Axonal T3 uptake and transport can trigger thyroid hormone signaling in the brain

FT3 and FT3/FT4 ratio are decreased and not compensated by levothyroxine treatment in TKI-treated thyroid cancer patients

Tyrosine kinase inhibitors (TKIs) can alter thyroid hormone levels in multiple cancer contexts. The aim of this study was to analyse the efficacy of levothyroxine (LT4) treatment to normalise thyroid hormone levels in thyroid cancer patients with increased thyroid-stimulating hormone (TSH) after TKI initiation. Forty-five consecutive patients were enrolled. One month after TKI initiation, TSH increased from 0.21 (IQR: 0.07-0.8) to 1.09 mIU/L (IQR: 0.16-4.1) (P < 0.0001), and the FT3/FT4 ratio decreased from 2.1 (IQR: 1.86-2.66) to 1.92 (IQR 1.69-2.3) (P < 0.001) compared to baseline. During the first year of TKI treatment, in 23 patients (51%), TSH increased >4 mIU/L with a concomitant decrease in both FT3 and FT4. An increase in the LT4 daily dose of 0.62 ± 0.44 μg/kg normalised TSH to baseline levels in all patients, but in 11 patients FT3 remained lower compared to baseline, i.e. 2.53 (IQR: 2.4-2.75) versus 3.5 pg/mL (IQR: 3.18-3.84), respectively (P = 0.001). Following this observation, in three consecutive patients with increased TSH and reduced FT3 during TKI treatment, we changed the LT4 monotherapy into an LT4 + LT3 combination at a ratio of 11:1. Two months later, serum TSH, FT3 and FT4 were all similar to baseline (P > 0.05 in all determinations). In conclusion, TKI-related increase in TSH was associated with decreased FT3 and FT3/FT4 ratio. This was not compensated by an increase in LT4 in half of the cases. The clinical impact of compensating for the low T3 levels in these patients should be addressed in prospective trials.

Serotonergic and Chemosensory Brain Areas and Sensory Ganglia Expressing Type 3 Deiodinase Mapped With Dio3Cre drivers

Thyroid hormone (triiodothyronine, T3) promotes neurodevelopment but under strict control because unconstrained exposure to T3 impairs brain and sensory functions. Thyroid hormone-inactivating type 3 deiodinase, encoded by Dio3, critically limits T3 signaling and controls diverse neural functions. Accordingly, understanding the cellular basis of T3 action requires identification of Dio3-expressing cell types but this is difficult because of low level, transient expression within the complexity of the nervous system. Here, we derived a knock-in Dio3Cre driver that sensitively labels Dio3-expressing cells in male and female mice. In this anatomical study, we identified Dio3 expression in the immature amygdala and other brain regions associated with emotion and motivation, and in serotonergic raphe nuclei, which influence many behavioral and physiological systems. Notably, expression in circumventricular organs, including the chemosensory subfornical organ and organum vasculosum laminae terminalis, suggested regulation of centers that lack a blood-brain barrier and directly sense signaling factors in the circulation. Expression in trigeminal, dorsal root, cochleovestibular, and other sensory ganglia highlighted contributions to sensory pathways. Although Dio3 expression declines during maturation, a conditional Dio3CreERt2 driver revealed neurons with T3-inducible expression in the adult brain, suggesting ongoing homeostatic functions. These Cre drivers indicate strategically located neuronal groups for control of T3 signaling in behavioral, chemosensory and sensory systems.

Hypothalamic Median Eminence Thyrotropin-Releasing Hormone-Degrading Ectoenzyme Activity Is Dispensable for Basal Thyroid Axis Activity in Lean Rodents

The amplitude of the phasic output of thyrotropin-releasing hormone (TRH) into the hypothalamus-pituitary portal capillaries is likely controlled by the TRH-degrading ectoenzyme (TRH-DE) expressed on the surface of median eminence (ME) β2-tanycytes. To extend this hypothesis, we performed experiments on adult rodents reared in standard conditions. TRH-DE was close to the putative sites of TRH release in the male rat external layer of the ME. In global Trhde knockout mice, basal hypothalamus-pituitary-thyroid (HPT) axis parameters were not altered but we detected an increased vimentin (a tanycyte marker) positive coverage of the portal vessels. We then overexpressed TRH-DE or a dominant negative isoform by microinjection of adeno-associated virus 1 (AAV1) vectors into the third ventricle of adult male rats. Two weeks after microinjection, cold-stress-induced serum TSH concentration was decreased if ME TRH-DE activity had been enhanced. However, the long-term modification of TRH-DE activity in the ME had only a small impact on basal serum TSH concentration but increased Trhr expression in the anterior pituitary of animals transduced with AAV1-TRH-DE. Thus, long-term modifications of ME TRH-DE activity lead to limited changes in serum TSH concentration in adult rodents reared in standard conditions, possibly because of adaptations of TRH communication in the ME and/or anterior pituitary.

