Enterohepatic circulation of triiodothyronine (T3) in rats: importance of the microflora for the liberation and reabsorption of T3 from biliary T3 conjugates

Gut microbial β-glucuronidases influence endobiotic homeostasis and are modulated by diverse therapeutics

Hormones and neurotransmitters are essential to homeostasis, and their disruptions are connected to diseases ranging from cancer to anxiety. The differential reactivation of endobiotic glucuronides by gut microbial β-glucuronidase (GUS) enzymes may influence interindividual differences in the onset and treatment of disease. Using multi-omic, in vitro, and in vivo approaches, we show that germ-free mice have reduced levels of active endobiotics and that distinct gut microbial Loop 1 and FMN GUS enzymes drive hormone and neurotransmitter reactivation. We demonstrate that a range of FDA-approved drugs prevent this reactivation by intercepting the catalytic cycle of the enzymes in a conserved fashion. Finally, we find that inhibiting GUS in conventional mice reduces free serotonin and increases its inactive glucuronide in the serum and intestines. Our results illuminate the indispensability of gut microbial enzymes in sustaining endobiotic homeostasis and indicate that therapeutic disruptions of this metabolism promote interindividual response variabilities.

Cross-talk between the gut microbiota and hypothyroidism: a bidirectional two-sample Mendelian randomization study

Background

Multiple observational studies suggest a connection between the composition of the gut microbiota and hypothyroidism. However, it has yet to be determined whether the gut microbiota has a causal effect on hypothyroidism.

Methods

To investigate the connection between the gut microbiota and hypothyroidism, two-sample Mendelian randomization was performed using data from a genome-wide association study meta-analysis (n = 18,430) conducted by the MiBioGen consortium. Summary statistics for hypothyroidism (26,342 cases and 59,827 controls) were obtained using the data from the FinnGen consortium R8 release data. To investigate the causal link between the gut microbiota and hypothyroidism, various methods, including MR-Egger, weighted median, weighted model, simple model, MR-PRESSO, and inverse variance weighted (IVW), were employed. The bacteria that were causally linked to hypothyroidism in forward Mendelian randomization analysis were subjected to reverse Mendelian randomization analysis. Cochran's Q statistics were utilized to gauge the heterogeneity of the instrumental variables.

Results

The results indicated that Akkermansia had a positive impact on hypothyroidism, with an odds ratio of 0.84 (95% CI 0.74-0.95, p = 0.01) based on the inverse variance-weighted estimates. Additionally, Anaerostipes (OR = 1.17, 95% CI 1.01-1.36, p = 0.04), Butyrivibrio (OR = 0.93, 95% CI 0.88-0.99, p = 0.02), Holdemania (OR = 0.89, 95% CI 0.81-0.99, p = 0.03), Intestinimonas (OR = 1.13, 95% CI 1.02-1.26, p = 0.03), Ruminiclostridium5 (OR = 1.19, 95% CI 1.01-1.41, p = 0.04), and Ruminococcaceae UCG-011 (OR = 0.91, 95% CI 0.84-0.99, p = 0.03) were identified. The gut microbiota was not significantly affected by hypothyroidism, as indicated by the results of the reverse MR analysis. There was no significant variation in the instrumental variables or horizontal pleiotropy.

Conclusion

The findings of this study using two-sample Mendelian randomization indicate a causal relationship between Akkermansia and hypothyroidism. Increased Akkermansia inhibits the onset and progression of hypothyroidism. Additional randomized controlled experiments are necessary to elucidate the beneficial impact of probiotics on hypothyroidism and their distinct protective mechanisms.

Intestinal microbiota regulates the gut-thyroid axis: the new dawn of improving Hashimoto thyroiditis

Intestinal microbiota plays an indispensable role in the host's innate immune system, which may be related to the occurrence of many autoimmune diseases. Hashimoto thyroiditis (HT) is one of the most common autoimmune diseases, and there is plenty of evidence indicating that HT may be related to genetics and environmental triggers, but the specific mechanism has not been proven clearly. Significantly, the composition and abundance of intestinal microbiota in patients with HT have an obvious difference. This phenomenon led us to think about whether intestinal microbiota can affect the progress of HT through some mechanisms. By summarizing the potential mechanism of intestinal microflora in regulating Hashimoto thyroiditis, this article explores the possibility of improving HT by regulating intestinal microbiota and summarizes relevant biomarkers as therapeutic targets, which provide new ideas for the clinical diagnosis and treatment of Hashimoto thyroiditis.

Maternal Administration of the CNS-Selective Sobetirome Prodrug Sob-AM2 Exerts Thyromimetic Effects in Murine MCT8-Deficient Fetuses

