Thyroid hormone receptors are down-regulated in skeletal muscle of patients with non-thyroidal illness syndrome secondary to non-septic shock

The relation between FT3 and long-term fatigue in patients with COVID-19

Background

Under the current pandemic of Corona Virus Disease 2019 (COVID-19), The relationship between fatigue and COVID-19 has been found. Infection with COVID-19 is associated with fatigue long after the acute phase of COVID-19. Understanding the association of thyroid hormones levels with post-COVID condition, such as fatigue, is necessary to improve quality of life.

Methods

This population-based cohort study was conducted in Dalian, China, from December 2022, to March 2023, using a Yidu Core platform in the First Affiliated Hospital of Dalian Medical University, that integrates medical records, laboratory tests, and all diagnosis and treatment information based on patients in hospital. Eligible individuals were 40 patients with COVID-19, Divided them into fatigue group and non-fatigue group following up by telephone using the FS-14 scale after 6 months. The primary outcomes were the diagnoses of fatigue. The association between thyroid hormones levels and post-COVID condition, such as fatigue, was assessed using logistic regression analysis.

Results

Compared with the non-fatigue group, the FT3 level in fatigue group was lower (p<0.05). FT3 was negatively correlated with fatigue after 6 months (OR 0.257, p<0.05). After adjusting for confounding factors such as age and gender, low FT3 was a risk factor for fatigue in patients with COVID-19, (OR 0.225, p<0.05). And the FT3 is less than 2.47 mol/L, it is the best critical value for predicting long-term fatigue, with a sensitivity of 92.3% and a specificity of 48.1%.

Conclusions

Most people still have fatigue 6 months after COVID-19 infection. FT3 serves as the important index to predict fatigue in patients with COVID-19. it should be closely monitored during infection.

T3 levels and thyroid hormone signaling

The clinical availability of tissue-specific biomarkers of thyroid hormone (TH) action constitutes a "holy grail" for the field. Scientists have investigated several TH-dependent markers, including the tissue content of triiodothyronine (T3)-the active form of TH. The study of animal models and humans indicates that the T3 content varies among different tissues, mostly due to the presence of low-affinity, high-capacity cytoplasmic T3 binding proteins. Nonetheless, given that T3 levels in the plasma and tissues are in equilibrium, T3 signaling is defined by the intracellular free T3 levels. The available techniques to assess tissue T3 are invasive and not clinically applicable. However, the tracer kinetic studies revealed that serum T3 levels can accurately predict tissue T3 content and T3 signaling in most tissues, except for the brain and pituitary gland. This is true not only for normal individuals but also for patients with hypo or hyperthyroidism-but not for patients with non-thyroidal illness syndrome. Given this direct relationship between serum and tissue T3 contents and T3 signaling in most tissues, clinicians managing patients with hypothyroidism could refocus attention on monitoring serum T3 levels. Future clinical trials should aim at correlating clinical outcomes with serum T3 levels.

The Role of Thyroid Hormones on Skeletal Muscle Thermogenesis

Nowadays obesity becomes a significant global problem. Hence, recently more and more attention has been paid to substances present in the body that have a significant impact on metabolic processes and thermogenesis, in the context of their potential use in the prevention and treatment of obesity. It is well known that the relationship between thyroid hormones and obesity is multilayered, however recently, more and more information about the possible relation between thyroid hormones and muscle metabolism has been published. The aim of this review is to present the most updated information on the physiological impact of thyroid hormones on muscle tissue, as well as pathological changes related to the occurrence of various types of thyroid disorders, including hypothyroidism, hyperthyroidism and sick euthyroid syndrome. However, the data in humans still remains insufficient, and further studies are needed to fully explore the thyroid-muscle cross-talk.

Perspective: Drawing on Findings From Critical Illness to Explain Myalgic Encephalomyelitis/Chronic Fatigue Syndrome

We propose an initial explanation for how myalgic encephalomyelitis / chronic fatigue syndrome (ME/CFS) could originate and perpetuate by drawing on findings from critical illness research. Specifically, we combine emerging findings regarding (a) hypoperfusion and endotheliopathy, and (b) intestinal injury in these illnesses with our previously published hypothesis about the role of (c) pituitary suppression, and (d) low thyroid hormone function associated with redox imbalance in ME/CFS. Moreover, we describe interlinkages between these pathophysiological mechanisms as well as “vicious cycles” involving cytokines and inflammation that may contribute to explain the chronic nature of these illnesses. This paper summarizes and expands on our previous publications about the relevance of findings from critical illness for ME/CFS. New knowledge on diagnostics, prognostics and treatment strategies could be gained through active collaboration between critical illness and ME/CFS researchers, which could lead to improved outcomes for both conditions.

