Preferential inhibition of thyroxine and 3,5,3'-triiodothyronine formation by propylthiouracil and methylmercaptoimidazole in thyroid peroxidase-catalyzed iodination of thyroglobulin

In vitro assays for characterization of distinct multiple catalytic activities of thyroid peroxidase using LC-MS/MS

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Multiple reactions catalyzed by thyroid peroxidase (TPO) were monitored by a battery of unique in vitro assays.

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Monoiodination and diiodination of L-Tyr to MIT and DIT was examined in a single assay.

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MIT to DIT and T3 to T4 monoiodination reactions were monitored separately.

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DIT to T4 conversion assay was used to study coupling of iodotyrosine phenolic rings.

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Distinct Km, Vmax, Kcat and Kcat/ Km values for each of the TPO catalysed reaction are presented.

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Differential response of 5 known inhibitors with specific TPO reactions was studied.

A diverse set of environmental contaminants have raised a concern about their potential adverse effects on endocrine signaling. Robust and widely accepted battery of in vitro assays is available to assess the disruption of androgenic and estrogenic pathways. However, such definitive systems to investigate effects on the disruption of thyroid pathways by the xenobiotics are not yet well established. One of the major “Molecular Initiating Events” (MIEs) in thyroid disruption involves targeting of thyroid peroxidase (TPO), a key enzyme involved in thyroid hormone synthesis. TPO catalyzes mono- and diiodination of L-Tyrosine (L-Tyr) to generate 3-Iodo-l-tyrosine (MIT) and 3,5-Diiodo-l-tyrosine (DIT), respectively, followed by the coupling of iodinated tyrosine rings to generate thyroid hormones, 3,3’5-Triiodo-l-thyronine (T3) and Levothyroxine (T4).

We sought to develop a robust, sensitive, and rapid in vitro assay systems to evaluate the effects of test chemicals on the multiple catalytic activities of thyroid peroxidase. Simple in vitro assays were designed to study TPO mediated distinct reactions using a single LC-MS/MS method.

Herein, we describe a battery of assays to investigate the iodination of L-Tyr to MIT and DIT, MIT to DIT as well as, T3 to T4 catalyzed by rat thyroid TPO. Importantly, two sequential reactions involving mono- and diiodination of L-Tyr could be analyzed in a single assay. The assay that monitors in vitro conversion of DIT to T4 was developed to study the coupling of tyrosine rings. Enzyme kinetics studies revealed distinct characteristics of multiple reactions catalyzed by TPO. Further, the known TPO inhibitors were used to assess their potency towards individual TPO substrates and reactions. The resultant half maximum inhibitory concentration (IC₅₀) values highlighted differential targeting of TPO catalyzed reactions by the same inhibitor. Overall results underscore the need to develop more nuanced approaches that account for distinct multiple catalytic activities of TPO.

Extrapolating In Vitro Screening Assay Data for Thyroperoxidase Inhibition to Predict Serum Thyroid Hormones in the Rat

Thyroperoxidase (TPO) is an enzyme essential for thyroid hormone (TH) synthesis and a target site for a number of xenobiotics that disrupt TH homeostasis. An in vitro high-throughput screening assay for TPO inhibition, the Amplex UltraRed-TPO (AUR-TPO), has been used to screen the ToxCast chemical libraries for this action. Output from this assay would be most useful if it could be readily translated into an in vivo response, namely a reduction of TH in serum. To this end, the relationship between TPO inhibition in vitro and serum TH decreases was examined in rats exposed to 2 classic TPO inhibitors, propylthiouracil (PTU) and methimazole (MMI). Serum and gland PTU, MMI, and TH levels were quantified using tandem liquid chromatography mass spectrometry. Thyroperoxidase activity was determined in thyroid gland microsomes treated with PTU or MMI in vitro and ex vivo from thyroid gland microsomes prepared from exposed animals. A quantitative model was constructed by contrasting in vitro and ex vivo AUR-TPO results and the in vivo time-course and dose-response analysis. In vitro:ex vivo correlations of AUR-TPO outputs indicated that less than 30% inhibition of TPO in vitro was sufficient to reduce serum T4 by 20%, a degree of regulatory significance. Although further testing of model estimates using other TPO inhibitors is essential for verification of these initial findings, the results of this study provide a means to translate in vitro screening assay results into predictions of in vivo serum T4 changes to inform risk assessment.