Background: Monocarboxylate transporter 8 (MCT8) deficiency is a rare X-linked disease where patients exhibit peripheral hyperthyroidism and cerebral hypothyroidism, which results in severe neurological impairments. These brain defects arise from a lack of thyroid hormones (TH) during critical stages of human brain development. Treatment options for MCT8-deficient patients are limited and none have been able to prevent or ameliorate effectively the neurological impairments. This study explored the effects of the TH agonist sobetirome and its CNS-selective amide prodrug, Sob-AM2, in the treatment of pregnant dams carrying fetuses lacking Mct8 and deiodinase type 2 (Mct8/Dio2 KO), as a murine model for MCT8 deficiency. Methods: Pregnant dams carrying Mct8/Dio2 KO fetuses were treated with 1 mg of sobetirome/kg body weight/day, or 0.3 mg of Sob-AM2/kg body weight/day for 7 days, starting at embryonic day 12.5 (E12.5). As controls, pregnant dams carrying wild-type and pregnant dams carrying Mct8/Dio2 KO fetuses were treated with daily subcutaneous injections of vehicle. Dams TH levels were measured by enzyme-linked immunosorbent assay (ELISA). Samples were extracted at E18.5 and the effect of treatments on the expression of triiodothyronine (T3)-dependent genes was measured in the placenta, fetal liver, and fetal cerebral cortex by real-time polymerase chain reaction. Results: Maternal sobetirome treatment led to spontaneous abortions. Sob-AM2 treatment, however, was able to cross the placental as well as the brain barriers and exert thyromimetic effects in Mct8/Dio2 KO fetal tissues. Sob-AM2 treatment did not affect the expression of the T3-target genes analyzed in the placenta, but it mediated thyromimetic effects in the fetal liver by increasing the expression of Dio1 and Dio3 genes. Interestingly, Sob-AM2 treatment increased the expression of several T3-dependent genes in the brain such as Hr, Shh, Dio3, Kcnj10, Klf9, and Faah in Mct8/Dio2 KO fetuses. Conclusions: Maternal administration of Sob-AM2 can cross the placental barrier and access the fetal tissues, including the brain, in the absence of MCT8, to exert thyromimetic actions by modulating the expression of T3-dependent genes. Therefore, Sob-AM2 has the potential to address the cerebral hypothyroidism characteristic of MCT8 deficiency from fetal stages and to prevent neurodevelopmental alterations in the MCT8-deficient fetal brain.

A Comprehensive Review of Thyroid Hormone Metabolism in the Gut and Its Clinical Implications

Background: The gut is a target organ of thyroid hormone (TH) that exerts its action via the nuclear thyroid hormone receptor α1 (TRα1) expressed in intestinal epithelial cells. THs are partially metabolized via hepatic sulfation and glucuronidation, resulting in the production of conjugated iodothyronines. Gut microbiota play an important role in peripheral TH metabolism as they produce and secrete enzymes with deconjugation activity (β-glucuronidase and sulfatase), via which TH can re-enter the enterohepatic circulation. Summary: Intestinal epithelium homeostasis (the finely tuned balance between cell proliferation and differentiation) is controlled by the crosstalk between triiodothyronine and TRα1 and the presence of specific TH transporters and TH-activating and -inactivating enzymes. Patients and experimental murine models with a dominant-negative mutation in the TRα exhibit gross abnormalities in the morphology of the intestinal epithelium and suffer from severe symptoms of a dysfunctional gastrointestinal tract. Over the past decade, gut microbiota has been identified as an essential factor in health and disease, depending on its compositional and functional profile. This has led to a renewed interest in the so-called gut-thyroid axis. Disruption of gut microbial homeostasis (dysbiosis) is associated with autoimmune thyroid disease (AITD), including Hashimoto's thyroiditis, Graves' disease, and Graves' orbitopathy. These studies reviewed here provide new insights into the gut microbiota roles in thyroid disease pathogenesis and may be an initial step toward microbiota-based therapies in AITD. However, it should be noted that cause-effect mechanisms remain to be proven, for which prospective cohort studies, randomized clinical trials, and experimental studies are needed. Conclusion: This review aims at providing a comprehensive insight into the interplay between TH metabolism and gut homeostasis.

What we know about the relationship between autoimmune thyroid diseases and gut microbiota: a perspective on the role of probiotics on pediatric endocrinology

INTRODUCTION

Autoimmune diseases account for a cumulative overall prevalence of about 3-5% worldwide. Among them, autoimmune thyroid diseases (ATDs) are the most common and comprise two main entities: Hashimoto's thyroiditis (HT) and Graves-Basedow disease (GD). The pathogenesis of ATDs remains not fully elucidated, however the role of microbioma has been proposed. Gut microbiota exert an important influence on the intestinal barrier, nutrient metabolism and immune system development and functions.

EVIDENCE ACQUISITION

In this review, we describe on the main features of ATDs in pediatrics, focusing on the reciprocal influence between gut microbiota, thyroid hormone metabolism and thyroid autoimmunity and consider the role of probiotics and other microbiota-targeted therapies in thyroid diseases with a perspective on pediatric endocrinology.

EVIDENCE SYNTHESIS

Microbiome affects both endogenous and exogenous thyroid hormone metabolism and influences the absorption of minerals important to the thyroid function, which are iodine, selenium, zinc and iron. The alteration of the gut microbiota, with the consequent modifications in the barrier function and the increased gut permeability, seems involved in the development of autoimmune and chronic inflammatory diseases, including ATDs. The supplementation with probiotics showed beneficial effects on the thyroid hormone and thyroid function because this strategy could restore the intestinal eubiosis and the good strain microorganism proliferation.

CONCLUSIONS

Even though the evidence about the interaction between microbiota and ATDs in pediatric patients is limited, the promising results obtained in the adult population, and in other autoimmune disorders affecting children, highlight the need of for further research in the pediatric field.

