greta.vandenberghe{at}med.kuleuven.ac.be.
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Abstract |
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Substitution of critically ill patients with high doses of GH, corticosteroids, or thyroid hormone has been found to have no or even a negative effect on clinical outcome (4, 5, 6, 7). In a recent study, it was shown that strict control of blood glucose levels less than 110 mg/dl with intensive insulin therapy markedly reduces morbidity and mortality in critically ill patients (8). Endocrine intervention with hypothalamic releasing factors, which restores pulsatile pituitary hormone secretion and normalizes peripheral hormone levels, may be another successful approach (3, 9).
In this study we focused on the peripheral metabolism of thyroid hormone in critically ill patients. In critical illness, serum T3 concentration decreases and serum rT3 increases, the magnitudes of these changes being related to the severity of disease (10). Although serum T4 and free T4 (FT4) may be increased in mild illnesses, serum T4 is decreased, and FT4 normal or decreased in severely ill patients (10).
In humans, peripheral thyroid hormone metabolism is mediated importantly by the three iodothyronine deiodinases D1, D2, and D3 (see Refs. 11 and 12 for reviews). D1 is present in liver, kidney, and thyroid and plays a key role in the production of serum T3 from T4 and in the breakdown of the metabolite rT3 (11, 12). D2 is present in brain, pituitary, thyroid, and skeletal muscle and also converts T4 by outer-ring deiodination to T3. In tissues such as the brain, D2 is important for local T3 production, but the enzyme in skeletal muscle may also contribute to plasma T3 production (11, 12). D3 is present in brain, skin, placenta, pregnant uterus, and various fetal tissues; it catalyzes the inactivation of T4 and T3 by inner-ring deiodination to rT3 and 3,3'-diiodothyronine (3,3'-T2), respectively (11, 12).
The strongly reduced circulating T3 levels in sick patients may be due in part to a decreased peripheral T4 deiodination by D1, D2, or both (12, 13, 14). The increase in serum rT3 levels is explained by a decrease in D1 activity because D1 is the principal pathway for rT3 clearance (15). Besides a decreased D1 activity, an impaired transport of T4 and rT3 into D1-containing tissues such as liver may be another important mechanism for the changes in thyroid hormone levels associated with illness (16). However, the possibility that an increased D3 activity contributes to the reduced serum T3 levels and increased rT3 levels should also be considered. Patients with D3-expressing hemangiomas may have very low serum T4 and T3 concentrations, combined with very high serum rT3 levels (17). The term consumptive hypothyroidism is used for this syndrome. There are hitherto no data on the eventual role of D3 induction in the low T3 syndrome of severe illness.
In this study, serum samples and liver and skeletal muscle biopsies were obtained from 80 patients within minutes after they died in a surgical intensive care unit (ICU). The patients had been randomized for intensive or conventional insulin therapy as recently described (8). Tissue D1, D2, and D3 gene expression and activity as well as serum TSH, T4, T3, and rT3 concentrations were measured. The correlations among these analytes as well as their correlations with several clinical parameters were calculated.
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Materials and Methods |
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Nonradioactive iodothyronines were obtained from Henning (Berlin, Germany). [3',5'-125I]T4 (with a specific activity of ca. 2000 mCi/µmol) was obtained from Amersham Pharmacia (Rozendaal, The Netherlands). [3'-125I]T3 and [3',5'-125I]rT3 (with both a specific activity of ca. 2000 mCi/µmol) were prepared by radioiodination of 3,5-T2 and 3,3'-T2, respectively (18). [125I]T4 and [125I]rT3 were purified immediately before use by Sephadex LH-20 (Amersham Pharmacia) chromatography (19). N-bromoacetyl-[125I]T3 (BrAc[125I]T3) was prepared as previously described (20). Its purity was checked by HPLC analysis. Protein molecular weight markers, and 6-n-propyl-2-thiouracil (PTU) were obtained from Sigma (Zwijndrecht, The Netherlands); dithiothreitol (DTT) from ICN (Zoetermeer, The Netherlands); electrophoresis grade SDS-PAGE reagents from Bio-Rad (Veenendaal, The Netherlands); Coomassie Brilliant Blue R-250 from Merck (Darmstadt, Germany); and TaqMan probes and primers from Biosource (Nivelles, Belgium). All other chemicals used in this study were of reagent grade.