Graves' disease: developments in first-line antithyroid drugs in the young

Introduction: First-line treatment for most young people with Graves' disease (GD) will include the administration of a thionamide antithyroid medication (ATD); Carbimazole (CBZ), Methimazole (MMZ), or rarely, propylthiouracil (PTU). GD is a challenge for families and clinicians because the likelihood of remission following a course of ATD is lower in young people when compared to adults, yet the risk of adverse events is higher. An overall consensus regarding the optimal ATD treatment regimen is lacking; how ATD are prescribed, for how long and how the associated risk of adverse events is managed varies between clinicians, units and nations. This partly reflects clinician and family uncertainty regarding outcomes.Areas covered: This review will focus on some of the key articles published in the field of thionamide ATD in children. It will highlight key issues that need to be discussed with families as well as addressing the approach and controversies in the treatment of GD. This article does not reflect a formal systematic review of the literature.Expert opinion: New strategies in areas such as immunomodulation may see the development of new antithyroid drug treatments that, either in isolation or in combination with thionamide therapy, may increase the likelihood of long-term remission.

ANNIVERSARY REVIEW: Antithyroid drug therapy: 70 years later

The thionamide antithyroid drugs were discovered in large part following serendipitous observations by a number of investigators in the 1940s who found that sulfhydryl-containing compounds were goitrogenic in animals. This prompted Prof. Edwin B Astwood to pioneer the use of these compounds to treat hyperthyroidism in the early 1940s and to develop the more potent and less toxic drugs that are used today. Despite their simple molecular structure and ease of use, many uncertainties remain, including their mechanism(s) of action, clinical role, optimal use in pregnancy and the prediction and prevention of rare but potentially life-threatening adverse reactions. In this review, we summarize the history of the development of these drugs and outline their current role in the clinical management of patients with hyperthyroidism.

Specific alteration of gene expression profile in rats by treatment with thyroid toxicants that inhibit thyroid hormone synthesis

Transcriptomics technologies have been used for risk assessment of chemicals, mainly to predict the modes of action (MOAs) of chemicals or identify biomarkers. Transcriptomics data may also be helpful to understand MOAs of chemicals at the molecular level in more detail. As an example of the known MOAs, there are two MOAs of thyroid toxicity: inhibition of thyroid hormone synthesis ("direct" effect) and hypermetabolism of thyroid hormone by enzyme induction in liver ("indirect" effect). In the present study, global profiles of gene expression were analyzed in rats treated with chemicals acting directly on the thyroid (thyroid peroxidase inhibitors such as propylthiouracil and methimazole) and chemicals acting indirectly on the thyroid (hepatic enzyme inducers such as phenobarbital and pregnenolone-16α-carbonitrile) using microarrays. Using a subtraction method between these two types of chemicals, we identified characteristic gene expression changes on the thyroid hormone synthesis pathway by direct-acting chemicals. Based on the functions of these genes, alterations of their expression seem to indicate the results of thyroid peroxidase inhibition, and might be helpful in more accurate evaluation of MOAs for thyroid toxicity.

Insulin Plant (Costus pictus) Extract Restores Thyroid Hormone Levels in Experimental Hypothyroidism

Background:

The aim of the present study was to investigate the preventive effect of Costus pictus leaf extract in experimental hypothyroidism.

Materials and Methods:

Forty male Wistar rats were randomly divided into four groups with ten rats in each group: Control (C), hypothyroid (H), control+extract (C+E), and hypothyroid+extract (H+E). Rats in C group did not receive any intervention throughout the experimental period. The rats in the C+E and H+E groups received pretreatment with C. pictus leaf extract for 4 weeks. Subsequently, for the next 6 weeks, rats in the H group received 0.05% propylthiouracil in drinking water while C+E group received C. pictus leaf extract and H+E group received propyl thiouracil and C. pictus leaf extract.