An update on non-thyroidal illness syndrome

The non-thyroidal illness syndrome (NTIS) was first reported in the 1970s as a remarkable ensemble of changes in serum TH (TH) concentrations occurring in probably any severe illness. Ever since, NTIS has remained an intriguing phenomenon not only because of the robustness of the decrease in serum triiodothyronine (T3), but also by its clear correlation with morbidity and mortality. In recent years, it has become clear that (parenteral) feeding in patients with critical illness should be taken into account as a major determinant not only of NTIS but also of clinical outcome. Moreover, both experimental animal and clinical studies have shown that tissue TH concentrations during NTIS do not necessarily reflect serum low TH concentrations and may decrease, remain unaltered, or even increase according to the organ and type of illness studied. These differential changes now have a solid basis in molecular studies on organ-specific TH transporters, receptors and deiodinases. Finally, the role of inflammatory pathways in these non-systemic changes has begun to be clarified. A fascinating role for TH metabolism in innate immune cells, including neutrophils and monocytes/macrophages, was reported in recent years, but there is no evidence at this early stage that this may be a determinant of susceptibility to infections. Although endocrinologists have been tempted to correct NTIS by TH supplementation, there is at present insufficient evidence that this is beneficial. Thus, there is a clear need for adequately powered randomized clinical trials (RCT) with clinically relevant endpoints to fill this knowledge gap.

Theory: Treatments for Prolonged ICU Patients May Provide New Therapeutic Avenues for Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS)

We here provide an overview of treatment trials for prolonged intensive care unit (ICU) patients and theorize about their relevance for potential treatment of myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS). Specifically, these treatment trials generally target: (a) the correction of suppressed endocrine axes, notably through a "reactivation" of the pituitary gland's pulsatile secretion of tropic hormones, or (b) the interruption of the "vicious circle" between inflammation, oxidative and nitrosative stress (O&NS), and low thyroid hormone function. There are significant parallels in the treatment trials for prolonged critical illness and ME/CFS; this is consistent with the hypothesis of an overlap in the mechanisms that prevent recovery in both conditions. Early successes in the simultaneous reactivation of pulsatile pituitary secretions in ICU patients-and the resulting positive metabolic effects-could indicate an avenue for treating ME/CFS. The therapeutic effects of thyroid hormones-including in mitigating O&NS and inflammation and in stimulating the adreno-cortical axis-also merit further studies. Collaborative research projects should further investigate the lessons from treatment trials for prolonged critical illness for solving ME/CFS.

Hypothesis: Mechanisms That Prevent Recovery in Prolonged ICU Patients Also Underlie Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS)

Here the hypothesis is advanced that maladaptive mechanisms that prevent recovery in some intensive care unit (ICU) patients may also underlie Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS). Specifically, these mechanisms are: (a) suppression of the pituitary gland's pulsatile secretion of tropic hormones, and (b) a "vicious circle" between inflammation, oxidative and nitrosative stress (O&NS), and low thyroid hormone function. This hypothesis should be investigated through collaborative research projects.

Paradigms of Dynamic Control of Thyroid Hormone Signaling

Thyroid hormone (TH) molecules enter cells via membrane transporters and, depending on the cell type, can be activated (i.e., T4 to T3 conversion) or inactivated (i.e., T3 to 3,3'-diiodo-l-thyronine or T4 to reverse T3 conversion). These reactions are catalyzed by the deiodinases. The biologically active hormone, T3, eventually binds to intracellular TH receptors (TRs), TRα and TRβ, and initiate TH signaling, that is, regulation of target genes and other metabolic pathways. At least three families of transmembrane transporters, MCT, OATP, and LAT, facilitate the entry of TH into cells, which follow the gradient of free hormone between the extracellular fluid and the cytoplasm. Inactivation or marked downregulation of TH transporters can dampen TH signaling. At the same time, dynamic modifications in the expression or activity of TRs and transcriptional coregulators can affect positively or negatively the intensity of TH signaling. However, the deiodinases are the element that provides greatest amplitude in dynamic control of TH signaling. Cells that express the activating deiodinase DIO2 can rapidly enhance TH signaling due to intracellular buildup of T3. In contrast, TH signaling is dampened in cells that express the inactivating deiodinase DIO3. This explains how THs can regulate pathways in development, metabolism, and growth, despite rather stable levels in the circulation. As a consequence, TH signaling is unique for each cell (tissue or organ), depending on circulating TH levels and on the exclusive blend of transporters, deiodinases, and TRs present in each cell. In this review we explore the key mechanisms underlying customization of TH signaling during development, in health and in disease states.