The relationships between the gut microbiota and its metabolites with thyroid diseases

Emerging studies have provided a preliminary understanding of the thyroid-gut axis, indicating that intestinal microbiota and its metabolites may act directly or indirectly on the thyroid by influencing intestinal microelements uptake, iodothyronine conversion and storage, and immune regulation, providing new insights into the pathogenesis of thyroid disorders and clinical management strategies. However, the research on gut microbiota and thyroid has only presented the tip of the iceberg. More robust clinical data and basic experiments are still required to elucidate the specific relationships and mechanisms in the future. Here we will characterize the associations between the microbiota and thyroid diseases to evaluate their potential implications in the pathophysiology and open up scientific avenues for future precision studies of the thyroid-gut axis.

Non-Invasive Measurement of Thyroid Hormones in Domestic Rabbits

Simple Summary

Thyroid hormones are essential for metabolism, energy homeostasis and reproduction. Hormones can be measured in various biological source materials: blood, urine, feces, saliva, hair, and others. The most common method for assessing hormone levels, including thyroid hormones, is a blood test, but this method has many limitations, especially in the diagnostic process of non-domestic animals. Non-invasive thyroid hormone measurement methods have been developed in the last decade. The aim of our study was to verify the usefulness of thyroid hormone analysis (total thyroxine, total triiodothyronine, free thyroxine, free triiodothyronine) in urine and feces of the domestic rabbit, comparing them with the serum. Results suggest that free triiodothyronine can be accurately and reliably measured in the feces and urine of the domestic rabbit.

Abstract

Thyroid hormones are essential for metabolism, energy homeostasis and reproduction. Hormones can be measured in various biological source materials: blood, feces, urine, saliva and others. The aim of our study was to verify usefulness of thyroid hormone analysis in the urine and feces of the domestic rabbit (Oryctolagus cuniculus f. domesticus), comparing them with the serum analyses. Samples were collected from 27 does in the age of 12–14 weeks. Total thyroxine (tT4), total triiodothyronine (tT3), free thyroxine (fT4) and free triiodothyronine (fT3) were tested using the radioimmunological method in serum, feces and urine. The highest concentration of tT4 was found in feces (104.72 ± 59.52 nmol/mg) and the lowest in urine (3.03 ± 3.11 nmol/mL). The highest tT3 concentration was found in blood serum (3.19 ± 0.64 nmol/L) and the lowest in urine (0.31 ± 0.43 nmol/L). The highest concentration of fT4 was observed in feces (43.71 ± 4.79 pmol/mg) and the lowest in blood serum (14.97 ± 3.42 pmol/L). The statistically highest concentration of fT3 (28.56 ± 20.79 pmol/L) was found in urine, whereas the lowest concentration of this hormone was found in feces (3.27 ± 1.33 pmol/mg). There was a positive and statistically significant correlation between serum and urine fT3 (r = 0.76) and a high positive correlation between serum and feces fT3 concentration (r = 0.62). Correlations between concentrations of other thyroid hormones between serum, urine and feces were found to be insignificant. The results suggest that fT3 can be accurately and reliably measured in the feces and urine of the domestic rabbit.

Circulating Thyroid Hormone Profile in Response to a Triiodothyronine Challenge in Familial Longevity

Context

Familial longevity is associated with higher circulating levels of thyrotropin (TSH), in the absence of differences in circulating thyroid hormones, and a lower thyroid responsivity to TSH, as previously observed in the Leiden Longevity Study (LLS). Further mechanisms underlying these observations remain unknown.

Objective

We hypothesized that members from long-lived families (offspring) have higher thyroid hormone turnover or less negative feedback effect on TSH secretion compared to controls.

Methods

In a case-control intervention study, 14 offspring and 13 similarly aged controls received 100 µg 3,5,3′-triiodothyronine (T3) orally. Their circulating T3, free T3 (fT3), and TSH levels were measured during 5 consecutive days. We compared profiles of circulating T3, fT3, and TSH between offspring and controls using general linear modeling (GLM) and calculated the percentage decline in TSH following T3 administration.

Results

Circulating T3 and fT3 levels increased to supraphysiologic values and normalized over the course of 5 days. There were no serious adverse events. T3 and fT3 concentration profiles over 5 days were similar between offspring and controls (T3 GLM P = .11, fT3 GLM P = .46). TSH levels decreased in a biphasic manner and started returning to baseline by day 5. The TSH concentration profile over 5 days was similar between offspring and controls (GLM P = .08), as was the relative TSH decline (%).

Conclusions

Members of long-lived families have neither higher T3 turnover nor diminished negative feedback of T3 on TSH secretion. The cause and biological role of elevated TSH levels in familial longevity remain to be elucidated.

The relationship between ingested thyroid hormones, thyroid homeostasis and iodine metabolism in humans and teleost fish