Shared Decisionmaking in the Treatment of Hypothyroidism

BACKGROUND

Hypothyroidism, a condition characterized by an underactive thyroid gland, affects millions worldwide, leading to cognitive and metabolic slowdowns. It is most prevalent in women and older adults, with causes including autoimmune thyroiditis, surgical thyroidectomy, and certain medications.

STANDARD OF CARE AND LIMITATIONS

The standard treatment involves synthetic levothyroxine (LT4) monotherapy, which alleviates symptoms by converting to the active hormone, T3. However, some patients continue to experience symptoms such as fatigue, mood disturbances, and poor quality of life despite normalized TSH levels. This persistence of symptoms may stem from misdiagnosis, inadequate dosing, or incomplete normalization of thyroid hormone signaling.

NEW FINDINGS

Research suggests that LT4 monotherapy may not fully restore T3 levels, leading to suboptimal symptom control. Consequently, combination therapy with LT4 and liothyronine (LT3) has been proposed as an alternative, aiming to balance T4 and T3 levels more effectively. Although randomized controlled trials have not identified significant differences in patient-reported outcomes between LT4 monotherapy and combination therapy, they indicate that patients may prefer the latter.

CONCLUSION

Guidelines from leading endocrinology organizations now recommend considering combination therapy for patients with persistent symptoms despite adequate LT4 dosing. A patient-centered approach, emphasizing shared decision-making and individualized treatment plans, is essential for optimizing outcomes in hypothyroidism management. Further research is needed to refine dosing strategies and identify the patients who would benefit most from combination therapy.

Treatment Preferences in Patients With Hypothyroidism

CONTEXT

Levothyroxine (L-T4) monotherapy is the standard of care for the treatment of hypothyroidism. A minority of L-T4-treated patients remain symptomatic and report better outcomes with combination therapy that contains liothyronine (L-T3) or with desiccated thyroid extract (DTE).

OBJECTIVE

This work aimed to assess patient preferences in the treatment of hypothyroidism.

METHODS

A systematic review, meta-analysis, meta-regression, and network meta-analysis of randomized controlled trials (RCTs) comparing treatments for adults with hypothyroidism (L-T4 vs L-T4 + L-T3 or DTE). Searches were conducted in PubMed, Embase, and Cochrane databases up to April 10, 2024. Data extraction and quality assessment were independently performed by 4 researchers.

RESULTS

Eleven RCTs (8 cross-over studies) with a total of 1135 patients were considered. Overall, 24% of patients preferred L-T4 vs 52% who preferred L-T4 + L-T3 or DTE; 24% had no preference. The meta-analysis confirmed the preference for combination therapy over L-T4 monotherapy (relative risk [RR]: 2.20; 95% CI, 1.38-3.52; P = .0009). Excluding 4 studies reduced the high heterogeneity (I2 = 81%) without affecting the results (RR: 1.97; 95% CI, 1.52-2.54; P < .00001; I2 = 24%). This preference profile remained when only crossover studies were considered (RR: 2.84; 95% CI, 1.50-5.39; P < .00001). Network meta-analysis confirmed the preference for DTE and L-T3 + L-T4 vs L-T4 alone.

CONCLUSION

Patients with hypothyroidism prefer combination therapy (L-T3 + L-T4 or DTE) over L-T4 monotherapy. The strength of these findings justifies considering patient preferences in the setting of shared decision-making in the treatment of hypothyroidism.

Variable transduction of thyroid hormone signaling in structures of the mouse brain

L-thyroxine (L-T4) monotherapy is the standard treatment for hypothyroidism, administered daily to normalize TSH levels. Once absorbed, T4 is converted to T3 to alleviate most symptoms. However, this treatment abnormally elevates plasma T4 levels in over 50% of patients. Using L-T4-treated Thyroid Hormone (TH) Action Indicator mice, which express a T3-regulated luciferase (Luc) reporter, we examined whether these T4 elevations disrupt TH signaling. Hypothyroid mice exhibited reduced Luc expression across brain regions, and L-T4 treatment failed to restore T3 signaling uniformly. There was also variability in the activity of type 2 deiodinase (D2), the enzyme that generates most brain T3. Intracerebroventricular T4 administration achieved higher elevation of Luc expression in the mediobasal hypothalamus compared to the cortex, and studies on cultured cortical astrocytes and hypothalamic tanycytes revealed cell-type-specific responses to T4. In tanycytes, exposure to T4 sustained D2 activity, leading to progressive T3 signaling, whereas in astrocytes, T4 exposure triggered a drop in D2 activity, limiting T3 production through a ubiquitin-dependent, self-limiting mechanism. The sustained D2 activity in tanycytes was linked to rapid deubiquitination by USP33, as confirmed using a ubiquitin-specific protease (USP) pan-inhibitor and USP33 knockout mice. In conclusion, the brain's response to L-T4 treatment is heterogeneous, influenced by cell-specific regulation of D2-mediated T3 production. While cortical astrocytes exhibit limited T3 signaling due to D2 ubiquitination, tanycytes coexpressing USP33 amplify T3 signaling by rescuing ubiquitinated D2 from proteasomal degradation. These findings provide mechanistic insights into the limitations of L-T4 therapy and highlight the need for tailored approaches to managing hypothyroidism.