Results:

Hypothyroid group rats exhibited dramatic increase in thyroid-stimulating hormone (TSH) levels with concomitant depletion in the levels of thyroid hormones. Treatment with the extract resulted in remarkable improvement in thyroid profile. Extract produced 10.59-fold increase in plasma free T3, 8.65-fold increase in free T4, and 3.59-fold decrease in TSH levels in H+E group in comparison with H group. Treatment with the extract ameliorated hypercholesterolemia, decreased levels of plasma C-reactive protein and tumor necrosis factor alpha, suppressed tissue oxidative stress and prevented hepatic and renal damage caused due to thyroid hormone depletion in the H+E group. Pentacyclic triterpenes alpha and beta amyrins were identified and quantified in the extract.

Conclusions:

This is the first study to reveal that C. pictus extract has therapeutic potential to restore thyroid hormone levels and prevent the biochemical complications due to thyroid hormone insufficiency in the animal model of experimental hypothyroidism.

SUMMARY

The preventive effect of Costus pictus leaf extract in experimental hypothyroidism was evaluated in the present study.

Hypothyroidism was induced in the experimental animals by giving 0.05% propylthiouracil in drinking water.

Hypothyroid rats exhibited dramatic increase in thyroid-stimulating hormone (TSH) levels with concomitant depletion in the levels of thyroid hormones.

Treatment with Costus pictus leaf extract in hypothyroid rats significantly improved the thyroid profile. It also ameliorated hypercholesterolemia, decreased the levels of plasma inflammatory markers, suppressed tissue oxidative stress and prevented hepatic and renal damage caused due to thyroid hormone depletion.

The possible active principles alpha and beta amyrins were identified and quantified in the extract through LC-MS.

Abbreviations Used: APCI: Atmospheric pressure chemical ionization; AST: Aspartate aminotransferase; ALT: Alanine aminotransferase; C group: Control group; C+E group: Control+extract group; C. pictus: Costus pictus; CRP: C-reactive protein; DPPH: 2,2-diphenyl-1-picrylhydrazyl; FRAP: Ferric reducing antioxidant power; HDL: High-density lipoprotein; H group: Hypothyroid group; H+E group: Hypothyroid+extract group; LDL: Low-density lipoprotein; LC-MS: Liquid chromatography-mass spectrometry; MDA: Malondialdehyde; PTU: 6-Propyl-2-thiouracil; SRM: Single reaction monitoring; TSH: Thyroid-stimulating hormone; TPTZ: 2,4,6-tri-(2-pyridyl)-5-triazine; TBA: 2-Thiobarbituric acid; TG: Triglyceride; TNFα: Tumor necrosis factor alpha; TAS: Total antioxidant status

Examination of an amphibian metamorphosis assay under an individual-separated exposure system using Silurana tropicalis tadpoles

We examined the validity of an amphibian (Silurana tropicalis) metamorphosis assay (a 28-day semistatic test) under an individual-separated exposure system, where tadpoles were individually held in small glass beakers. We first conducted a comparative rearing experiment for 28 days between this exposure system and the traditional individual-grouped exposure system, both of which held 30 tadpoles (stages 49 and 50) in dechlorinated tap water (a control solution). The former system served to reduce interindividual variability in regard to three morphological measures (developmental stage, hind limb length, and total body length). Under this system, we tested thyroxine (T4, 1μg/L) and propylthiouracil (PTU, 75mg/L) for 28 days of exposure. The morphological data collected at 7-day intervals indicated that significant metamorphic acceleration and retardation were consistently induced in the tadpoles exposed to T4 and PTU, respectively. In addition, the thyroid glands of the tadpoles exposed to T4 and PTU clearly exhibited atrophy and hypertrophy accompanied with severe follicular cell hyperplasia, respectively. Our results are in agreement with the historical data generated from previous studies employing the traditional exposure system, thus indicating the validity of our alternative testing protocol.