Oral l-thyroxine (T4) therapy is used to treat human hypothyroidism but T4 fed to teleost fish does not raise plasma thyroid hormone (TH) levels nor induce growth, even though oral 3,5,3'-triiodo-l-thyronine (T3) is effective. This suggests a major difference in TH metabolism between teleosts and humans, often used as a starting thyroid model for lower vertebrates. To gain further insight on the proximate (mechanistic) and ultimate (survival value) factors underlying this difference, the several steps in TH homeostasis from intestinal TH uptake to hypothalamic-hypophyseal regulation were compared between humans and teleosts, and following dietary TH challenges. A major proximate factor limiting trout T4 uptake is a potent constitutive thiol-inhibited intestinal complete T4 deiodination that is ineffective for T3. At the hepatic level, T4 deiodination, conjugation and extensive biliary excretion with negligible T4 enterohepatic recycling can further block teleost T4 uptake to plasma. Such protection of plasma T4 from dietary T4 may be particularly critical for piscivorous fish consuming thyroid tissue, rich in T4 but not T3. It would prevent disruption by unregulated ingested T4 of the characteristic acute and transient changes in teleost plasma T4 due to diel rhythms, food intake and stress-related factors. These marked natural short-term fluctuations in teleost plasma T4 levels are enabled by the relatively small and rapidly-cleared plasma T4 pool, stemming largely from properties of the plasma T4-binding proteins. Humans, however, due mainly to plasma T4-binding globulin, have a relatively massive circulating pool of T4 and an extremely well-buffered free T4 level, consistent with the major TH role in regulating basal metabolic rate. Furthermore, this large well-buffered and slowly-cleared plasma T4 pool, in conjuction with enterohepatic recycling and relaxation of hypothalamic-hypophyseal negative feedback, allows humans to temporarily 'store' ingested T4 in plasma, thereby sparing endogenous TH secretion and conserving thyroidal iodine reserves. Indeed, iodine conservation is likely the key ultimate factor determining the divergent evolution of the human and teleost systems. For humans, ingested iodine in the form of I^-, or TH and their derivatives, is the sole iodine source and may be limiting in many environments. However, most freshwater teleosts, in addition to their ability to assimilate dietary I^-, can derive sufficient I^- from their copious gill irrigation, with no selective advantage in absorbing dietary T4 which would disrupt their natural acute and transient changes in plasma T4. Thus T4 may act also as a vitamin (vitamone) in humans but not in teleosts; in contrast, T3, naturally ingested at much lower levels, may act as a vitamone in both humans and teleosts.

Structure-Activity Relationships of Central Nervous System Penetration by Fatty Acid Amide Hydrolase (FAAH)-Targeted Thyromimetic Prodrugs

Thyroid hormone (TH) action is of clinical interest in treating demyelinating diseases of the central nervous system (CNS). Two amide prodrugs of sobetirome, a potent thyroid hormone agonist, were previously shown to significantly improve CNS selective distribution of the parent drug through hydrolysis in the CNS by fatty acid amide hydrolase (FAAH). This concept is elaborated upon here with a series of 29 amide prodrugs targeting FAAH. We identify that conservative aliphatic modifications such as the N-methyl (4), N-ethyl (5), N-fluoroethyl (15), and N-cyclopropyl (18) substantially favor selective CNS distribution of the parent drug in mice. Additionally, lead compounds exhibit moderate to good rates of hydrolysis at FAAH in vitro suggesting both enzymatic and physicochemical properties are important parameters for optimization. Both 4 and 15 were orally bioavailable while retaining appreciable CNS parent drug delivery following an oral dose. The pharmacokinetic parameters of 4 over 24 h postdose (i.v. and p.o.) were determined.

Treatment of Hypothyroid Patients With L-Thyroxine (L-T4) Plus Triiodothyronine Sulfate (T3S). A Phase II, Open-Label, Single Center, Parallel Groups Study on Therapeutic Efficacy and Tolerability

Sodium salt of levothyroxine (L-T4) is the treatment of choice of hypothyroidism. Yet, L-T4 monotherapy produces supoptimal 3,5,3'-triiodothyronine (T3)/T4 ratio in serum, as compared to normal subjects, and a minority of hypothyroid individuals on L-T4 complain for an incomplete well-being. Orally administered 3,5,3'-triiodothyronine sulfate (T3S) can be converted to T3 in humans, resulting in steady-state serum T3 concentrations for up to 48 h. In this study (EudraCT number 2010-018663-42), 36 thyroidectomized hypothyroid patients receiving 100 (group A), 125 (group B), or 150 μg (group C) L-T4 were enrolled in a 75 days study in which 25 μg L-T4 were replaced by 40 μg of T3S. A significant, progressive reduction in mean FT4 values was observed, being the largest in the group A and the smallest in group C, while no relevant variations in FT3 and total T3 serum values were observed in the three groups. TSH serum levels increased in all groups, the highest value being observed in group A. Lipid parameters did not show clinically significant changes in all groups. No T3S-related changes in the safety laboratory tests were recorded. No adverse event was judged as related to experimental treatment, and no patient discontinued the treatment. Twelve patients judged the L-T4+T3S treatment better than L-T4 alone, while no patient reported a preference for L-T4 over the combined treatment. In conclusion, the results of this study indicate that a combination of L-T4+T3S in hypothyroid subjects may allow mainteinance of normal levels of serum T3, with restoration of a physiological FT4/FT3 ratio and no appearance of adverse events. Further studies are required to verify whether the LT4+T3S chronic combined treatment of hypothyroidism is able to produce additional benefits over L-T4 monotherapy.