Changes in Hepatic TRβ Protein Expression, Lipogenic Gene Expression, and Long-Chain Acylcarnitine Levels During Chronic Hyperthyroidism and Triiodothyronine Withdrawal in a Mouse Model

BACKGROUND

Thyroid hormone (TH) has important roles in regulating hepatic metabolism. It was previously reported that most hepatic genes activated by a single triiodothyronine (T3) injection became desensitized after multiple injections, and that approximately 10% of target genes did not return to basal expression levels after T3 withdrawal, despite normalization of serum TH and thyrotropin (TSH) levels. To determine the possible mechanism(s) for desensitization and incomplete recovery of hepatic target gene transcription and their effects on metabolism, mRNA and/or protein expression levels of key regulators of TH action were measured, as well as metabolomic changes after chronic T3 treatment and withdrawal.

METHODS

Adult male mice were treated with daily injections of T3 (20 μg/100 g body weight) for 14 days followed by the cessation of T3 for 10 days. Livers were harvested at 6 hours, 24 hours, and 14 days after the first T3 injection, and at 10 days after withdrawal, and then analyzed by quantitative reverse transcription polymerase chain reaction, Western blotting, and metabolomics.

RESULTS

Although TH receptor (TRα and TRβ) mRNAs decreased slightly after chronic T3 treatment, only TRβ protein decreased before returning to basal expression level after withdrawal. The expression of other regulators of TH action was unchanged. TRβ protein expression was also decreased in adult male monocarboxylate transporter-8 (Mct8)-knockout mice, an in vivo model of chronic intrahepatic hyperthyroidism. Previously, increased hepatic long-chain acylcarnitine levels were found after acute TH treatment. However, in this study, long-chain acylcarnitine levels were unchanged after chronic T3, and paradoxically increased after T3 withdrawal. Pathway analyses of the previous microarray results showed upregulation of lipogenic genes after acute T3 treatment and withdrawal. Phosphorylation of acetyl-CoA carboxylase also decreased after T3 withdrawal.

CONCLUSIONS

Decreased hepatic TRβ protein expression occurred after chronic T3 exposure in adult male wild-type and Mct8-knockout mice. Gene array pathway and metabolomics analyses showed abnormalities in hepatic lipogenic gene expression and acylcarnitine levels, respectively, after withdrawal, despite normalization of serum TSH and TH levels. These findings may help explain the variable clinical presentations of some patients during hyperthyroidism and recovery, since TRβ protein, target gene expression, and metabolic adaptive changes can occur in individual tissues without necessarily being reflected by circulating TH and TSH concentrations.

Thyroid Allostasis-Adaptive Responses of Thyrotropic Feedback Control to Conditions of Strain, Stress, and Developmental Programming

The hypothalamus-pituitary-thyroid feedback control is a dynamic, adaptive system. In situations of illness and deprivation of energy representing type 1 allostasis, the stress response operates to alter both its set point and peripheral transfer parameters. In contrast, type 2 allostatic load, typically effective in psychosocial stress, pregnancy, metabolic syndrome, and adaptation to cold, produces a nearly opposite phenotype of predictive plasticity. The non-thyroidal illness syndrome (NTIS) or thyroid allostasis in critical illness, tumors, uremia, and starvation (TACITUS), commonly observed in hospitalized patients, displays a historically well-studied pattern of allostatic thyroid response. This is characterized by decreased total and free thyroid hormone concentrations and varying levels of thyroid-stimulating hormone (TSH) ranging from decreased (in severe cases) to normal or even elevated (mainly in the recovery phase) TSH concentrations. An acute versus chronic stage (wasting syndrome) of TACITUS can be discerned. The two types differ in molecular mechanisms and prognosis. The acute adaptation of thyroid hormone metabolism to critical illness may prove beneficial to the organism, whereas the far more complex molecular alterations associated with chronic illness frequently lead to allostatic overload. The latter is associated with poor outcome, independently of the underlying disease. Adaptive responses of thyroid homeostasis extend to alterations in thyroid hormone concentrations during fetal life, periods of weight gain or loss, thermoregulation, physical exercise, and psychiatric diseases. The various forms of thyroid allostasis pose serious problems in differential diagnosis of thyroid disease. This review article provides an overview of physiological mechanisms as well as major diagnostic and therapeutic implications of thyroid allostasis under a variety of developmental and straining conditions.