The KCNQ1-KCNE2 K⁺ channel is required for adequate thyroid I⁻ uptake

The KCNQ1 α subunit and the KCNE2 β subunit form a potassium channel in thyroid epithelial cells. Genetic disruption of KCNQ1-KCNE2 causes hypothyroidism in mice, resulting in cardiac hypertrophy, dwarfism, alopecia, and prenatal mortality. Here, we investigated the mechanistic requirement for KCNQ1-KCNE2 in thyroid hormone biosynthesis, utilizing whole-animal dynamic positron emission tomography. The KCNQ1-specific antagonist (-)-[3R,4S]-chromanol 293B (C293B) significantly impaired thyroid cell I(-) uptake, which is mediated by the Na(+)/I(-) symporter (NIS), in vivo (dSUV/dt: vehicle, 0.028 ± 0.004 min(-1); 10 mg/kg C293B, 0.009 ± 0.006 min(-1)) and in vitro (EC(50): 99 ± 10 μM C293B). Na(+)-dependent nicotinate uptake by SMCT, however, was unaffected. Kcne2 deletion did not alter the balance of free vs. thyroglobulin-bound I(-) in the thyroid (distinguished using ClO(4)(-), a competitive inhibitor of NIS), indicating that KCNQ1-KCNE2 is not required for Duox/TPO-mediated I(-) organification. However, Kcne2 deletion doubled the rate of free I(-) efflux from the thyroid following ClO(4)(-) injection, a NIS-independent process. Thus, KCNQ1-KCNE2 is necessary for adequate thyroid cell I(-) uptake, the most likely explanation being that it is prerequisite for adequate NIS activity.

Low Thyroid Function Leads to Cardiac Atrophy With Chamber Dilatation, Impaired Myocardial Blood Flow, Loss of Arterioles, and Severe Systolic Dysfunction

Background— Although thyroid dysfunction has been linked to heart failure, it is not clear whether hypothyroidism alone can cause heart failure.

Methods and Results— Hypothyroidism was induced in adult rats by treatment with 0.025% propylthiouracil (PTU) for 6 weeks (PTU-S) and 1 year (PTU-L). Echocardiographic measurements, left ventricular (LV) hemodynamics, isolated myocyte length (KOH method), myocardial blood flow (fluorescent microspheres), arteriolar morphometry, and gene expression (Western blot) were determined. Heart weight, heart rate, LV systolic blood pressure, LV ejection fraction, LV fractional shortening, and systolic wall thickness were reduced in PTU-S and PTU-L rats. LV internal diameter in systole increased by 40% in PTU-S and 86% in PTU-L. LV internal dimension in diastole was increased in PTU-S and PTU-L rats, but only PTU-L rats showed a significant increase in myocyte length due to series sarcomere addition. Resting and maximum (adenosine) myocardial blood flow were reduced in both PTU-S and PTU-L rats. Impaired blood flow was due to a large reduction in arteriolar length density and small arterioles in PTU-S and PTU-L ( P <0.05 or greater for all of the above comparisons). Expression of sarcoplasmic/endoplasmic reticulum Ca 2+ -ATPase (SERCA)-2a and α-myosin heavy chain were reduced in hypothyroidism, whereas phospholamban and β-myosin heavy chain were increased.

Conclusions— Hypothyroidism led to severe, progressive systolic dysfunction and increased chamber diameter/wall thickness ratio despite a reduction in cardiac mass. Chamber dilatation in PTU-L rats was due to series sarcomere addition, typical of heart failure. Hypothyroidism resulted in impaired myocardial blood flow due to a dramatic loss of arterioles. Thus, we have identified 2 important new mechanisms by which low thyroid function may lead to heart failure.

High pre-therapy [99mTc]pertechnetate thyroid uptake, thyroid size and thyrostatic drugs: predictive factors of failure in [131I]iodide therapy in Graves' disease

BACKGROUND AND OBJECTIVE

Several factors may interfere with the success rate of radioiodine therapy (RIT) in Graves' disease. Our aim was to evaluate, retrospectively, some of these factors in the outcome of RIT.

METHODS

Patient gender, age at diagnosis, ophthalmopathy, disease duration, thyroid size, drug used as clinical treatment, thionamide withdrawal period during RIT preparation, FT4, TSH and [99mTc]pertechnetate thyroid uptake prior to RIT were studied as potential interference factors for RIT success. Eighty-two Graves' disease patients were submitted to RIT after thionamide treatment failure. Prior to RIT, 67 patients were receiving methimazole and 15 propylthiouracil. Thirty-three patients received thionamides during RIT; in 49 patients the medication was withdrawn for 2-30 days. [99mTc]pertechnetate thyroid uptake was determined before RIT. Fixed doses of 370 MBq of [131I]iodide were administered to all patients.