Transport, Metabolism, and Function of Thyroid Hormones in the Developing Mammalian Brain

Ever since the discovery of thyroid hormone deficiency as the primary cause of cretinism in the second half of the 19th century, the crucial role of thyroid hormone (TH) signaling in embryonic brain development has been established. However, the biological understanding of TH function in brain formation is far from complete, despite advances in treating thyroid function deficiency disorders. The pleiotropic nature of TH action makes it difficult to identify and study discrete roles of TH in various aspect of embryogenesis, including neurogenesis and brain maturation. These challenges notwithstanding, enormous progress has been achieved in understanding TH production and its regulation, their conversions and routes of entry into the developing mammalian brain. The endocrine environment has to adjust when an embryo ceases to rely solely on maternal source of hormones as its own thyroid gland develops and starts to produce endogenous TH. A number of mechanisms are in place to secure the proper delivery and action of TH with placenta, blood-brain interface, and choroid plexus as barriers of entry that need to selectively transport and modify these hormones thus controlling their active levels. Additionally, target cells also possess mechanisms to import, modify and bind TH to further fine-tune their action. A complex picture of a tightly regulated network of transport proteins, modifying enzymes, and receptors has emerged from the past studies. TH have been implicated in multiple processes related to brain formation in mammals-neuronal progenitor proliferation, neuronal migration, functional maturation, and survival-with their exact roles changing over developmental time. Given the plethora of effects thyroid hormones exert on various cell types at different developmental periods, the precise spatiotemporal regulation of their action is of crucial importance. In this review we summarize the current knowledge about TH delivery, conversions, and function in the developing mammalian brain. We also discuss their potential role in vertebrate brain evolution and offer future directions for research aimed at elucidating TH signaling in nervous system development.

Enteric Microbiota⁻Gut⁻Brain Axis from the Perspective of Nuclear Receptors

Nuclear receptors (NRs) play a key role in regulating virtually all body functions, thus maintaining a healthy operating body with all its complex systems. Recently, gut microbiota emerged as major factor contributing to the health of the whole organism. Enteric bacteria have multiple ways to influence their host and several of them involve communication with the brain. Mounting evidence of cooperation between gut flora and NRs is already available. However, the full potential of the microbiota interconnection with NRs remains to be uncovered. Herewith, we present the current state of knowledge on the multifaceted roles of NRs in the enteric microbiota–gut–brain axis.

"With a little help from my friends" - The role of microbiota in thyroid hormone metabolism and enterohepatic recycling

The gut microbiota is composed of over 1200 species of anaerobes and aerobes bacteria along with bacteriophages, viruses and fungal species. Increasing evidence indicates that the intestinal microbiota, beside digestive equilibrium, is also crucial for immunologic, hormonal and metabolic homeostasis. The intestinal microbiota interacts with distant organs by signals which may be part of the bacteria themselves or their metabolites. Dysbiosis has been observed in inflammatory or autoimmune disorders such as multiple sclerosis or type 1 diabetes as well as in obesity and type 2 diabetes. Functional thyroid disorders were associated with bacterial overgrowth and a different microbial composition. Although thyroid metabolism was apparently disregarded, the interference of microbiota on peripheral iodothyronine homeostasis is an intriguing issue. In this review we focused on the interactions of intestinal microbiota with thyroid-related micronutrients and with the metabolic steps of endogenous and exogenous iodothyronines.

Ester-to-amide rearrangement of ethanolamine-derived prodrugs of sobetirome with increased blood-brain barrier penetration

Current therapeutic options for treating demyelinating disorders such as multiple sclerosis (MS) do not stimulate myelin repair, thus creating a clinical need for therapeutic agents that address axonal remyelination. Thyroid hormone is known to play an important role in promoting developmental myelination and repair, and CNS permeable thyromimetic agents could offer an increased therapeutic index compared to endogenous thyroid hormone. Sobetirome is a clinical stage thyromimetic that has been shown to have promising activity in preclinical models related to MS and X-linked adrenoleukodystrophy (X-ALD), a genetic disease that involves demyelination. Here we report a new series of sobetirome prodrugs containing ethanolamine-based promoieties that were found to undergo an intramolecular O,N acyl migration to form the pharmacologically relevant amide species. Several of these systemically administered prodrugs deliver more sobetirome to the brain compared to unmodified sobetirome. Pharmacokinetic properties of the parent drug sobetirome and amidoalcohol prodrug 3 are described and prodrug 3 was found to be more potent than sobetirome in target engagement in the brain from systemic dosing.

Thiazopyr and Thyroid Disruption: Case Study Within the Context of the 2006 IPCS Human Relevance Framework for Analysis of a Cancer Mode of Action

Thiazopyr increases the incidence of male rat thyroid follicular-cell tumors; however, it is not carcinogenic in mice. Thiazopyr is not genotoxic. Thiazopyr exerts its carcinogenic effect on the rat thyroid gland secondary to enhanced metabolism of thyroxin leading to hormone imbalance. The relevance of these rat tumors to human health was assessed by using the 2006 IPCS Human Relevance Framework. The postulated rodent tumor mode of action was tested against the Bradford Hill criteria and was found to satisfy the conditions of dose and temporal concordance, biological plausibility, coherence, strength, consistency, and specificity that fits with a well-established mode of action for thyroid follicular-cell tumors. Although the postulated mode of action could theoretically operate in humans, marked quantitative differences in the inherent susceptibility for neoplasia to thyroid hormone imbalance in rats allows for the conclusion that thiazopyr does not pose a carcinogenic hazard to humans.

Increased prevalence of subclinical hypothyroidism in common bile duct stone patients

CONTEXT

Earlier, we have shown an increased prevalence of previously diagnosed hypothyroidism in common bile duct (CBD) stone patients and a delayed emptying of the biliary tract in hypothyroidism, explained partly by the missing prorelaxing effect of T(4) on the sphincter of Oddi contractility.

OBJECTIVE

In this study, the prevalence of previously undiagnosed subclinical hypothyroidism in CBD stone patients was compared with nongallstone controls.