Subjects
This study was part of a large randomized, controlled study on intensive insulin treatment in ICU patients (n = 1548), of which the major clinical outcomes have been published in detail elsewhere (8). On admission, patients were randomly assigned to either strict normalization of blood glucose (80–110 mg/dl) with intensive insulin therapy or the conventional approach, in which insulin infusion is initiated only when blood glucose exceeds 215 mg/dl, to maintain blood glucose levels between 180 and 200 mg/dl. Maximal insulin dose was arbitrarily set at 50 IU/h. The study protocol has been approved by the Ethical Review Board of the University of Leuven School of Medicine, and patients were included after informed consent from the closest family member.
A total of 80 patients was included in this study. Blood samples were obtained from 65 patients, liver biopsies from 66 patients, and skeletal muscle (rectus abdominis) biopsies from 66 patients within minutes after death [25.2 ± 20.0 (SD), range (5–97 min) for liver and 20.7 ± 19.7, range (0–95 min) for skeletal muscle]. From 51 patients blood, liver, and skeletal muscle samples were available. All patients had been randomized for conventional or intensive insulin treatment (8). Thirty-one patients had been treated with thyroid hormone during the course of their critical illness when they had a serum T4 concentration less than 50 nmol/liter in the face of a normal thyroxine-binding globulin and concomitantly clinical symptoms of hypothyroidism, defined as coma or central nervous system suppression, failure to wean from the ventilator, or hemodynamic instability, which were unexplained and resistant to conventional supportive therapy. In these cases, thyroid hormone treatment consisted of an iv bolus of 150 µg T4 daily plus 0.6 µg T3 per kg body weight per 24 h as a continuous iv infusion. All patients included in this study had died in the ICU, and the cause of death was determined both clinically by the attending ICU physician and postmortem examination. The pathologist was unaware of insulin treatment allocation. Relevant patients’ characteristics are summarized in Table 1
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The care of patients in the ICU often comprises infusion of heparin, either systemically or locally to prevent clotting of vascular access, which substantially interferes with the assay used to quantify free concentrations of thyroid hormone (21). Therefore, we refrained from measuring serum FT4 and free T3 in this study. Serum total T4, total T3, and TSH were measured by chemoluminescence assays (Vitros ECi Immunodiagnostic System, Ortho-Clinical Diagnostics, Amersham, UK). The rT3 was measured by RIA as previously described (22). Within-assay coefficients of variation amounted to 4% for TSH, 2% for T4, 2% for T3, and 3–4% for rT3. Normal values for TSH, T4, T3, and rT3 were determined in 270 healthy individuals. Mean ± 2 SD was used as the normal range for T4, T3, and rT3, whereas the 95% confidence interval was used for TSH.
Deiodinase activities
Human liver and skeletal muscle samples were homogenized on ice in 10 volumes of PE buffer [0.1 M phosphate and 2 mM EDTA (pH 7.2)] using a Polytron (Kinematica AG, Lucerne, Switzerland). Homogenates were snap frozen in aliquots and stored at -80 C until further analysis. Protein concentration was measured with the Bio-Rad protein assay using BSA as the standard following the manufacturer’s instructions.
