Circulating 3-T1AM and 3,5-T2 in Critically Ill Patients: A Cross-Sectional Observational Study

Minor perturbations of thyroid homeostasis and major cardiovascular endpoints—Physiological mechanisms and clinical evidence

It is well established that thyroid dysfunction is linked to an increased risk of cardiovascular morbidity and mortality. The pleiotropic action of thyroid hormones strongly impacts the cardiovascular system and affects both the generation of the normal heart rhythm and arrhythmia. A meta-analysis of published evidence suggests a positive association of FT4 concentration with major adverse cardiovascular end points (MACE), but this association only partially extends to TSH. The risk for cardiovascular death is increased in both subclinical hypothyroidism and subclinical thyrotoxicosis. Several published studies found associations of TSH and FT4 concentrations, respectively, with major cardiovascular endpoints. Both reduced and elevated TSH concentrations predict the cardiovascular risk, and this association extends to TSH gradients within the reference range. Likewise, increased FT4 concentrations, but high-normal FT4 within its reference range as well, herald a poor outcome. These observations translate to a monotonic and sensitive effect of FT4 and a U-shaped relationship between TSH and cardiovascular risk. Up to now, the pathophysiological mechanism of this complex pattern of association is poorly understood. Integrating the available evidence suggests a dual etiology of elevated FT4 concentration, comprising both ensuing primary hypothyroidism and a raised set point of thyroid function, e. g. in the context of psychiatric disease, chronic stress and type 2 allostatic load. Addressing the association between thyroid homeostasis and cardiovascular diseases from a systems perspective could pave the way to new directions of research and a more personalized approach to the treatment of patients with cardiovascular risk.

Endocrine interventions in the intensive care unit

Following the onset of any life-threatening illness that requires intensive medical care, alterations within the neuroendocrine axes occur which are thought to be essential for survival, as they postpone energy-consuming anabolism, activate energy-producing catabolic pathways, and optimize immunological and cardiovascular functions. The hormonal changes present in the acute phase of critical illness at least partially resemble those of the fasting state, and recent evidence suggests that they are part of a beneficial, evolutionary-conserved adaptive stress response. However, a fraction of patients who survive the acute phase of critical illness remain dependent on vital organ support and enter the prolonged phase of critical illness. In these patients, the hypothalamic-pituitary-peripheral axes are functionally suppressed, which may have negative consequences by which recovery may be hampered and the risk of morbidity and mortality in the long-term increased. Most randomized controlled trials of critically ill patients that investigated the impact on the outcome of treatment with peripheral hormones did not reveal a robust morbidity or mortality benefit. In contrast, small studies of patients in the prolonged phase of critical illness documented promising results with the infusion of hypothalamic-releasing hormones. The currently available data corroborate the need for well-designed and adequately powered RCTs to further investigate the impact of these releasing factors on patient-centered outcomes.

Determination of 3-iodothyronamine (3-T1AM) in mouse liver using liquid chromatography-tandem mass spectrometry

3-iodothyronamine (3-T₁AM) has been suggested as a novel chemical messenger and potent trace amine-associated receptor 1 ligand in the CNS that occurs naturally as endogenous metabolite of the thyroid hormones. Discrepancies and variations in 3-T₁AM plasma and tissue concentrations have nonetheless caused controversy regarding the existence and biological role of 3-T₁AM. These discussions are at least partially based on potential analytical artefacts caused by differential decay kinetics of 3-T₁AM and the widely used deuterated quantification standard D₄-T₁AM. Here, we report a novel LC-MS/MS method for the quantification of 3-T₁AM in biological specimens using stable isotope dilution with ¹³C₆-T₁AM, a new internal standard that showed pharmacodynamic properties comparable to endogenous 3-T₁AM. The method detection limit (MDL) and method quantification limit (MQL) of 3-T₁AM were 0.04 and 0.09 ng/g, respectively. The spike-recoveries of 3-T₁AM were between 85.4% and 94.3%, with a coefficient of variation of 3.7-5.8%. The intra-day and inter-day variations of 3-T₁AM were 8.45-11.2% and 3.58-5.73%, respectively. Endogenous 3-T₁AM liver values in C57BL/6J mice were 2.20 ± 0.49 pmol/g with a detection frequency of 50%. Higher liver 3-T₁AM values were found when C57BL/6J mice were treated with N-acetyl-3-iodothyronamine or O-acetyl-3-iodothyronamine. Overall, our new stable isotope dilution LC-MS/MS method improves both the sensitivity and selectivity compared with existing methods. The concomitant possibility to quantify additional thyroid hormones such as thyroxine, 3,5,3'-triiodo-_L-thyronine, 3,3',5'-triiodo-_L-thyronine, 3,3'-diiodo-_L-thyronine, and 3,5-diiodo-_L-thyronine further adds to the value of our novel method in exploring the natural occurrence and fate of 3-T₁AM in biological tissues and fluids.

