Concise Review invited by Clara Alvarez; Reviewing Editor: Clara Alvarez

The authors declare that there is not a conflict of interest with this manuscript.

Correspondence to:
Joaquin Lado Abeal
Division of Endocrinology, Department of Internal Medicine
TexasTech University Health Sciences Center, School of Medicine
3601 4th Street STOP 9410
Lubbock, Texas 79430-9410 - USA
Telephone 806 7433155-283
e-mail: joaquin.lado@ttuhsc.edu

ABSTRACT
Nonthyroidal Illness Syndrome (NTIS), also called Sick Euthyroid Syndrome or Low 3,5,3’-triiodothyronine (T3) syndrome, refers to a thyroid phenotype consisting of low T3, normal-high reverse T3 (rT3), normal-low thyroxine (T4), and inappropriately normal or low serum thyroid stimulating hormone (TSH) observed during illness and starvation. The pathophysiology of NTIS involves a central hypothyroidism together with changes in thyroid hormone production, transport, cellular uptake, and metabolism, and a decrease in thyroid hormone receptor levels. These changes reduce the bioavailability and probably also the actions of thyroid hormones, thus creating a clinical situation of hypothyroidism. Whether such changes are directed to protect the organism from the actions of thyroid hormones in catabolic situations, or whether the changes are themselves harmful and therefore indicate thyroid hormone replacement treatment in patients with NTIS, is a question of debate (Hot Thyroidol. 2009: e11).

Introduction
The term Nonthyroidal Illness Syndrome (NTIS) refers to characteristic changes in thyroid hormone levels during illness and starvation (1). These changes are low T3 and normal-high rT3 in serum and tissues, normal or low serum T4 and inappropriately normal or low serum TSH relative to serum levels of T4 and T3 (2) (Figure 1).



Figure 1. Serum thyroid hormone levels in a group of patients with NTIS related to septic shock, and controls. Patients with NTIS have lower serum TSH and free T3, higher reverse of T3, and no differences were observed in serum free T4 levels, relative to controls. Yellow bars indicate hormonal normal range values.

NTIS can be considered as a part of the neuroendocrine response to severe stress that also includes an increase in serum glucocorticoid levels and a decrease in serum gonadotropins. It remains unclear whether NTIS is a beneficial adaptive response to reduce energy consumption, or a form of secondary hypothyroidism that requires thyroid hormone treatment (3-6).
Patients with NTIS and low serum T4 have an increased probability of death (7-10), probably because the duration and magnitude of NTIS are dependent on the severity of the underlying disease.
However there is no clinical evidence that under these circumstances thyroid hormone treatment is advantageous, nor indeed disadvantageous (6). In the few prospective studies conducted to assess the response to thyroid hormone therapy of patients with severe NTIS no reduction in mortality was observed (11, 12); an increase in mortality in the treated group was reported in one study (13), and no changes in other clinical outcomes were observed (12, 14). Nevertheless some authors advocate T3 therapy in cardiac transplantation recipients since T3 seems to lead to a rapid restoration of energy stores, together with an improvement in myocardial function, a reduction in the incidence of inadequate heart function early after transplantation, and an increase in graft survival (15). T3 administration to humans is associated with improved hemodynamics, reduced peripheral vascular resistance, and increased cardiac output. T3 administration to adults during coronary artery surgery neither changes the requirement for inotropic drugs nor increases the incidence of arrhythmia (16).
Studies in experimental animals have shown that T3 decreases during a period of regional myocardial ischemia and that T3 administration is associated with an improved left ventricular ejection fraction, an important indicator of outcome after acute myocardial infarction (15,17). Taken together, these studies suggest that T3 administration to patients with cardiac-related NTIS is safe, well tolerated, and potentially of benefit to these patients.
A recent study in children that underwent cardiac bypass surgery for congenital heart lesions showed that all the chidren presented NTIS within the first day postoperatively. NTIS changes correlated to prolonged hospital stay and T3 levels within 6-14 h from surgery predicted the patients outcome (18).
However the beneficial role of thyroid hormone replacement in children with NTIS remains unknown.