Liver D1 activities were determined as described earlier (23) by duplicate incubations of homogenates (10 µg protein) for 30 min at 37 C with 0.1 µM [3',5'-125I]rT3 (100,000 cpm) in a final volume of 0.1 ml PED10 buffer (PE + 10 mM DTT). To validate the specificity of the D1 assay, some incubations were also carried out in the presence of 0.1 mM of the D1 inhibitor PTU or excess unlabeled rT3 (1 µM). Reactions were stopped by addition of 0.1 ml 5% (wt/vol) BSA in water on ice. The protein-bound iodothyronines were precipitated by addition of 0.5 ml ice-cold 10% (wt/vol) trichloroacetic acid in water. Following centrifugation, 125I- was isolated from the supernatant by chromatography on Sephadex LH-20 minicolumns (24). Skeletal muscle D1 activities were assayed similarly, using 200 µg homogenate protein in a 60-min incubation.
Liver and skeletal muscle D2 activities were assayed as earlier described (25) by duplicate incubation of 200 µg homogenate protein for 60 min at 37 C with 1 nM [3',5'-125I]T4 (100,000 cpm) in a final volume of 0.1 ml PED25 buffer (PE + 25 mM DTT). The incubations were carried out in the presence of 0.1 µM unlabeled T3, to prevent inner-ring deiodination of the labeled T4 substrate by D3, if present, and in the absence or presence of 0.1 µM unlabeled T4, which is sufficient to saturate D2. Deiodination of labeled T4 in the absence minus that in the presence of excess unlabeled T4 represents D2 activity. The further procedure for the quantitation of 125I- production was the same as described above for the D1 assay.
Tissue D3 activities were measured as described earlier (24) by duplicate incubation of liver (100 µg protein) or skeletal muscle (200 µg protein) homogenate for 60 min at 37 C with 1 nM [3'-125I]T3 (200,000 cpm) in a final volume of 0.1 ml PED50 buffer. To validate the D3 assay, some incubations were also carried out in the presence of 10 or 100 nM unlabeled T3. Reactions were stopped by addition of 0.1 ml ice-cold methanol. After centrifugation, 0.15 ml of the supernatant was added to 0.1 ml 0.02 M ammonium acetate (pH 4), and 0.1 ml of the mixture was applied to a 4.6 x 250 mm Symmetry C18 column connected to an Alliance HPLC system (Waters, Etten-Leur, The Netherlands). The column was eluted with a linear gradient of acetonitrile (28%–42% in 15 min) in 0.02 M ammonium acetate (pH 4.0) at a flow of 1.2 ml/min. The radioactivity in the eluate was measured on-line using a Radiomatic A-500 flow scintillation detector (Packard, Meriden, CT).
Affinity labeling
Affinity labeling of liver D1 was done as previously described (23) in samples with low, intermediate, or high D1 activity. Briefly, homogenate (100 µg protein) was incubated for 15 min at 37 C with 100,000 cpm (
25 fmol) BrAc[125I]T3 in 75 µl PED1 buffer (PE + 1 mM DTT). To specifically block affinity labeling of D1 protein, some incubations were carried out in the presence of 10 µM rT3 and 100 µM PTU (23). Labeling was stopped by the addition of SDS-loading buffer and heating the mixture for 5 min at 80 C. Proteins (100 µg per lane) were separated by SDS-PAGE gel electrophoresis in a 12% resolving, 5% stacking gel (23). Gels were stained with Coomassie Brilliant Blue R-250, dried at 80 C under vacuum, and autoradiographed by exposure overnight at -80 C to Biomax imaging film (Eastman Kodak, Rochester, NY).
RNA isolation and reverse transcription (RT)
RNA was isolated from liver samples using the High Pure RNA tissue kit (Roche Diagnostics, Almere, The Netherlands) according to the manufacturer’s protocol. RNA concentrations were determined using the RiboGreen RNA quantitation kit (Molecular Probes, Leiden, The Netherlands). All samples were diluted to 0.1 µg/µl, and 1 µg was used for cDNA synthesis using the TaqMan RT kit (Roche Diagnostics).