RESULTS

Eleven patients became euthyroid; 40 became hypothyroid and 31 remained hyperthyroid. There was no association between outcome and age at diagnosis, gender, ophthalmopathy, pre-RIT FT4, TSH, antithyroid antibodies or thyrostatic drug. Multiple logistic regression showed higher probability of treatment success in patients with thyroid mass <53 g (odds ratio (OR)=8.9), with pre-RIT thyroid uptake <12.5% (OR=4.1) and in patients who withdrew thionamide before RIT (OR=4.9).

CONCLUSIONS

Fixed doses of 370 MBq of radioiodine seem to be practical and effective for treating Graves' disease patients with [99mTc]pertechnetate uptake <12.5% and thyroid mass <53 g. This treatment is clearly not recommended for patients with large goitre. In contrast to what could be expected, patients with a high pre-RIT thyroid uptake presented a higher rate of RIT failure.

Progress towards Development of an Amphibian-Based Thyroid Screening Assay Using Xenopus laevis. Organismal and Thyroidal Responses to the Model Compounds 6-Propylthiouracil, Methimazole, and Thyroxine

In response to the initial Endocrine Disruptor Screening and Testing Advisory Committee (EDSTAC) recommendations, research was conducted on the development of a Xenopus laevis based tail resorption assay for evaluating thyroid axis disruption. This research highlighted key limitations associated with relying on tail resorption as a measure of anti/thyroid activity. The most critical limitation being that tail tissues of tadpoles at metamorphic climax are insensitive to perturbation by thyroid axis agonists/antagonists. To improve upon the initial proposal, we have conducted experiments comparing the sensitivity of pre-metamorphic (stage 51) and pro-metamorphic (stage 54) larvae to the model thyroid axis disruptors methimazole (control, 6.25, 12.5, 25, 50, 100 mg/l), 6-propylthiouracil (PTU) (control, 1.25, 2.5, 5, 10, and 20 mg/l), and thyroxine (T4) (0.25, 0.5, 1, 2, 4 microg/l). Exposures were conducted using two different experimental designs. For experimental design 1, tadpoles were exposed to methimazole or PTU starting at either NF stage 51 or NF 54 for 14 days. For experimental design 2, tadpoles were exposed to PTU or T4 starting at NF stage 51 or NF 54 for 14 and 21 days, respectively. Methimazole and PTU, which are thyroid hormone synthesis inhibitors, both caused a concentration dependent delay in larval development. As determined from this endpoint, there were only minor differences in sensitivity observed among the two stages examined. Further, both compounds caused concentration dependent changes in thyroid gland morphology. These changes were characterized as reduced colloid, glandular hypertrophy, and cellular hyperplasia and hypertrophy. Treatment failed to negatively affect growth, even in tadpoles that experienced significant metamorphic inhibition. T4 treatment resulted in a concentration dependent increase in developmental rate, as would be expected. Similar to studies with methimazole, there were no differences in sensitivity among the two developmental stages examined. These results indicate that tadpoles in the early stages of metamorphosis are sensitive to thyroid axis disruption and that development of a short-term, diagnostic amphibian-based thyroid screening assay shows considerable promise.

Role of the constitutive androstane receptor in xenobiotic-induced thyroid hormone metabolism

The induction of hepatic drug metabolizing enzymes alters not only the metabolism of the xenobiotic substances that induce them but also the metabolism of various endogenous hormones. The xenobiotic receptor constitutive androstane receptor (CAR) (NR1I3) mediates the well-studied induction of CYP2B genes and other drug-metabolizing enzymes by phenobarbital (PB), an antiepileptic drug that has been shown to alter thyroid hormone (TH) levels. Here we show that CAR is required for PB-mediated disruption of TH homeostasis and the induction of thyroid follicular cell proliferation. Treatment with PB or the more potent and more effective CAR ligand 1, 4-bis-[2-(3, 5,-dichloropyridyloxy)] benzene resulted in universal induction of thyroid hormone glucuronidation and sulfation pathways in a CAR-dependent manner. This resulted in a decrease in serum T4 concentration and a concomitant increase in serum TSH levels. CAR activation also decreased serum T3 levels in mice in which T3 production was blocked. The increase in serum TSH levels resulted in the stimulation of thyroid-follicular cell proliferation. These results highlight the central role of the xenosensor CAR in drug-hormone interactions.