Pitfalls in the measurement and interpretation of thyroid function tests

Thyroid function tests (TFTs) are amongst the most commonly requested laboratory investigations in both primary and secondary care. Fortunately, most TFTs are straightforward to interpret and confirm the clinical impression of euthyroidism, hypothyroidism or hyperthyroidism. However, in an important subgroup of patients the results of TFTs can seem confusing, either by virtue of being discordant with the clinical picture or because they appear incongruent with each other [e.g. raised thyroid hormones (TH), but with non-suppressed thyrotropin (TSH); raised TSH, but with normal TH]. In such cases, it is important first to revisit the clinical context, and to consider potential confounding factors, including alterations in normal physiology (e.g. pregnancy), intercurrent (non-thyroidal) illness, and medication usage (e.g. thyroxine, amiodarone, heparin). Once these have been excluded, laboratory artefacts in commonly used TSH or TH immunoassays should be screened for, thus avoiding unnecessary further investigation and/or treatment in cases where there is assay interference. In the remainder, consideration should be given to screening for rare genetic and acquired disorders of the hypothalamic-pituitary-thyroid (HPT) axis [e.g. resistance to thyroid hormone (RTH), thyrotropinoma (TSHoma)]. Here, we discuss the main pitfalls in the measurement and interpretation of TFTs, and propose a structured algorithm for the investigation and management of patients with anomalous/discordant TFTs.

Septic shock non-thyroidal illness syndrome causes hypothyroidism and conditions for reduced sensitivity to thyroid hormone

Non-thyroidal illness syndrome (NTIS) is part of the neuroendocrine response to stress, but the significance of this syndrome remains uncertain. The aim of this study was to investigate the effect of lipopolysaccharide (LPS)-induced NTIS on thyroid hormone (TH) levels and TH molecular targets, as well as the relationship between septic shock nuclear factor kappa-light-chain-enhancer of activated B cells (NF-kB) activation and TH receptor β (THRB) gene expression at a multi-tissue level in a pig model. Prepubertal domestic pigs were given i.v. saline or LPS for 48 h. Serum and tissue TH was measured by chemiluminescence and RIA. Expression of THRs and cofactors was measured by real-time PCR, and deiodinase (DIO) activity was measured by enzyme assays. Tissue NF-kB nuclear binding activity was evaluated by EMSA. LPS-treated pigs had decreased TH levels in serum and most tissues. DIO1 expression in liver and kidney and DIO1 activity in kidney decreased after LPS. No changes in DIO2 activity were observed between groups. LPS induced an increase in hypothalamus, thyroid, and liver DIO3 activity. Among the other studied genes, monocarboxylate transporter 8 and THRB were the most commonly repressed in endotoxemic pigs. LPS-induced NF-kB activation was associated with a decrease in THRB gene expression only in frontal lobe, adrenal gland, and kidney cortex. We conclude that LPS-induced NTIS in pigs is characterized by hypothyroidism and tissue-specific reduced TH sensitivity. The role of NF-kB in regulating THRB expression during endotoxemia, if any, is restricted to a limited number of tissues.

New Insights toward the Acute Non-Thyroidal Illness Syndrome

The non-thyroidal illness syndrome (NTIS) refers to changes in serum thyroid hormone levels observed in critically ill patients in the absence of hypothalamic-pituitary-thyroid primary dysfunction. Affected individuals have low T3, elevated rT3, and inappropriately normal TSH levels. The pathophysiological mechanisms are poorly understood but the acute and chronic changes in pituitary-thyroid function are probably the consequence of the action of multiple factors. The early phase seems to reflect changes occurring primarily in the peripheral thyroid hormone metabolism, best seen in humans since 80-90% of the circulating T3 are derived from the pro-hormone T4. The conversion of T4 to T3 is catalyzed by type 1 (D1) and type 2 (D2) deiodinases via outer-ring deiodination. In contrast, type 3 deiodinase (D3) catalyzes the inactivation of both T4 and T3. Over the last decades, several studies have attempted to elucidate the mechanisms underlying the changes on circulating thyroid hormones in NTIS. Increased inflammatory cytokines, which occurs in response to virtually any illness, has long been speculated to play a role in derangements of deiodinase expression. On the other hand, oxidative stress due to augmented reactive oxygen species (ROS) generation is characteristic of many diseases that are associated with NTIS. Changes in the intracellular redox state may disrupt deiodinase function by independent mechanisms, which might include depletion of the as yet unidentified endogenous thiol cofactor. Here we aim to present an updated picture of the advances in understanding the mechanisms that result in the fall of thyroid hormone levels in the acute phase of NTIS.