Real-time RT-PCR
D1 and D3 mRNA levels were determined in a set of liver samples with low, intermediate, and high deiodinase activities. The ABI PRISM 7700 sequence detection system (Applied Biosystems, Nieuwerkerk aan den IJssel, The Netherlands) was used, which uses TaqMan chemistry for highly accurate quantitation of mRNA levels. Sequences and concentrations of the primers and probes are given in Table 2. Hepatic D1 and D3 mRNA levels are expressed relative to those of the glyceraldehyde-3-phosphate dehydrogenase housekeeping gene. The glyceraldehyde-3-phosphate dehydrogenase probe and primers were provided as preoptimized control system (Applied Biosystems).
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Ct (i.e. the Ct value of the target gene minus the Ct value of the housekeeping gene). Statistical analysis
Data were analyzed using the statistical program SPSS 10.0.7 for Windows (SPSS Inc., Chicago, IL). Logarithmic transformations were applied to normalize variables and minimize the influence of outliers, when appropriate. All analyses were done on the whole group as well as on subgroups treated or not treated with thyroid hormone. Data were analyzed using one-way ANOVA tests, with a post hoc Fisher’s least significant difference test for multiple comparisons, t tests, Mann-Whitney U tests, and linear regression analyses, when appropriate.
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Results |
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). Patients who were treated with thyroid hormone had higher serum T3 and lower TSH levels than patients who were not treated with thyroid hormone. Serum T4 and rT3 levels were not different between these groups. No significant correlation was observed among serum TSH, T4, T3, and rT3 and postmortem time.
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Significant D3 activities were detected in most tissue samples, with ranges of 0.1–9.2 and 0.1–1.7 fmol/min per milligram protein in liver and skeletal muscle, respectively. In both tissues, D3 activity was progressively saturated by increasing the T3 concentration from 1 to 10 and 100 nM, providing approximate Michaelis constant values of 3.6 ± 0.6 nM T3 in liver and 2.3 ± 0.6 nM T3 for skeletal muscle (mean ± SD, n = 4). These values are in close agreement with previous reports (11, 12). There was a significant correlation between D3 expression in liver and skeletal muscle (R = 0.54, P < 0.001), but in some patients high expression in liver was observed with low expression in skeletal muscle and vice versa. No significant correlation was observed between deiodinase activities and postmortem time.
Serum thyroid hormone levels and tissue deiodinase activities were not different between the patients who had received intensive or conventional insulin therapy (Table 4). This has been found in the whole group and separately in the groups of patients who did or did not receive thyroid hormone treatment.
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) Serum T3/rT3 was not correlated with muscle D3 activity.
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Liver D3 activity showed a positive correlation with serum rT3 (R = 0.40, P = 0.016) and the rT3/T4 ratio (R = 0.55, P = 0.001) in the group of patients who had not been treated with thyroid hormone. These correlations were completely absent in the group of patients who had received thyroid hormone treatment. Skeletal muscle D3 activity showed a positive correlation with the serum rT3/T4 ratio (R = 0.55, P = 0.001). No other correlations were found for liver or skeletal muscle D3 activity with serum iodothyronine levels.
Hepatic D1 activity showed a significant correlation with cause of death, being lowest in the patients who had died of a cardiovascular collapse, with successive increases in the patients who had died of multiple organ failure (MOF) with sepsis or MOF with systemic inflammatory response syndrome (SIRS) and being highest in the patients who had died of severe brain damage (ANOVA P < 0.01). Liver D1 activities in the latter group were similar to those determined in normal liver samples (data not shown). The serum T3/rT3 ratio showed a similar positive correlation with cause of death (ANOVA P < 0.01), whereas liver D3 activity showed an insignificant negative correlation (ANOVA P = 0.2) with cause of death. These relationships were strongest in the group of patients who had not been treated with thyroid hormone (Fig. 2), but similar correlations were also found in the group of patients who had been treated with thyroid hormone. D3 activities in skeletal muscle did not correlate with cause of death.