Hypothalamic-pituitary function in NTIS
The inappropriately normal or low serum TSH levels in NTIS patients suggest an impairment of hypothalamic-pituitary function. Low TRH mRNA expression levels have been found in the periventricular nuclei of the hypothalamus in NTIS patients (19). A decrease in pulsatile TSH secretion as well as an increase in basal but not in pulsatile TSH secretion after TRH infusion have been found in prolonged critically ill patients (19). The finding that thyroid axis alterations are partially reversed by the combined infusion of TRH and GH secretagogues (20) provides further evidence of the importance of the role of central hypothyroidism in NTIS. Increased levels of proinflammatory cytokines, endogenous glucocorticoids and glucagon typically seen in critically ill patients, together with administration of glucocorticoids and dopaminergic drugs, could directly suppress TRH secretion, the pituitary response to TRH, and TSH secretion (19-21). In humans, administration of tumor necrosis factor-alpha (TNF-α) (22), interleukin-6 (IL6) (23) and interferon alpha (24) causes a decrease in the plasma concentration of TSH and T3 and an increase in rT3, resembling the biochemical changes in NTIS. In rats, interleukin-1alpha (IL1-α) decrease hypothalamic pro-TRH, pituitary TSH beta gene expression and plasma TSH levels, while IL6 reduce plasma TSH without decrease hypothalamic pro-TRH and pituitary TSH beta expression, indicating that IL1-α affects the synthesis and release of TRH and TSH, and that IL6 affects TSH release (25). Fasting decreases TSH secretion in humans (26), while starvation decreases thyroid function in rats due to a reduction in the synthesis and release of TRH and TSH (27); an increase in adrenal glucocorticoid secretion is probably responsible for TSH suppression during fasting and also for the reduced TSH levels seen in NTIS (26).
The molecular mechanisms of the impairment of hypothalamic–pituitary function in NTIS are not well characterized. Bacterial lipopolysaccharide (LPS) administration to rats induces type-2 iodothyronine deiodinase (DIO2) activity in tanycytes located in the mediobasal hypothalamus, and it has been hypothesized that the increase in DIO2 activity could increase the conversion of T4 into T3, creating a local hyperthyroidism that prevents an increase in TRH and/or TSH secretion in response to low T3 (28). Although a decrease in TRH and an increase in DIO2 mRNA expression in the hypothalamus were found in a model of prolonged critically ill rabbits, the hypothalamic concentration of T4 was low and of T3 was low-normal, with no changes observed in thyroid hormone receptor beta (THRB) or alpha (THRA) mRNA expression (29). Moreover, in autopsy samples from humans with severe NTIS a decrease in T3 concentration was found at the levels of the hypothalamus and the pituitary (30).
These studies indicate that in NTIS there is a relatively hypothyroid state at the hypothalamus, arguing against an increase in hypothalamic conversion of T4 to T3 as a cause of low TRH mRNA expression in NTIS.

Thyroid hormone production and thyroid gland function in NTIS
Although T4 production is decreased in NTIS patients, thyroid gland function in NTIS has not been systematically studied. As already mentioned, NTIS thyroid axis alterations are partially reversed by the combined infusion of TRH and GH secretagogues (20), indicating that the thyroid gland is able to release thyroid hormones if appropriately stimulated by TSH. These observations do not, however, rule out a role of the thyroid gland in the pathophysiology of NTIS, especially in severe cases with low T4 levels. Patients with NTIS have altered TSH glycosylation which is associated with reduced biological activity (31), and lethal NTIS is associated with major morphological changes of the thyroid gland including loss of colloid and reductions in follicular size and thyroid weight (32). Finally, cytokines affect thyroid cell function in several ways (21,26), including a decrease in basal and TSHstimulated iodide uptake by IL1 and TNF, inhibition of thyroglobulin synthesis by IL1, TNF-α and interferon gamma (IFN-γ), a decrease in thyroperoxidase expression by IL1, IL6 and IFN-γ, and a decrease in T3 secretion by IL1, TNF-α and IFN-γ.
Low serum T4 levels in patients with NTIS are due not only to a decreased daily T4 production but also to a reduction of T4 binding to carrier proteins (33) which is mainly due to a decrease in serum concentration and binding affinity of thyroid binding globulin (TBG). In NTIS there is an increased cleavage of TBG by elastases and an increase in the levels of non-esterified fatty acids (NEFA) which compete and displace T4 for binding to TBG.