3-Iodothyronamine and Derivatives: New Allies Against Metabolic Syndrome?

In the two decades since its discovery, a large body of evidence has amassed to highlight the potential of 3-iodothyronamine (T1AM) as an antiobesity drug, whose pleiotropic signaling actions profoundly impact energy metabolism. In the present review, we recapitulate the most relevant properties of T1AM, including its structural and functional relationship to thyroid hormone, its endogenous levels, molecular targets, as well as its genomic and non-genomic effects on metabolism elicited in experimental models after exogenous administration. The physiological and pathophysiological relevance of T1AM in the regulation of energy homeostasis and metabolism is also discussed, along with its potential therapeutic applications in metabolic disturbances. Finally, we examine a number of T1AM analogs that have been recently developed with the aim of designing novel pharmacological agents for the treatment of interlinked diseases, such as metabolic and neurodegenerative disorders, as well as additional synthetic tools that can be exploited to further explore T1AM-dependent mechanisms and the physiological roles of trace amine-associated receptor 1 (TAAR1)-mediated effects.

3,5-T2-A Janus-Faced Thyroid Hormone Metabolite Exerts Both Canonical T3-Mimetic Endocrine and Intracrine Hepatic Action

Over the last decades, thyroid hormone metabolites (THMs) received marked attention as it has been demonstrated that they are bioactive compounds. Their concentrations were determined by immunoassay or mass-spectrometry methods. Among those metabolites, 3,5-diiodothyronine (3,5-T2), occurs at low nanomolar concentrations in human serum, but might reach tissue concentrations similar to those of T4 and T3, at least based on data from rodent models. However, the immunoassay-based measurements in human sera revealed remarkable variations depending on antibodies used in the assays and thus need to be interpreted with caution. In clinical experimental approaches in euthyroid volunteers and hypothyroid patients using the immunoassay as the analytical tool no evidence of formation of 3,5-T2 from its putative precursors T4 or T3 was found, nor was any support found for the assumption that 3,5-T2 might represent a direct precursor for serum 3-T1-AM generated by combined deiodination and decarboxylation from 3,5-T2, as previously documented for mouse intestinal mucosa. We hypothesized that lowered endogenous production of 3,5-T2 in patients requiring T4 replacement therapy after thyroidectomy or for treatment of autoimmune thyroid disease, compared to production of 3,5-T2 in individuals with intact thyroid glands might contribute to the discontent seen in a subset of patients with this therapeutic regimen. So far, our observations do not support this assumption. However, the unexpected association between high serum 3,5-T2 and elevated urinary concentrations of metabolites related to coffee consumption requires further studies for an explanation. Elevated 3,5-T2 serum concentrations were found in several situations including impaired renal function, chronic dialysis, sepsis, non-survival in the ICU as well as post-operative atrial fibrillation (POAF) in studies using a monoclonal antibody-based chemoluminescence immunoassay. Pilot analysis of human sera using LC-linear-ion-trap-mass-spectrometry yielded 3,5-T2 concentrations below the limit of quantification in the majority of cases, thus the divergent results of both methods need to be reconciliated by further studies. Although positive anti-steatotic effects have been observed in rodent models, use of 3,5-T2 as a muscle anabolic, slimming or fitness drug, easily obtained without medical prescription, must be advised against, considering its potency in suppressing the HPT axis and causing adverse cardiac side effects. 3,5-T2 escapes regular detection by commercially available clinical routine assays used for thyroid function tests, which may be seriously disrupted in individuals self-administering 3,5-T2 obtained over-the counter or from other sources.