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and 4). Liver D3 activity was higher in patients who had been treated with inotropes (Mann-Whitney U test, P < 0.05) or RRT (P = 0.056) (Figs. 3 and 4). Muscle D3 activity was also higher in patients who had been treated with inotropes (Mann-Whitney U test, P < 0.05) (Fig. 4). Correction for treatment with dopamine did not affect these correlations. Liver D1 activity was negatively correlated with plasma urea (R = -0.35, P = 0.01) and with plasma creatinine (R = -0.27, P = 0.06) (Fig. 5). A remarkably strong, negative correlation was found for liver D1 activity with plasma total bilirubin (R = -0.54, P < 0.0001) (Fig. 6). D3 activities in liver and skeletal muscle showed no relationship with plasma urea, creatinine, or bilirubin levels. No correlation was found of liver D1, liver D3, or skeletal muscle D3 activity with plasma C-reactive protein levels.
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). Affinity labeling of D1 but not of other proteins was blocked by the addition of PTU and unlabeled rT3 (not shown), in support of the specificity of the affinity labeling of D1 with BrAc[125I]T3. Liver D1 and D3 mRNA levels determined by real-time quantitative RT-PCR were significantly correlated with the corresponding deiodinase activities (R = 0.74, P < 0.001, for D1; R = 0.78, P < 0.001 for D3) (Fig. 8).
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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In healthy subjects, about 20% of circulating T3 is secreted by the thyroid. Estimates of the contribution of liver and kidney D1 to peripheral T3 production vary from 15% to 80% (12), with the remainder of extrathyroidal T3 production originating in D2-containing tissues such as skeletal muscle. The contribution of D1 seems to be highest in hyperthyroid patients, whereas D2 may play a more important role in euthyroid and, in particular, hypothyroid subjects (12). A low D1 activity will result in not only a decreased production of T3 from T4 but also reduced clearance of rT3 because D1 is the principal pathway for rT3 clearance (12). A decreased uptake of T4 and rT3 into D1-expressing tissues is another possible mechanism to lower serum T3 and increase serum rT3 levels (10, 15, 16). Furthermore, the activity of the different deiodinases greatly depends on thiol cofactors, although the physiological cofactor(s) for each deiodinase has not been identified (11, 12). In our study, tissue deiodinase activities were determined in the presence of excess exogenous cofactor (DTT). Our studies, therefore, do not provide information about changes in the availability of the natural cofactors as a possible mechanism for the changes in peripheral thyroid hormone metabolism in critical illness.
In normal healthy subjects, only D1 is expressed in liver and preliminary data suggest that D2 activity is expressed in skeletal muscle (12). In liver of critically ill patients, we found D1 activity, which, except for patients who died acutely from severe brain damage, was low, compared with values observed in healthy individuals, and also substantial D3 activity. As expected, D2 activity could not be detected in liver. Surprisingly, skeletal muscle of critically ill patients showed a significant D3 activity, whereas both muscle D1 and D2 activities were negligible. In general, the expression of D1 and D3 is up-regulated and that of D2 is down-regulated by thyroid hormone (11, 12). Based on the low serum thyroid hormone levels in our patients, a high D2 expression and a low D1 and D3 expression would be expected. Although we did observe mostly low hepatic D1 expression, D2 activity in skeletal muscle was undetectable, and liver and skeletal muscle samples expressed substantial D3 activities. Hence, a role of regulators other than thyroid hormone is suggested.