PATIENTS

All patients were clinically euthyreotic and without a history of thyroid function abnormalities. CBD stones were diagnosed at endoscopic retrograde cholangiopancreatography (group 1; n = 303) or ruled out by previous medical history, liver function tests, and ultrasonography (control group II; n = 142).

MAIN OUTCOME MEASURES

Serum free FT(4) and TSH (S-TSH) were analyzed; S-TSH above the normal range (>6.0 mU/liter) was considered as subclinical and S-TSH 5.0-6.0 mU/liter as borderline-subclinical hypothyroidism.

RESULTS

A total of 5.3 and 5.0% (total 10.2%; 31 of 303) of the CBD stone patients were diagnosed to have subclinical and borderline-subclinical hypothyroidism, compared with 1.4% (P = 0.05) and 1.4% (total 2.8%, four of 142; P = 0.026) in the control group, respectively. In women older than 60 yr, the prevalence of subclinical hypothyroidism was 11.4% in CBD stone and 1.8% in control patients (P = 0.032) and subclinical plus borderline-subclinical hypothyroidism 23.8% in CBD stone and 1.8% in control patients (P = 0.012).

CONCLUSION

Subclinical hypothyroidism is more common in the CBD stone patients, compared with nongallstone controls, supporting our hypothesis that hypothyroidism might play a role in the forming of CBD stones. At minimum, women older than 60 yr with CBD stones should be screened for borderline or overt subclinical hypothyroidism.

Induction of T(4) UDP-GT activity, serum thyroid stimulating hormone, and thyroid follicular cell proliferation in mice treated with microsomal enzyme inducers

The microsomal enzyme inducers phenobarbital (PB), pregnenolone-16 alpha-carbonitrile (PCN), 3-methylcholanthrene (3MC), and Aroclor 1254 (PCB) are known to induce thyroxine (T(4)) glucuronidation and reduce serum T(4) concentrations in rats. Also, microsomal enzyme inducers that increase serum TSH (i.e., PB and PCN) also increase thyroid follicular cell proliferation in rats. Little is known about the effects of these microsomal enzyme inducers on T(4) glucuronidation, serum thyroid hormone concentrations, serum TSH, and thyroid gland growth in mice. Therefore, we tested the hypothesis that microsomal enzyme inducers induce T(4) UDP-GT activity, resulting in reduced serum T(4) concentrations, as well as increased serum TSH and thyroid follicular cell proliferation in mice. B6C3F male mice were fed a control diet or a diet containing PB (600, 1200, 1800, or 2400 ppm), PCN (250, 500, 1000, or 2000 ppm), 3MC (62.5, 125, 250, or 500 ppm), or PCB (10, 30, 100, or 300 ppm) for 21 days. All four inducers increased liver weight and hepatic microsomal UDP-GT activity toward chloramphenicol, alpha-naphthol, and T(4). PB and PCB decreased serum total T(4), but PCN and 3MC did not. Serum thyroid stimulating hormone was markedly increased by PCN and 3MC treatments, and slightly increased by PB and PCB treatments. All four microsomal enzyme inducers dramatically increased thyroid follicular cell proliferation in mice. The findings suggest that PB, PCN, 3MC, and PCB disrupt thyroid hormone homeostasis in mice.

Identification of thyroid hormone transporters in humans: different molecules are involved in a tissue-specific manner

We have recently identified that rat organic anion transporters, polypeptide2 (oatp2) and oatp3, both of which transport thyroid hormones. However, in humans the molecular organization of the organic anion transporters has diverged, and the responsible molecule for thyroid hormone transport has not been clarified, except for human liver-specific transporter (LST-1) identified by us. In this study we isolated and characterized a novel human organic anion transporter, OATP-E from human brain. The isolated complementary DNA encodes a polypeptide of 722 amino acids with 12 transmembrane domains. A rat counterpart, oatp-E, was also identified. Homology analysis and the phylogenetic tree analysis revealed that OATP-E/oatp-E is a subfamily of the organic anion transporter. Human OATP-E transported 3,3',5-triiodo-L-thyronine (K(m), 0.9 microM), thyronine, and rT(3) in a Na(+)-independent manner. Although the clone was isolated from the brain, OATP-E messenger RNA was abundantly expressed in various peripheral tissues. The rat counterpart, oatp-E, also transported 3,3',5-triiodo-L-thyronine. In addition, in this study we revealed that human OATP, which is exclusively expressed in the brain, transported 3,3',5-triiodo-L-thyronine (K(m), 6.5 microM), T(4) (K(m), 8.0 microM), and rT(3). These data suggest that in humans, several different molecules are involved in transporting thyroid hormone: OATP in the brain, LST-1 in the liver, and OATP-E in peripheral tissues.

Effects of microsomal enzyme inducers on thyroid follicular cell proliferation and thyroid hormone metabolism

The effects of microsomal enzyme inducers on thyroid hormone homeostasis and the thyroid gland are of concern. We have investigated the effects of microsomal enzyme inducers on thyroid follicular cell proliferation and thyroid hormone metabolism in rats. We have shown that small increases in serum TSH can result in large increases in thyroid follicular cell proliferation. Furthermore, only those microsomal enzyme inducers that increase serum TSH--that is, phenobarbital (PB) and pregnenolone-16alpha-carbonitrile (PCN)-increase thyroid follicular cell proliferation, whereas those microsomal enzyme inducers that do not increase serum TSH--that is, 3-methylcholanthrene (3MC) and Aroclor 1254 (PCB)-do not increase thyroid follicular cell proliferation. Deiodination does not appear to be the reason why serum T3 concentrations are maintained in microsomal enzyme inducer-treated rats. We have also shown that those microsomal enzyme inducers that increase serum TSH increase T3 UDP-glucuronosyltransferase (UGT) activity, whereas those microsomal enzyme inducers that do not increase serum TSH do not increase T3 UGT activity. This finding suggests that induction of T3 glucuronidation, rather than T4 glucuronidation, mediates increases in serum TSH of microsomal enzyme inducer treated rats.