Evaluation of mechanisms inducing thyroid toxicity and the ability of the enhanced OECD Test Guideline 407 to detect these changes

The OECD has developed an "enhanced Test Guideline 407" (TG 407) protocol for detecting endocrine effects during the course of a 28-day testing scheme. This protocol has gone through a validation process with (anti)estrogenic and (anti)androgenic compounds and substances that affect the thyroid (thyroxine and propylthiouracil). This review investigates whether a 28-day testing scheme would show up alterations in the thyroid-related parameters of the "enhanced TG 407" (T3, T4, TSH, thyroid weight and histopathology), irrespective of the mode of action. For each mode of action, a generally accepted reference chemical was selected and an in-depth literature survey was carried out, and the chemical was evaluated for treatment-related changes of thyroid-dependent parameters. The following model chemicals were selected: ion perchlorate, blockage of iodine uptake; propylthiouracil, inhibition of thyroid hormone synthesis; excess of iodine, blockage of thyroid hormone release; pyrazole, thyroid cytotoxicity; minocycline, thyroid pigmentation; amiodarone, inhibition of TSH synthesis; diethylstilbestrol, competition for thyroid hormone binding globulin; selenium-deficient diet, inhibition of thyroxine deiodination; FD&C Red No. 3, inhibition of peripheral 5'-deiodinase; cadmium, lipid peroxidation; phenobarbital, increase in thyroxine conjugation and biliary excretion; temelastine, thyroxine accumulation. Test data for treatments lasting approximately one month were available for most of these model chemicals, and these demonstrated the expected thyroid-related changes. Thus, it can be concluded that a 28-day testing scheme allows for the detection of thyroid-disrupting chemicals. The literature data also were evaluated according to whether preference can be given to any of the thyroid-related parameters (thyroid/pituitary hormones, thyroid weight and histopathology) with regard to dose-related sensitivities. Due to different study designs (such as treatment duration, application mode, dose selection and parameters used), no clear picture emerged. Therefore, consideration should be given to all of these parameters, which should also help to define the mode of action. Overall, this literature review provides support for the contention that the newly developed "enhanced TG 407" test protocol is well suited to the detection of chemicals that affect the thyroid gland.

Propylthiouracil reduces the effectiveness of radioiodine treatment in hyperthyroid patients with Graves' disease

In order to assess the effect of propylthiouracil (PTU) or methimazole (MMI) pretreatment on patient outcome after radioiodine therapy, we examined 100 patients with Graves' disease 3, 6, 9, and 12 months after administration of a 10-mCi standard single dose of 131I. They were assigned to one of three groups: no drug (ND) treatment (30 cases); MMI (45 cases); and PTU (25 cases). Antithyroid drugs (ATD) were withdrawn 15 days before radioiodine administration. The groups were similar concerning age, gender, ATD pretreatment duration, goiter size, and initial serum triiodothyronine (T3), thyroxine (T4), free thyroxine (FT4), antithyroid autoantibody levels, 24-hour radioiodine uptake and 131I dose administered per gram of thyroid tissue. ND and MMI groups presented a similar rate of cure of 73.3% and 77.8% respectively (p = NS). In contrast, the PTU group showed a rate of cure of only 32% (p < 0.05). Logistic regression analysis indicated that PTU administration (p = 0.003) and thyroid size (p = 0.02) were the variables related to radioiodine therapy failure. Our data demonstrate that the chance of 131I treatment failure is higher in individuals using PTU than in patients using MMI or not using any ATD before radioiodine (odds ratio [OR] 5.84; 95% confidence interval [CI] 1.82-18.76) suggesting that PTU should be avoided in the treatment of patients with Graves' disease.