To reduce the confounding effect of variable concentrations of T4 and T4-binding proteins, we mainly focused on the correlation of tissue deiodinase activities with serum iodothyronine ratios. Because a low D1 expression conceivably reduces T3 production and rT3 clearance and because elevated D3 expression enhances T3 clearance and rT3 production, the serum T3/rT3 ratio is the parameter that most accurately reflects the result of altered peripheral thyroid hormone metabolism during critical illness. Our data suggest an important role of liver D1 in inducing the altered thyroid hormone levels of critically ill patients because liver D1 activity was positively correlated with serum T3/rT3 ratio and negatively correlated with the serum rT3/T4 ratio. Surprisingly, liver D1 activity also showed a negative correlation with the serum T3/T4 ratio. This is due in part to the positive correlation between liver D1 activity and serum T4 levels in the thyroid hormone-treated group, which may be explained by the positive control of D1 expression by thyroid hormone, although this is thought to be mediated by T3 (12, 27). Liver D3 activity correlated positively with serum rT3/T4 and tended to correlate negatively with serum T3/rT3. Normally, D3 is present only in human liver during fetal development, in which it protects the fetus from undue exposure to thyroid hormone, suggesting that pathological conditions in adult life may be associated with changes in deiodinase expression, in particular that of D3, to levels occurring during fetal development (24, 28). There are two ways by which D3 decreases T3 availability: It prevents conversion of T4 to T3 by catalyzing the conversion of T4 to rT3 instead, and it also catalyzes the degradation of T3 to 3,3'-T2. Therefore, induction of liver D3 in critically ill patients is likely to contribute to the low serum T3 and high serum rT3 levels in critically ill patients.
There is evidence that D2 is expressed in skeletal muscle of healthy subjects (25). D2 activity is regulated by substrate-induced enzyme inactivation (12). Hence, it may be surprising that we failed to detect significant D2 activity in skeletal muscle of critically ill patients, particularly in view of the low serum T4 levels. However, critically ill patients have strongly increased serum rT3 concentrations, which may also inactivate D2 (29). Another explanation for the lack of D2 activity is the short half-life of functional D2 protein (<1 h in euthyroid conditions), which may cause rapid postmortem D2 inactivation (12). However, because in this study tissue biopsies were taken within minutes after death, postmortem decay of D2 activity should be minimal. The shortest interval between entry in the ICU and isolation of tissue samples was between 24 and 48 h. Thus, in view of the short half-life of the D2 protein, it may well have disappeared from skeletal muscle if its expression is acutely suppressed in severe illness. It is also possible that D2 expression varies between different types of skeletal muscle, but this remains to be investigated. Like D1, D2 is a thyroid hormone-activating enzyme. Therefore, the lack of D2 activity in skeletal muscle, as does the down-regulation of hepatic D1 activity, may contribute to low serum T3 levels in critically ill patients.
Another surprising finding was the expression of substantial D3 activity in skeletal muscle. Muscle D3 activity was significantly correlated with serum rT3/T4 ratio, but not with T3/rT3 ratio, in critically ill patients. Although D3 activity has been reported previously in fetal rat muscle (30), this is the first report of D3 expression in human skeletal muscle. By converting T4 to rT3 and T3 to 3,3'-T2, D3 may lower local thyroid hormone levels in skeletal muscle. It has been shown recently that expression of D3 in hemangiomas may result in very low serum T4 and T3 and very high rT3 levels (17). Because skeletal muscle is such an abundant tissue in humans, it is likely that induction of D3 in skeletal muscle also contributes to the low serum T4 and T3 and high rT3 levels in critical illness.
No significant differences in tissue deiodinase expression were observed between patients who had been treated with intensive or conventional insulin therapy. This seems to contradict the marked beneficial effects of intensive insulin therapy on morbidity and mortality in intensive care patients (8). However, it should be noted that the current study involved postmortem samples. The illness of patients who died after being intensively treated with insulin was therefore at least as severe as that of patients who died after having received conventional insulin therapy.