Deiodinase expression and activity
In humans, approximately 80% of T3 is produced by extra-thyroidal enzymatic deiodination of T4, mainly in the liver and kidney by type-1 iodothyronine deiodinase (DIO1), and in skeletal muscle by DIO2 (34). In post-mortem analyses of patients with NTIS, low T3 and high serum and tissue rT3 have been found to be related to decreased activities of liver DIO1 and skeletal muscle DIO2, and to increased activities of type-3 iodothyronine deiodinase (DIO3) in the liver and in skeletal muscle (2,35). Similar findings have been reported in an animal model of prolonged critical illness (36). In patients with septic shock and NTIS there is a decrease in skeletal muscle DIO2 mRNA expression but not in DIO2 activity, and an increase in DIO3 activity. However, no changes in DIO1 or DIO3 activity were observed at the level of subcutaneous adipose tissue, suggesting that the changes in deiodinase activity occurring in patients with septic shock and NTIS are tissue specific (37). The changes in deiodinase activity in NTIS patients could be attributable to increases in serum glucocorticoids and proinflammatory cytokines (38-41), and/or to a decrease in thyroid hormone bioavailability within particular tissues. TNF-α, IL1-α and IL6 inhibit DIO1 activity in rodents (21) and in human hepatocarcinoma cell lines (40). It has been proposed that one of the causes of a decrease in DIO1 activity could be due to competition between the promoters of DIO1 and the multiple genes regulated by those cytokines for the few cellular coactivators available, such as SCR-1 (42), or for transcription factors such as NF-kB (43). Activated NF-kB, a transcription factor that plays a pivotal role in immune and inflammatory responses, is a potential molecular factor at the root of NTIS in patients with increased cytokines (44,45); in vitro studies using HepG2 cells have shown that NF-kB activation attenuates the induction of DIO1 by T3 (44) and that the decrease in DIO1 mRNA expression by IL1-β is mediated by the simultaneous activation of NF-kB and the activator protein-1 (AP-1) pathway (43).
Deiodinases are selenoproteins, while selenium deficiency is frequently observed in sick patients and selenium deficiency decreases DIO1 activity (45). Mutations in SECIS binding protein 2 (SBP2), a protein that incorporates selenocysteine into the catalytic center of deiodinases, also cause a decrease both in deiodinase activity and in serum T3 (46). However, selenium administration to critically ill patients has not been found to alter serum thyroid hormone levels (47), nor have changes been found in SBP2 mRNA expression levels in skeletal muscle or subcutaneous adipose tissue samples from patients with septic shock NTIS, relative to controls (37). These results suggest that selenium does not play a major role in the pathophysiology of NTIS.

Although changes in peripheral deiodination may be necessary, it seems that they are not sufficient to cause NTIS. Mice deficient in deiodinase activity have normal serum T3 levels and an increased serum T4 concentration (48). Thus decreased DIO1 gene expression and low DIO1 activity observed in NTIS patients could be an effect rather than a cause of low T3. Recent studies have questioned the physiological relevance of DIO2 activity in human skeletal muscle due to its normally low activity (49), while DIO2 activity in skeletal muscle biopsies from patients with septic shock NTIS is not significantly different from controls (37). Continuous infusion of TRH and GHRP2 has been found to increase T4 and T3 but not rT3 serum levels, suggesting that central hypothyroidism is a major component of NTIS syndrome (20).