Thyroid Hormone and Alzheimer Disease: Bridging Epidemiology to Mechanism

The identification of critical factors that can worsen the mechanisms contributing to the pathophysiology of Alzheimer disease is of paramount importance. Thyroid hormones (TH) fit this criterion. Epidemiological studies have identified an association between altered circulating TH levels and Alzheimer disease. The study of human and animal models indicates that TH can affect all the main cellular, molecular, and genetic mechanisms known as hallmarks of Alzheimer disease. This is true not only for the excessive production in the brain of protein aggregates leading to amyloid plaques and neurofibrillary tangles but also for the clearance of these molecules from the brain parenchyma via the blood-brain barrier and for the escalated process of neuroinflammation-and even for the effects of carrying Alzheimer-associated genetic variants. Suboptimal TH levels result in a greater accumulation of protein aggregates in the brain. The direct TH regulation of critical genes involved in amyloid beta production and clearance is remarkable, affecting the expression of multiple genes, including APP (related to amyloid beta production), APOE, LRP1, TREM2, AQP4, and ABCB1 (related to amyloid beta clearance). TH also affects microglia by increasing their migration and function and directly regulating the immunosuppressor gene CD73, impacting the immune response of these cells. Studies aiming to understand the mechanisms that could explain how changes in TH levels can contribute to the brain alterations seen in patients with Alzheimer disease are ongoing. These studies have potential implications for the management of patients with Alzheimer disease and ultimately can contribute to devising new interventions for these conditions.

The Thr92Ala polymorphism in the type 2 deiodinase gene is linked to depression in patients with COVID-19 after hospital discharge

Background

The Thr92Ala-DIO2 polymorphism has been associated with clinical outcomes in hospitalized patients with COVID-19 and neuropsychiatric diseases. This study examines the impact of the Thr92Ala-DIO2 polymorphism on neuropsychological symptoms, particularly depressive symptoms, in patients who have had moderate to severe SARS-CoV-2 infection and were later discharged.

Methods

Our prospective cohort study, conducted from June to August 2020, collected data from 273 patients hospitalized with COVID-19. This included thyroid function tests, inflammatory markers, hematologic indices, and genotyping of the Thr92Ala-DIO2 polymorphism. Post-discharge, we followed up with 68 patients over 30 to 45 days, dividing them into depressive (29 patients) and non-depressive (39 patients) groups based on their Beck Depression Inventory scores.

Results

We categorized 68 patients into three groups based on their genotypes: Thr/Thr (22 patients), Thr/Ala (41 patients), and Ala/Ala (5 patients). Depressive symptoms were less frequent in the Thr/Ala group (29.3%) compared to the Thr/Thr (59.1%) and Ala/Ala (60%) groups (p = 0.048). The Thr/Ala heterozygous genotype correlated with a lower risk of post-COVID-19 depression, as shown by univariate and multivariate logistic regression analyses. These analyses, adjusted for various factors, indicated a 70% to 81% reduction in risk.

Conclusion

Our findings appear to be the first to show that heterozygosity for Thr92Ala-DIO2 in patients with COVID-19 may protect against post-COVID-19 depression symptoms up to 2 months after the illness.

Gene polymorphisms and thyroid hormone signaling: implication for the treatment of hypothyroidism

INTRODUCTION

Mutations and single nucleotide polymorphisms (SNPs) in the genes encoding the network of proteins involved in thyroid hormone signaling (TH) may have implications for the effectiveness of the treatment of hypothyroidism with LT4. It is conceivable that loss-of-function mutations or SNPs impair the ability of LT4 to be activated to T3, reach its targets, and ultimately resolve symptoms of hypothyroidism. Some of these patients do benefit from therapy containing LT4 and LT3.

METHODS

Here, we reviewed the PubMed and examined gene mutations and SNPs in the TH cellular transporters, deiodinases, and TH receptors, along with their impact on TH signaling, and potential clinical implications.

RESULTS

In some mechanisms, such as the Thr92Ala-DIO2 SNP, there is a compelling rationale for reduced T4 to T3 activation that limits the effectiveness of LT4 to restore euthyroidism. In other mechanisms, a potential case can be made but more studies with a larger number of individuals are needed.