Development of cholinergic neurons in rat brain regions: dose-dependent effects of propylthiouracil-induced hypothyroidism

The effects of hypothyroidism on development of cholinergic system in brain regions (prefrontal cortex and hippocampus) were evaluated by measuring choline acetyltransferase (ChAT) activity and hemicholinium-3 binding to the high-affinity choline transporter. Various degrees of thyroid deficiency were produced by perinatal exposure to propylthiouracil (PTU) in drinking water ranging from 5 ppm (mg/l) to 25 ppm beginning at gestational day 18 until postnatal day 21. ChAT, a marker for cholinergic nerve terminals, was reduced by PTU in a dose-dependent manner. Concomitant with the enzyme deficits, hemicholinium-3 binding was elevated, suggesting an increase in neuronal impulse activity. Although similar changes were seen in both brain regions examined, the magnitude and duration of these changes were more definitive in the prefrontal cortex. Nonetheless, these neurochemical alterations appeared to be recoverable when the rats returned to a euthyroid state, and no further changes were observed as the animals reached adulthood. In comparison, data reported in a succeeding article indicate that deficits in cognitive function were first seen in weanling hypothyroid rats, but that the behavioral impairments lasted well into adulthood when thyroid status and cholinergic parameters in the brain appeared to have recovered to normal. These results suggest that alterations of cholinergic system caused by perinatal hypothyroidism are associated with neurobehavioral deficits at weaning, and these developmental deviations may cause permanent impairment of cognitive function despite recovery from the hormonal imbalance at adult ages.

Induction of metamorphosis in the sand dollar Peronella japonica by thyroid hormones

The larva of the sand dollar Peronella japonica lacks a mouth and gut, and undergoes metamorphosis into a juvenile sand dollar without feeding. In the present study, it was found that thyroid hormones accelerate the metamorphosis of P. japonica larvae. The contents of thyroid hormones in larvae increased gradually during development. Thiourea and potassium perchlorate, inhibitors of thyroid hormone synthesis, delayed larval metamorphosis and simultaneously repressed an increase in the content of thyroxine in the larval body. These results suggest that the P. japonica larva has a system for synthesis of thyroid hormones that act as factors for inducing metamorphosis.

Antithyroid drugs for the treatment of hyperthyroidism caused by Graves' disease

Therapy for Graves' disease is not straightforward and often involves complex decision making. Long-term antithyroid drug therapy is appealing because it is nonablative, but it is not for everyone. The physician must weigh the advantages and disadvantages of antithyroid drug treatment and help the patient arrive at an individualized therapeutic strategy that is appropriate and cost-effective.

Pituitary and thyroid gland lesions induced by 2-mercaptobenzimidazole (2-MBI) inhalation in male Fischer-344 rats

2-Mercaptobenzimidazole (2-MBI) is a member of the thioureylene compound family known for their potent antithyroid activity. Male Fischer-344 rats were exposed to 0.0, 3.13, 12.5, and 50.0 mg/m3 of 2-MBI via inhalation for 13 wk. Follicular cell hyperplasia occurred in the thyroid glands of rats from the 3 2-MBI treatment groups. Thyrotrophs in the pituitary glands from rats in these 3 groups were hyperplastic, and they had varying numbers of hypertrophic cells with either eosinophilic stippled cytoplasm or with eosinophilic globules within 1 or more large vacuoles that displaced the nucleus. These cells were compared by immunohistochemistry and ultrastructural examination to "thyroidectomy cells" within the anterior pituitary glands of thyroparathyroidectomized rats and were determined to be identical to them. Immunohistochemical staining for the beta-chain of thyroid-stimulating hormone confirmed that the hyperplastic and hypertrophic cells were thyrotrophs. Ultrastructurally, hypertrophic cells in 2-MBI-treated rats and thyroidectomy cells in thyroparathyroidectomized rats had expanded cytoplasm containing either increased profiles of rough endoplasmic reticulum (RER) or one or more widely dilated cisternae of RER, which displaced other cellular organelles. 2-MBI appears to act comparably to other thioureylene compounds that have been shown to produce low serum concentrations of triiodothyronine and thyroxine, which results in thyrotroph hypertrophy and hyperplasia, resultant thyroid hyperplasia, and subsequent goiter.