A Thyroid Hormone-Independent Molecular Fingerprint of 3,5-Diiodothyronine Suggests a Strong Relationship with Coffee Metabolism in Humans

Background: In numerous studies based predominantly on rodent models, administration of 3,5-diiodo-L-thyronine (3,5-T2), a metabolite of the thyroid hormones (TH) thyroxine (T4) and triiodo-L-thyronine (T3), was reported to cause beneficial health effects, including reversal of steatohepatosis and prevention of insulin resistance, in most instances without adverse thyrotoxic side effects. However, the empirical evidence concerning the physiological relevance of endogenously produced 3,5-T2 in humans is comparatively poor. Therefore, to improve the understanding of 3,5-T2-related metabolic processes, we performed a comprehensive metabolomic study relating serum 3,5-T2 concentrations to plasma and urine metabolite levels within a large general population sample. Methods: Serum 3,5-T2 concentrations were determined for 856 participants of the population-based Study of Health in Pomerania-TREND (SHIP-TREND). Plasma and urine metabolome data were generated using mass spectrometry and nuclear magnetic resonance spectroscopy, allowing quantification of 613 and 578 metabolites in plasma and urine, respectively. To detect thyroid function-independent significant 3,5-T2-metabolite associations, linear regression analyses controlling for major confounders, including thyrotropin and free T4, were performed. The same analyses were carried out using a sample of 16 male healthy volunteers treated for 8 weeks with 250 μg/day levothyroxine to induce thyrotoxicosis. Results: The specific molecular fingerprint of 3,5-T2 comprised 15 and 73 significantly associated metabolites in plasma and urine, respectively. Serum 3,5-T2 concentrations were neither associated with classical thyroid function parameters nor altered during experimental thyrotoxicosis. Strikingly, many metabolites related to coffee metabolism, including caffeine and paraxanthine, formed the clearest positively associated molecular signature. Importantly, these associations were replicated in the experimental human thyrotoxicosis model. Conclusion: The molecular fingerprint of 3,5-T2 demonstrates a clear and strong positive association of the serum levels of this TH metabolite with plasma levels of compounds indicating coffee consumption, therefore pointing to the liver as an organ, the metabolism of which is strongly affected by coffee. Furthermore, 3,5-T2 serum concentrations were found not to be directly TH dependent. Considering the beneficial health effects of 3,5-T2 administration observed in animal models and those of coffee consumption demonstrated in large epidemiological studies, one might speculate that coffee-stimulated hepatic 3,5-T2 production or accumulation represents an important molecular link in this connection.

Nonthyroidal Illness Syndrome Across the Ages

In conditions of acute illness, patients present with reduced plasma T3 concentrations without a concomitant rise in TSH. In contrast, plasma concentrations of the inactive hormone rT3 increase, whereas plasma concentrations of T4 remain low-normal. This constellation of changes, referred to as nonthyroidal illness syndrome (NTIS), is present across all ages, from preterm neonates and over-term critically ill infants and children to critically ill adults. Although the severity of illness strongly correlates with the severity of the NTIS phenotype, the causality of this association remains debated, and pathophysiological mechanisms remain incompletely understood. In the acute phase of illness, NTIS appears to be caused predominantly by an increased peripheral inactivation of thyroid hormones, in which reduced nutritional intake plays a role. Current evidence suggests that these acute peripheral changes are part of a beneficial adaptation of the body to reduce expenditure of energy and to activate the innate immune response, which is important for survival. In contrast, in more severely ill and prolonged critically ill patients, an additional central suppression of the thyroid hormone axis alters and further aggravates the NTIS phenotype. Recent studies suggest that this central suppression may not be adaptive. Whether treatment of this central component of NTIS in prolonged critically ill patients, with the use of hypothalamic releasing factors, improves outcome remains to be investigated in large randomized control trials.