Thyroid hormone tissue uptake and actions
Patients with fatal NTIS show decreased T4 and T3 in most tissues (30), caused in part by reduced uptake (50). Several factors, including NEFA and bilirubin, have been identified as inhibitors of cellular T4 transport, although their mechanisms of action remain unknown (51). Also NTIS observed in patients with anorexia nervosa or in obese patients under caloric restriction seems to be related to carbohydrate restriction, free unsaturated fatty acid increase and low tissular ATP levels that could decrease thyroid hormone uptake and metabolism (52). However, the administration of T4 plus T3 to NTIS patients previous to death increased iodothyronine levels in liver and skeletal muscle (2) showing that thyroid hormone bioavailability is not limited by reduced tissue uptake if appropriate replacement therapy is given. The thyroid hormone transporters monocarboxylate transported 8 (MCT8, SLC16A2) and MCT10 (TATA1, SLC16A10) do not seem to play a role in controlling thyroid hormone uptake in skeletal muscle, liver or kidney (37,53), although MCT8 might play a role in thyroid hormone uptake at the level of adipose tissue (37).
Thyroid hormone action depends on the tissue distribution and expression levels of thyroid hormone receptors (TRs), ligand regulated transcription factors that bind to the thyroid hormone response elements (TREs) of target genes. TRs are encoded by the THRA and THRB genes that by alternative splicing give rise to different TR isoforms (54). TRs bind to TREs predominantly as heterodimers with the retinoid X receptor (RXR) and together they also bind to other regulatory proteins that act as corepressors and co-activators modulating TR transcription function. Nuclear receptor co-repressor, NCOR1, and silencing mediator of retinoid and TR, SMRT, recruit histone deacetylases and inhibit the basal transcription machinery (55). Steroid receptor co-activator, SRC1, has intrinsic histone acetyl transferase activity and mediates the chromatin remodeling that allows transcription (56). TR expression is reduced in skeletal muscle and subcutaneous adipose tissue of patients with septic shock NTIS (37), and similar findings have been obtained in mice after LPS administration leading to an acute decrease in TRα1, TRα2 and TRβ proteins and mRNA expression in white fat and the liver and heart (57,58). Interestingly, a previous study in humans found an increase in TRα and TRβ mRNA concentrations in peripheral mononuclear cells from patients with NTIS due to liver disease or chronic renal failure, but no differences were observed in cells from patients with severe acute illness recruited within 48 h of admission to ICU (59). A recent study of liver biopsies from critically ill patients who died in ICU reported that the ratio of mRNA expression of the two THRA isoforms TRα1 and TRα2 was higher in patients with more severe disease (60); interestingly, however, in six patients who died after a very short stay in ICU, the ratio was not different from that seen in controls.
Together, these data show that THR gene expression decreases during the acute phase of severe illness. Skeletal muscle and subcutaneous adipose tissue express the three RXR isoforms, RXRA, RXRB and RXRG, and patients with septic shock NTIS show a decrease in RXRG mRNA expression in skeletal muscle and adipose tissue, and an increase in RXRA expression in skeletal muscle, but no changes in RXRB expression in either skeletal muscle or adipose tissue (37). RXRs have an important metabolic role in skeletal muscle, increasing the uptake and oxidation of saturated fatty acids, and the ratio of unsaturated to saturated fatty acids. Moreover, RXRG-deficient mice have increased skeletal muscle lipoprotein lipase activity and lower triglyceride levels (61). The RXR isoform expression changes seen in skeletal muscle and subcutaneous adipose tissue samples from septic shock NTIS patients may be directed at decreasing insulin resistance and increasing ATP production from the fat within skeletal muscle during septic shock.
In conclusion, NTIS forms a part of the neuroendocrine response to moderate-severe illness and starvation, caused by impairment of hypothalamic-pituitary function and of the physiological mechanisms producing T3 in peripheral tissues: NTIS is a systemic inflammatory reaction triggered by a variety of deleterious agents and by a reduction in energy intake. Depending on the severity of the noxious agent, extent of damage and time taken to recover, several molecular changes affecting transport, tissue uptake, metabolism and action of thyroid hormones may be observed (Figure 2). Whether such changes are beneficial and directed towards protecting the cell from thyroid hormone action under catabolic conditions, or whether they are harmful and thyroid hormone supplementation should be given to NTIS patients, is still a question of debate.



Figure 2. Thyroid hormone metabolism and action in normal (A) and NTIS (B) subjects. A decrease in serum thyroid hormone levels, cellular uptake and tissue levels of thyroid hormones, expression of MCT8, DIO1, DIO2, thyroid hormone receptors THRA and THRB, and RXRG, together with an increase in activity of DIO3, have been described in tissue samples from patients with NTIS.

Acknowledgements
This study was supported by Ministerio de Educación SAF2006-02542 (JLA), Xunta de Galicia PGIDIT04PXIC20801PN (JLA), PGIDIT06PXIB208360PR (JLA). We thank Xiao Hui Liao from the Thyroid Study Unit at the University of Chicago for her comments.