Effects of microsomal enzyme inducers on outer-ring deiodinase activity toward thyroid hormones in various rat tissues

Microsomal enzyme inducers, such as phenobarbital (PB), pregnenolone-16alpha-carbonitrile (PCN), 3-methylcholanthrene (3MC), and Aroclor 1254 (PCB) are more effective at reducing serum thyroxine (T(4)) than serum triiodothyronine (T(3)). It is possible that rats treated with PB and PCN maintain serum T(3) by increasing serum TSH, which stimulates the thyroid gland to synthesize more T(3). However, it is unclear how serum T(3) is maintained in rats treated with 3MC or PCB, because serum TSH is not increased in these rats. We hypothesized that increased conversion of T(4) to T(3), catalyzed by outer-ring deiodinases (ORD) type-I and -II, is the reason serum T(3) is maintained in rats treated with 3MC or PCB. Furthermore, 3MC and PCB do not increase serum TSH, whereas PB and PCN do, because type-II ORD activity in the pituitary of 3MC- and PCB-treated rats is increased greater than in rats treated with PB or PCN. To test these two hypotheses, male Sprague-Dawley rats were fed either a basal diet or a diet containing PB (300, 600, 1200, or 2400 ppm), PCN (200, 400, 800, or 1600 ppm), 3MC (50, 100, 200, or 400 ppm), or PCB (25, 50, 100, or 200 ppm) for 7 days. Type-I ORD activity was measured in thyroid, kidney, and liver, whereas type-II ORD activity was measured in brown adipose tissue, pituitary, and brain. Type-I ORD activity in thyroid was not affected by PB, 3MC, or PCB treatments, and was slightly increased by PCN. Type-I ORD activity in kidney was not affected by PB, PCN, or 3MC treatments, and was reduced by PCB treatment. Type-I ORD activity in liver was reduced by PB, PCN, 3MC, and PCB treatments. Type-II ORD activity in brown adipose tissue was unaffected by any of the four treatments. Type-II ORD activity in pituitary was unaffected by PB or 3MC treatments, and was increased by PCN or PCB treatments. Type-II ORD activity in brain was unaffected by PB treatment, and was increased by PCN, 3MC, and PCB treatments. Overall, total ORD activity, calculated by summation of ORD activities in thyroid, kidney, liver, brown adipose tissue, pituitary, and brain, was reduced rather than increased by the four microsomal enzyme inducers. In conclusion, increased conversion of T(4) to T(3) is not the reason serum T(3) concentration is maintained in 3MC- or PCB-treated rats. Furthermore, the reason serum TSH is not increased in 3MC- and PCB-treated rats is the result of mechanisms other than increased type-II ORD activity in pituitary.

Effects of thyroid status and thyrostatic drugs on hepatic glucuronidation of lodothyronines and other substrates in rats : Induction of phenol UDP-glucuronyltransferase by methimazole

Glucuronidation of iodothyronines in rat liver is catalyzed by at least three UDP-glucuronyltransferases (UGTs): bilirubin UGT, phenol UGT, and androsterone UGT. Bilirubin and phenol UGT activities are regulated by thyroid hormone, but the effect of thyroid status on hepatic glucuronidation of iodothyronines is unknown. We examined the effects of hypothyroidism induced by treatment of rats with propylthiouracil (PTU) or methimazole (MMI) or by thyroidectomy as well as the effects of T4-induced hyperthyroidism on the hepatic UGT activities for T4, T3, bilirubin,p-nitrophenol (PNP), and androsterone. Bilirubin UGT activity was increased in MMI- or PTU-induced hypothyroid and thyroidectomized rats, and decreased in hyperthyroid animals. T4 and, to a lesser extent, T3 UGT activities were increased in MMI- or PTU-induced hypothyroid rats, and T4 but not T3 glucuronidation also showed a significant increase in thyroidectomized rats. T4 but not T3 UGT activity was slightly decreased in hyperthyroid rats. While PNP UGT activity was decreased in thyroidectomized rats and increased in hyperthyroid animals, it was also markedly increased by MMI and slightly increased by PTU-induced hypothyroidism. In T4-substituted rats, MMI did not affect T4, T3, bilirubin and androsterone UGT activities but again strongly induced PNP UGT activity, indicating that this represented a direct induction of PNP UGT by the drug independent of its thyrostatic action. Androsterone UGT activity was hardly affected by thyroid status. Our results suggest a modest, negative control of the hepatic glucuronidation of thyroid hormone by thyroid status, which may be mediated by changes in bilirubin UGT activity. To our knowledge, this is the first report of the marked induction of a hepatic enzyme by MMI, which is not mediated by its thyroid hormone-lowering effect.