Liver D1 activity and serum T3/rT3 ratio showed a significant relationship with cause of death. Cause of death was categorized in four different groups, with group 1 having the most severe, and group 4 the least hemodynamic instability before death. Hepatic D1 activity and serum T3/rT3 were lowest in patients who died of a cardiovascular collapse (group 1) and were highest in patients who died of severe brain damage (group 4). Liver D3 showed an opposite, insignificant correlation with cause of death. Because ICU stay was shorter in groups 1 and 4 than in groups 2 and 3 (MOF with sepsis and MOF with SIRS), these correlations cannot be explained by the duration of the illness. These data suggest that the observed changes in liver deiodinase activity may relate to poor tissue perfusion. For this reason, we analyzed the relation of tissue deiodinase activities with several other clinical parameters related to a decreased tissue perfusion, such as decreased kidney function (plasma urea and creatinine), the need for renal replacement therapy (dialysis or hemofiltration), the need for inotropes, and impaired liver function (plasma total bilirubin). Liver D1 activity was negatively correlated with plasma urea and creatinine. Patients with acute renal failure requiring dialysis/hemofiltration and those requiring inotropes for hemodynamic stability had lower liver D1 activities and higher liver D3 activities than patients who did not require these treatments. Skeletal muscle D3 activity was also higher in patients treated with inotropes. Although dopamine is known to inhibit TSH secretion and peripheral T4 to T3 conversion (31), correction for dopamine did not affect these correlations. Liver D1 activity showed a very strong negative correlation with plasma total bilirubin, whereas no correlation was shown with inflammation as reflected by an elevated CRP level. To our knowledge, there are no previous data suggesting a relation between deiodinase activity and tissue hypoxia, but a pH dependence of D1 activity has been shown in perfused rat livers (32). Because D1 is responsible for the activation and D3 for the inactivation of thyroid hormone, regulation of deiodinase activities by cellular hypoxia may be a tissue-specific way to alter thyroid hormone bioactivity during limited oxygen supply.
Our data regarding hepatic deiodinase activities were substantiated by a good correlation with liver D1 and D3 mRNA levels and affinity labeling of the D1 protein with N- bromoacetyl-[125I]T3. This suggests that the changes in liver D1 and D3 expression in severely ill patients are largely exerted at the pretranslational level.
In conclusion, this is the first report on the relationships among tissue deiodinase activities, thyroid hormone levels, and clinical parameters in a large group of critically ill patients. Liver D1 activity is down-regulated, and liver and skeletal muscle D3, both not present in healthy subjects, are induced, particularly in those disease states with poor tissue perfusion and unrelated to inflammation. The data suggest that low D1 activity plays an important role in the altered thyroid hormone levels during critical illness. The induction of D3 in liver and skeletal muscle of critically ill patients suggests a role of this enzyme in pathophysiology, which may be more important than previously thought. Indeed, high D3 activity in an abundant tissue such as skeletal muscle is likely to lower circulating T4 and T3 and increase rT3 levels, hence contributing to the low T3 syndrome of severe illness.
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Footnotes |
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Abbreviations: 3,3'-T2, 3,3'-diiodothyronine; Ct, cycle threshold; D1, D2, and D3, type 1, 2, and 3 iodothyronine deiodinase; DTT, dithiothreitol; FT4, free T4; ICU, intensive care unit; MOF, multiple organ failure; PED, phosphate, EDTA; PTU, 6-n-propyl-2-thiouracil; RRT, renal replacement therapy; RT, reverse transcription; SIRS, systemic inflammatory response syndrome.
Received December 20, 2002.
Accepted April 4, 2003.
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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S. A. Huang, M. A. Mulcahey, A. Crescenzi, M. Chung, B. W. Kim, C. Barnes, W. Kuijt, H. Turano, J. Harney, and P. R. Larsen Transforming Growth Factor-{beta} Promotes Inactivation of Extracellular Thyroid Hormones via Transcriptional Stimulation of Type 3 Iodothyronine Deiodinase Mol. Endocrinol., December 1, 2005; 19(12): 3126 - 3136. [Abstract] [Full Text] [PDF]
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R. P. Peeters, S. van der Geyten, P. J. Wouters, V. M. Darras, H. van Toor, E. Kaptein, T. J. Visser, and G. Van den Berghe Tissue Thyroid Hormone Levels in Critical Illness J. Clin. Endocrinol. Metab., December 1, 2005; 90(12): 6498 - 6507. [Abstract] [Full Text] [PDF]
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