DISCUSSION/CONCLUSION

Understanding the clinical impact of the genetic makeup of LT4-treated patients may help in the preemptive identification of those individuals that would benefit from therapy containing LT3.

Impaired T3 uptake and action in MCT8-deficient cerebral organoids underlie Allan-Herndon-Dudley syndrome

Patients with mutations in the thyroid hormone (TH) cell transporter monocarboxylate transporter 8 (MCT8) gene develop severe neuropsychomotor retardation known as Allan-Herndon-Dudley syndrome (AHDS). It is assumed that this is caused by a reduction in TH signaling in the developing brain during both intrauterine and postnatal developmental stages, and treatment remains understandably challenging. Given species differences in brain TH transporters and the limitations of studies in mice, we generated cerebral organoids (COs) using human induced pluripotent stem cells (iPSCs) from MCT8-deficient patients. MCT8-deficient COs exhibited (i) altered early neurodevelopment, resulting in smaller neural rosettes with thinner cortical units, (ii) impaired triiodothyronine (T3) transport in developing neural cells, as assessed through deiodinase-3-mediated T3 catabolism, (iii) reduced expression of genes involved in cerebral cortex development, and (iv) reduced T3 inducibility of TH-regulated genes. In contrast, the TH analogs 3,5-diiodothyropropionic acid and 3,3',5-triiodothyroacetic acid triggered normal responses (induction/repression of T3-responsive genes) in MCT8-deficient COs, constituting proof of concept that lack of T3 transport underlies the pathophysiology of AHDS and demonstrating the clinical potential for TH analogs to be used in treating patients with AHDS. MCT8-deficient COs represent a species-specific relevant preclinical model that can be utilized to screen drugs with potential benefits as personalized therapeutics for patients with AHDS.

Thyroid hormone biosynthesis and its role in brain development and maintenance

Thyroid hormones are critical modulators in the physiological processes necessary to virtually all tissues, with exceptionally fundamental roles in brain development and maintenance. These hormones regulate essential neurodevelopment events, including neuronal migration, synaptogenesis, and myelination. Additionally, thyroid hormones are crucial for maintaining brain homeostasis and cognitive function in adulthood. This chapter aims to offer a comprehensive understanding of thyroid hormone biosynthesis and its intricate role in brain physiology. Here, we described the mechanisms underlying the biosynthesis of thyroid hormones, their influence on various aspects of brain development and ongoing maintenance, and the proteins in the brain that are responsive to these hormones. This chapter was geared towards broadening our understanding of thyroid hormone action in the brain, shedding light on potential therapeutic targets for neurodevelopmental and neurodegenerative disorders.

Nongenomic roles of thyroid hormones and their derivatives in adult brain: are these compounds putative neurotransmitters?

We review the evidence regarding the nongenomic (or non-canonical) actions of thyroid hormones (thyronines) and their derivatives (including thyronamines and thyroacetic acids) in the adult brain. The paper seeks to evaluate these compounds for consideration as candidate neurotransmitters. Neurotransmitters are defined by their (a) presence in the neural tissue, (b) release from neural tissue or cell, (c) binding to high-affinity and saturable recognition sites, (d) triggering of a specific effector mechanism and (e) inactivation mechanism. Thyronines and thyronamines are concentrated in brain tissue and show distinctive patterns of distribution within the brain. Nerve terminals accumulate a large amount of thyroid hormones in mature brain, suggesting a synaptic function. However, surprisingly little is known about the potential release of thyroid hormones at synapses. There are specific binding sites for thyroid hormones in nerve-terminal fractions (synaptosomes). A notable cell-membrane binding site for thyroid hormones is integrin αvβ3. Furthermore, thyronines bind specifically to other defined neurotransmitter receptors, including GABAergic, catecholaminergic, glutamatergic, serotonergic and cholinergic systems. Here, the thyronines tend to bind to sites other than the primary sites and have allosteric effects. Thyronamines also bind to specific membrane receptors, including the trace amine associated receptors (TAARs), especially TAAR1. The thyronines and thyronamines activate specific effector mechanisms that are short in latency and often occur in subcellular fractions lacking nuclei, suggesting nongenomic actions. Some of the effector mechanisms for thyronines include effects on protein phosphorylation, Na+/K+ ATPase, and behavioral measures such as sleep regulation and measures of memory retention. Thyronamines promptly regulate body temperature. Lastly, there are numerous inactivation mechanisms for the hormones, including decarboxylation, deiodination, oxidative deamination, glucuronidation, sulfation and acetylation. Therefore, at the current state of the research field, thyroid hormones and their derivatives satisfy most, but not all, of the criteria for definition as neurotransmitters.