Role of sulfation in thyroid hormone metabolism

The type I iodothyronine deiodinase (ID-I) in liver and kidney converts the prohormone thyroxine (T4) by outer ring deiodination (ORD) to bioactive 3,3',5-triiodothyronine (T3) or by inner ring deiodination (IRD) to inactive 3,3',5-triiodothronine (rT3), while it also catalyzes the IRD of T3 and the ORD of rT3, with the latter as the preferred substrate. Sulfation of the phenolic hydroxyl group blocks the ORD of T4, while it strongly stimulates the IRD of both T4 and T3, indicating that sulfation is an important step in the irreversible inactivation of thyroid hormone. This review summarizes recent studies concerning this interaction between sulfation and deiodination of iodothyronines, the characterization of iodothyronine sulfotransferase activities, the measurement of iodothyronine sulfates in humans and animals, and the possible physiological importance of iodothyronine sulfation.

Sulfate conjugates of iodothyronines in developing sheep: effect of fetal hypothyroidism

We recently showed that thyroxine sulfate (T4S) and 3,3',5-triiodothyronine sulfate (T3S) were major thyroid hormone metabolites in ovine fetuses and neonates. To further characterize the sulfation pathway in ovine fetuses, we measured 3,3',5'-triiodothyronine (rT3S) in serum and other body fluids in samples obtained from fetal (n = 23, 94-145 days of gestational age, term = 150 days), newborn (n = 6), and adult (n = 6) sheep. In addition, T3S, T4S, and rT3S levels were measured in tissue fluids and serum samples obtained from ovine fetuses 13 days after total thyroidectomy (Tx) conducted at gestational age of 110-113 days (n = 5). Sham-operated twin fetuses served as controls (n = 5). The relative order of mean rT3S concentration for various tissue fluids in fetuses were meconium > bile > serum > allantoic fluid > urine or amniotic fluid. Peak mean tissue fluid levels generally occurred at 110-130 days gestation. In hypothyroid fetuses, significant decreases in the mean serum concentrations of T4S and rT3S, but not T3S, were noted. The mean rT3S level also was decreased significantly in allantoic fluid, bile, and meconium, whereas T4S and T3S levels were reduced only in bile of the Tx fetuses. These data demonstrate that sulfation is a major pathway in thyroid hormone metabolism in both euthyroid and hypothyroid ovine fetuses.

Binding and degradation of 3,5,3'-triiodothyronine and thyroxine by rat intestinal bacteria

Intestinal bacteria hydrolyze conjugates of thyroxine (T4) and 3,5,3'-triiodothyronine (T3) secreted in bile, but it is not clear whether they have any other role in metabolism, storage, transport, or action of thyroid hormone in the intestines. We have examined aspects of T3 and T4 binding and degradation processes in fresh feces and cecum contents, obtained from normal control rats and from rats partially decontaminated by treatment with oral antibiotics for 2-3 wk. Samples were homogenized in phosphate buffer, fractionated, and subjected to various test conditions and incubated at 37 degrees C with 125I-labeled T3 (T3*) or T4 (T4*) for 2 or 24 h. Supernatants of high-speed centrifuged incubates were chromatographed to test for degradation products, and percentage binding was measured in the pellets. Substantial binding of T3* and T4* was found in all control rat feces and cecum content samples by 2 h, but binding was absent or significantly reduced in partially decontaminated rat samples. Bacterial binding of T3* and T4* were further shown to be competitive with graded doses of bovine serum albumin. Considerable degradation of T3* and T4* to labeled iodide (I*) only was also observed in feces and cecum content samples and was much greater in control rat than in corresponding partially decontaminated rat samples. Light had no effects in our system and heat reduced I* production. Propylthiouracil and sodium ipodate had little effect or equivocal effects, but dithiothreitol substantially inhibited I* production.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of 3,5,3'-triiodo-L-thyronine and L-thyroxine absorption from the intestinal lumen of the fasted rainbow trout, Oncorhynchus mykiss

The absorptions of 3,5,3'-triiodo-L-thyronine (T3) and L-thyroxine (T4) from the intestinal lumen of the rainbow trout were compared in vivo. Tracer doses of [(125)I]T4 ((+)T4) or [(125)I]T3 ((*)T3) were injected through an anal cannula into the duodenum of trout fasted for 3 days at 12°C, and radioactivity was measured in blood and tissues at 4-48 h. (*)T3 was removed more extensively than (*)T4 from the intestinal lumen and more radioactivity was absorbed into the blood and tissues of u+T3-injected trout than (*)T4-injected trout. HPLC analysis showed that a high proportion of the radioactivity in the plasma, liver, kidney and intestinal lumen of (*)T3-injected trout remained as the parent (*)T3. However, in (*)T4-injected trout most plasma radioactivity was in the form of (125)I(-), and by 24 h a high proportion of luminal radioactivity was (125)I(-). By 48 h, over 4% of the injected (*)T3 and 1% of the injected (*)T4 dose resided in the gall bladder, primarily as derivatives of (*)T3 or (*)T4. We conclude that T3 is absorbed more effectively than T4 from the intestinal lumen of fasted trout, indicating the potential for an enterohepatic T3 cycle.

The role of sulfation in thyroid hormone metabolism

Sulfate conjugation is a significant metabolic reaction for thyroxine and especially so for triiodothyronine and lower iodothyronines in rats. Triiodothyronine sulfation has also been demonstrated in humans. Sulfation accelerates the deiodinative breakdown of iodothyronines by the type I iodothyronine deiodinase in liver and thus represents a rate-limiting step in one of the elimination pathways of thyroid hormone.