Novel thyroid hormones

The field of thyroid hormone signaling has grown more complex in recent years. In particular, it has been suggested that some thyroid hormone derivatives, tentatively named "novel thyroid hormones" or "active thyroid hormone metabolites", may act as independent chemical messengers. They include 3,5-diiodothyronine (T2), 3-iodothyronamine (T1AM), and several iodothyroacetic acids, i.e., 3,5,3',5'-thyroacetic acid (TA4), 3,5,3'-thyroacetic acid (TA3), and 3-thyroacetic acid (TA1). We summarize the present knowledge on these compounds, namely their biosynthetic pathways, endogenous levels, molecular targets, and the functional effects elicited in experimental preparations or intact animals after exogenous administration. Their physiological and pathophysiological role is discussed, and potential therapeutic applications are outlined. The requirements needed to qualify these substances as chemical messengers must still be validated, although promising evidence has been collected. At present, the best candidate to the role of independent chemical messenger appears to be T1AM, and its most interesting effects concern metabolism and brain function. The responses elicited in experimental animals have suggested potential therapeutic applications. TA3 has an established role in thyroid hormone resistance syndromes, and is under investigation in Allen-Herndon-Dudley syndrome. Other potential targets are represented by obesity and dyslipidemia (for T2 and T1AM); dementia and degenerative brain disease (for T1AM and TA1); cancer (for T1AM and TA4). Another intriguing and unexplored question is the potential relevance of these metabolites in the clinical picture of hypothyroidism and in the response to replacement therapy.

The Colorful Diversity of Thyroid Hormone Metabolites

Since the discovery of L-thyroxine, the main secretory product of the thyroid gland, and its major metabolite T3, which exerts the majority of thyroid hormone action via ligand-dependent modulation of the function of T3 receptors in nuclei, mitochondria, and other subcellular compartments, various other T4-derived endogenous metabolites have been identified in blood and tissues of humans, animals, and early protochordates. This review addresses major historical milestones and experimental findings resulting in the discovery of the key enzymes of thyroid hormone metabolism, the three selenoprotein deiodinases, as well as the decarboxylases and amine oxidases involved in formation and degradation of recently identified endogenous thyroid hormone metabolites, i.e. 3-iodothyronamine and 3-thyroacetic acid. The concerted action of deiodinases 2 and 3 in regulation of local T3 availability is discussed. Special attention is given to the role of the thyromimetic "hot" metabolite 3,5-T2 and the "cool" 3-iodothyronamine, especially after administration of pharmacological doses of these endogenous thyroid hormone metabolites in various animal experimental models. In addition, available information on the biological roles of the two major acetic acid derivatives of thyroid hormones, i.e. Tetrac and Triac, as well as sulfated metabolites of thyroid hormones is reviewed. This review addresses the consequences of the existence of this broad spectrum of endogenous thyroid hormone metabolites, the "thyronome," beyond the classical thyroid hormone profile comprising T4, T3, and rT3 for appropriate analytical coverage and clinical diagnostics using mass spectrometry versus immunoassays for determination of total and free concentrations of thyroid hormone metabolites in blood and tissues.

Association Between 3-Iodothyronamine (T1am) Concentrations and Left Ventricular Function in Chronic Heart Failure

CONTEXT

Thyroid hormone metabolites might affect the heart. The endogenous aminergic metabolite 3-iodothyronamine (T1am) reduces left ventricular ejection fraction (LVEF) in rodents.

OBJECTIVE

To investigate concentration of T1am and its association with LVEF and biomarkers of heart function in patients with chronic heart failure (CHF) without thyroid disease, including patients with cardiac cachexia (nonedematous weight loss >5% over 6 months).