Beyond low plasma T3: local thyroid hormone metabolism during inflammation and infection

Decreased serum thyroid hormone concentrations in severely ill patients were first reported in the 1970s, but the functional meaning of the observed changes in thyroid hormone levels, together known as nonthyroidal illness syndrome (NTIS), remains enigmatic. Although the common view was that NTIS results in overall down-regulation of metabolism in order to save energy, recent work has shown a more complex picture. NTIS comprises marked variation in transcriptional and translational activity of genes involved in thyroid hormone metabolism, ranging from inhibition to activation, dependent on the organ or tissue studied. Illness-induced changes in each of these organs appear to be very different during acute or chronic inflammation, adding an additional level of complexity. Organ- and timing-specific changes in the activity of thyroid hormone deiodinating enzymes (deiodinase types 1, 2, and 3) highlight deiodinases as proactive players in the response to illness, whereas the granulocyte is a novel and potentially important cell type involved in NTIS during bacterial infection. Although acute NTIS can be seen as an adaptive response to support the immune response, NTIS may turn disadvantageous when critical illness enters a chronic phase necessitating prolonged life support. For instance, changes in thyroid hormone metabolism in muscle during critical illness may be relevant for the pathogenesis of myopathy associated with prolonged ventilator dependence. This review focuses on NTIS as a timing-related and organ-specific response to illness, occurring independently from the decrease in serum thyroid hormone levels and potentially relevant for disease progression.

Thyroid axis function and dysfunction in critical illness

Acutely ill patients typically present with low circulating T3 and increased reverse T3. When illness is severe and prolonged, also pulsatile TSH secretion and circulating T4 levels are low. This constellation of changes within the thyroid axis is referred to as the low T3 syndrome or non-thyroidal illness syndrome (NTI), and comprises both peripheral and central alterations in the thyroid axis. Acute alterations are dominated by changes in thyroid hormone binding, in thyroid hormone uptake by the cell and in the activity of the type-1 and type-3 deiodinase enzymes. Prolonged critical illness is associated with a neuroendocrine dysfunction characterized by suppressed hypothalamic thyrotropin-releasing hormone (TRH) expression, resulting in reduced stimulation of the thyrotropes whereby thyroidal hormone release is impaired. During prolonged critical illness, several tissue responses could be interpreted as compensatory to low thyroid hormone availability, such as increased expression of monocarboxylate transporters, upregulation of type 2 deiodinase activity and increased sensitivity at the receptor level. Whether the low T3 syndrome should be treated and which compound should be used remains to be further studied.

Triiodothyronine administration ameliorates the demyelination/remyelination ratio in a non-human primate model of multiple sclerosis by correcting tissue hypothyroidism

Remyelination failure is a key landmark in chronic progression of multiple sclerosis (MS), the most diffuse demyelinating disease in human, but the reasons for this are still unknown. It has been shown that thyroid hormone administration in the rodent models of acute and chronic demyelinating diseases improved their clinical course, pathology and remyelination. In the present study, we translated this therapeutic attempt to experimental allergic encephalomyelitis (EAE) in the non-human primate Callithrix Jacchus (marmoset). We report that short protocols of triiodothyronine treatment shifts the demyelination/remyelination balance toward remyelination, as assessed by morphology, immunohistochemistry and molecular biology, and improves the clinical course of the disease. We also found that severely ill animals display hypothyroidism and severe alteration of deiodinase and thyroid hormone receptor mRNAs expression in the spinal cord, which was completely corrected by thyroid hormone treatment. We therefore suggest that thyroid hormone treatment improves myelin sheath morphology in marmoset EAE, by correcting the dysfunction of thyroid hormone cellular effectors.