METHODS

Cross-sectional study. CHF was characterized by LVEF <45% and symptoms. Three groups were included (n = 19 in each group, matched on age, sex, and kidney function): patients with cachexia (CAC), patients without (non-CAC), and control (C) patients with prior myocardial infarction and LVEF >45%. T1am was measured by a monoclonal antibody-based chemiluminescence immunoassay. N-amino terminal pro-BNP (NT-proBNP) concentrations were also analyzed.

RESULTS

Mean (SD) LVEF: CAC, 32 ± 9%; non-CAC, 38 ± 8%; and C, 60 ± 8% (P < 0.0001). TSH, T4, and T3 levels did not differ between groups and did not correlate to T1am. Serum T1am (nmol/L) concentrations were higher in CHF: CAC (mean ± SD), 12.4 ± 6.6; non-CAC, 9.1 ± 5; and C, 7.3 ± 2.9. A negative association between T1am and LVEF was present after adjusting for sex, age, T3, and estimated glomerular filtration rate (P = 0.03). Further, serum T1am levels tended to be associated with NT-proBNP (P = 0.053).

CONCLUSION

Serum T1am levels were increased in patients with CHF and numerically highest (although nonsignificant) in patients with cardiac cachexia. Increasing T1am concentrations were independently associated with reduced LVEF, suggesting a direct effect on the human heart.

3-Iodothyronamine-A Thyroid Hormone Metabolite With Distinct Target Profiles and Mode of Action

The rediscovery of the group of thyronamines (TAMs), especially the first detailed description of their most prominent congener 3-iodothyronamine (3T1AM) 14 years ago, boosted research on this thyroid hormone metabolite tremendously. TAMs exert actions partly opposite to and distinct from known functions of thyroid hormones. These fascinating metabolic, anapyrexic, cytoprotective, and brain effects quickly evoked the hope to use hormone-derived TAMs as a therapeutic option. The G protein-coupled receptor (GPCR) TAAR1, a member of the trace amine-associated receptor (TAAR) family, was identified as the first target and effector of TAM action. The initial enthusiasm on pharmacological actions of exogenous TAMs elicited many questions, such as sites of biosynthesis, analytics, modes of action, inactivation, and role of TAMs in (patho)physiology. Meanwhile, it became clear that TAMs not only interact with TAAR1 or other TAAR family members but also with several aminergic receptors and non-GPCR targets such as transient receptor potential channels, mitochondrial proteins, and the serum TAM-binding protein apolipoprotein B100, thus classifying 3T1AM as a multitarget ligand. The physiological mode of action of TAMs is still controversial because regulation of endogenous TAM production and the sites of its biosynthesis are not fully elucidated. Methods for 3T1AM analytics need further validation, as they revealed different blood and tissue concentrations depending on detection principles used such as monoclonal antibody-based immunoassay vs liquid chromatography- matrix-assisted laser desorption/ionization mass spectrometry or time-of-flight mass spectrometry. In this review, we comprehensively summarize and critically evaluate current basic, translational, and clinical knowledge on 3T1AM and its main metabolite 3-iodothyroacetic acid, focusing on endocrine-relevant aspects and open but highly challenging issues.

Thyroid hormone and the brain: Mechanisms of action in development and role in protection and promotion of recovery after brain injury

Thyroid hormone (TH) is essential for normal brain development and may also promote recovery and neuronal regeneration after brain injury. TH acts predominantly through the nuclear receptors, TH receptor alpha (THRA) and beta (THRB). Additional factors that impact TH action in the brain include metabolism, activation of thyroxine (T4) to triiodothyronine (T3) by the enzyme 5'-deiodinase Type 2 (Dio2), inactivation by the enzyme 5-deiodinase Type 3 (Dio3) to reverse T3 (rT3), which occurs in glial cells, and uptake by the Mct8 transporter in neurons. Traumatic brain injury (TBI) is associated with inflammation, metabolic alterations and neural death. In clinical studies, central hypothyroidism, due to hypothalamic and pituitary dysfunction, has been found in some individuals after brain injury. TH has been shown, in animal models, to be protective for the damage incurred from brain injury and may have a role to limit injury and promote recovery. Although clinical trials have not yet been reported, findings from in vitro and in vivo models inform potential treatment strategies utilizing TH for protection and promotion of recovery after brain injury.

Thyroid Hormones and Derivatives: Endogenous Thyroid Hormones and Their Targets

More than a century after the discovery of L-Thyroxine, the main thyroid hormone secreted solely by the thyroid gland, several metabolites of this iodinated, tyrosine-derived ancestral hormone have been identified. These are utilized as hormones during development, differentiation, metamorphosis, and regulation of most biochemical reactions in vertebrates and their precursor species. Among those metabolites are the thyromimetically active 3,3',5-Triiodo-L-thyronine (T3) and 3,5-Diiodo-L-thronine, reverse-T3 (3,3',5'-Triiodo-L-thyronine) with still unclear function, the recently re-discovered thyronamines (e.g., 3-Iodo-thyronamine), which exert in part T3-antagonistic functions, the thyroacetic acids (e.g., Tetrac and Triac), as well as various sulfated or glucuronidated metabolites of this panel of iodinated signaling compounds. In the blood most of these hydrophobic metabolites are tightly bound to the serum distributor proteins thyroxine binding globulin (TBG), transthyretin (TTR), albumin or apolipoprotein B100. Cellular import and export of these charged, highly hydrophobic amino acid derivatives requires a number of cell-membrane transporters or facilitators such as MCT8 or MCT10 and members of the OATP and LAT families of transporters. Depending on their structure, the thyroid hormone metabolites exert their cellular action by binding and thus modulating the function of various receptors systems (e.g., ανβ3 integrin receptor and transient receptor potential channels (TRPM8) of the cell membrane), in part linked to intracellular downstream kinase signaling cascades, and several isoforms of membrane-associated, mitochondrial or nuclear thyroid hormone receptors (TR), which are members of the c-erbA family of ligand-modulated transcription factors. Intracellular deiodinase selenoenzymes, which obligatory are membrane integrated enzymes, ornithine decarboxylase and monoamine oxidases control local availability of biologically active thyroid hormone metabolites. Inactivation of thyroid hormone metabolites occurs mainly by deiodination, sulfation or glucuronidation, reactions which favor their renal or fecal elimination.

3,5-Diiodothyronine: A Novel Thyroid Hormone Metabolite and Potent Modulator of Energy Metabolism

Over 30 years of research has demonstrated that 3,5-diiodo-L-thyronine (3,5-T2), an endogenous metabolite of thyroid hormones, exhibits interesting metabolic activities. In rodent models, exogenously administered 3,5-T2 rapidly increases resting metabolic rate and elicits short-term beneficial hypolipidemic effects; however, very few studies have evaluated the effects of endogenous and exogenous T2 in humans. Further analyses on larger cohorts are needed to determine whether 3,5-T2 is a potent additional modulator of energy metabolism. In addition, while several lines of evidence suggest that 3,5-T2 mainly acts through Thyroid hormone receptors (THRs)- independent ways, with mitochondria as a likely cellular target, THRs-mediated actions have also been described. The detailed cellular and molecular mechanisms through which 3,5-T2 elicits a multiplicity of actions remains unknown. Here, we provide an overview of the most recent literature on 3,5-T2 bioactivity with a particular focus on short-term and long-term effects, describing data obtained through in vivo and in vitro approaches in both mammalian and non-mammalian species.

The classic pathways of thyroid hormone metabolism

Thyroid hormones (TH) are crucial for growth and development and play an important role in energy homeostasis. Although serum TH levels are relatively constant in the physiological state, TH bioavailability at the tissue and cellular level is dependent on local TH metabolism. Circulating TH produced by the thyroid can be metabolized by a number of different pathways resulting in 1) activation of TH 2) deactivation of TH or 3) excretion of TH and subsequent metabolites. These pathways play an essential role in determining local TH levels and action. The major classical pathways of TH metabolism are deiodination, sulfation, glucuronidation, and ether-link cleavage. This review provides an overview of these pathways, their relative contributions to TH levels in the serum and in various organs and the changes in these pathways elicited by fasting and illness.

Neuronal effects of thyroid hormone metabolites

Thyroid hormones and their metabolites are active regulators of gene expression, mitochondrial function and various other physiological actions in different organs and tissues. These actions are mediated by a spatio-temporal regulation of thyroid hormones and metabolites within a target cell. This spatio-temporal resolution as well as classical and non-classical actions of thyroid hormones and metabolites is accomplished and regulated on multiple levels as uptake, local activation and signaling of thyroid hormones. In this review, we will give an overview of the systems involved in regulating the presence and activity of thyroid hormones and their metabolites within the brain, specifically in neurons. While a wealth of data on thyroxin (T₄) and 3,5,3'-triiodothyronine (T₃) in the brain has been generated, research into the presence of action of other thyroid hormone metabolites is still sparse and requires further investigations.

Recovery of 3-Iodothyronamine and Derivatives in Biological Matrixes: Problems and Pitfalls

BACKGROUND

Difficulties have been reported in quantitating 3-iodothyronamine (T1AM) in blood or serum, and tentatively attributed to problems in extraction or other pre-analytical steps. For this reason, even cell culture experiments have often be performed with unphysiological protein-free media. The aim of this study was to evaluate the recovery of exogenous T1AM added to a standard cell culture medium, namely Dulbecco's minimum essential medium (DMEM) supplemented with fetal bovine serum (FBS), and to other biological matrixes.

METHODS

Cell culture media (Krebs-Ringer buffer, DMEM, FBS, DMEM + FBS, used either in the absence or in the presence of NG108-15 cells) and other biological matrixes (rat brain and liver homogenates, human plasma, and blood) were spiked with T1AM and/or deuterated T1AM (d4-T1AM) and incubated for times ranging from 0 to 240 minutes. Samples were then extracted using a liquid/liquid method and analyzed using liquid chromatography coupled to mass spectrometry in order to assay T1AM and its metabolites, namely 3-iodothyroacetic acid (TA1), thyronamine, thyroacetic acid, N-acetyl-T1AM, and T1AM esters.

RESULTS

In FBS-containing buffers, T1AM decreased exponentially over time, with a half-life of 6-17 minutes, depending on FBS content, and after 60 minutes, it averaged 0-10% of the baseline. T1AM metabolites were not detected, except for minimum amounts of TA1. Notably, d4-T1AM decreased over time at a much lower rate, reaching 50-70% of the baseline at 60 minutes. These effects were completely abolished by protein denaturation and partly reduced by semicarbazide. In the presence of cells, T1AM concentration decreased virtually to 0 within 60 minutes, but TA1 accumulated in the incubation medium, with quantitative recovery. Spontaneous decrease in T1AM concentration with isotopic difference was confirmed in rat organ homogenates and human blood.

CONCLUSIONS

These results suggest binding and sequestration of T1AM and/or its aldehyde derivative by blood and tissue proteins, with significant isotope effects. These issues might account for the technical problems complicating the analytical assays of endogenous T1AM.

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.

Torpor: The Rise and Fall of 3-Monoiodothyronamine from Brain to Gut-From Gut to Brain?

3-Monoiodothyronamine (T1AM), first isolated from rat brain, is reported to be an endogenous, rapidly acting metabolite of thyroxine. One of its numerous effects is the induction of a "torpor-like" state in experimental animals. A critical analysis of T1AM, to serve as an endogenous cryogen, is given. The proposed biosynthetic pathway for formation of T1AM, which includes deiodinases and ornithine decarboxylase in the upper intestinum, is an unusual one. To reach the brain via systemic circulation, enterohepatic recycling and passage through the liver may occur. The possible role of gut microbiota is discussed. T1AM concentrations in human serum, measured by a specific monoclonal assay are up to three orders of magnitude higher compared to values obtained by MS/MS technology. The difference is explained by the presence of a high-affinity binder for T1AM (Apolipoprotein B-100) in serum, which permits the immunoassay to measure the total concentration of the analyte but limits MS/MS technology to detect only the unbound (free) analyte, a view, which is contested here.