Biological evaluation of compounds with -NCS- group or derived from thiazole and imidazole. Activity on prostaglandin synthetase complex

The effects of compounds with activity against thyroid peroxidase were tested on the activity of hydroperoxidase and cyclo-oxygenase of the prostaglandin synthetase complex in-vitro. Active compounds were found to inhibit the peroxidase, and the cyclo-oxygenase function. These compounds were also found to have anti-inflammatory activity as demonstrated by the reduction of carrageenan-induced oedema of the hind paw of the rat. Indomethacin and non-steroidal anti-inflammatory drugs tested under the same conditions were shown to have activity towards the cyclo-oxygenase rather than the peroxidase function of the prostaglandin synthetase complex. A common feature of the active compounds was the presence of an -NCS- linkage or free -SH group.

Localization of gastric peroxidase and its inhibition by mercaptomethylimidazole, an inducer of gastric acid secretion

Mercaptomethylimidazole (MMI) is a potent inducer of gastric acid secretion which is associated with significant inhibition of peroxidase activity of rat gastric mucosa in vivo. A time-dependent increase in acid secretion correlates well with time-dependent decrease in the peroxidase activity. In a chamber experiment in vitro using isolated gastric mucosa, MMI stimulates acid secretion, showing an almost linear response up to 600 microM. The time-dependent increase in acid secretion is also correlated with time-dependent inhibition of the peroxidase activity. This effect is not mediated through oxidation of MMI by flavin-containing mono-oxygenase, which is absent from gastric mucosa. The peroxidase has been localized mainly in parietal cells isolated and purified from gastric mucosa by controlled digestion with collagenase followed by Percoll-density-gradient centrifugation. Peroxidase activity was further localized in the outer membrane of the purified mitochondria of the parietal cell by some membrane-impermeant reagents, indicating outward orientation of the enzyme. MMI can inhibit the peroxidase activity of both the parietal cell and its mitochondria in a concentration-dependent manner. The possible involvement of the parietal-cell peroxidase-H2O2 system in MMI-induced acid secretion may be suggested.

Antithyroid drug effects on function and growth of FRTL-5 cells

Although antithyroid drugs (ATDs) are known to exert their effects by inhibiting iodide organification within the thyroid follicular cell, a full understanding of their mechanisms of action is lacking. In this study the effects of methimazole (MMI) and propylthiouracil (PTU) on thyrotropin (TSH) and thyroid-stimulating immunoglobin (TSI)-stimulated cAMP production and growth in FRTL-5 cells was investigated. MMI, but not PTU, inhibited TSH-stimulated cAMP production, but only at the very highest concentration (10(-3) M): 0.3 +/- 0.01 vs 0.79 +/- 0.13 pmol/micrograms protein (p < 0.01). Neither MMI nor PTU inhibited TSI-stimulated cAMP production at any dose. Neither MMI nor PTU exhibited an inhibitory effect on TSH- or TSI-stimulated cell growth, as measured by [3H]-thymidine incorporation. These observations suggest that high concentrations of MMI may act to control thyroid function by inhibiting receptor-mediated cAMP production. Although decreases in thyroid gland size frequently occur during ATD therapy, neither MMI nor PTU exhibited any effect on TSH- or TSI-stimulated thyroid cell growth.

Antithyroid effects of propylthiouracil and sulfamonomethoxine in rats and monkeys

Experiments were performed to investigate the species difference in antithyroid effects of propylthiouracil (PTU) and sulfamonomethoxine (SMM). Sprague-Dawley rats were administered 30 mg/kg of PTU, or 30 or 270 mg/kg of SMM orally for 5 weeks, while squirrel monkeys received 30 mg/kg of PTU or 270 mg/kg of SMM. In rats receiving 30 mg/kg of PTU, a decrease of both serum 3,5,3'-triiodothyronine and thyroxine concentrations and of 131I incorporation into the thyroid hormone precursors was observed, together with elevation of serum thyroid-stimulating hormone concentration, an increase in thyroid weight, and hyperplasia of the follicular epithelium of the thyroid gland. Similar changes were seen in rats receiving 270 mg/kg of SMM; however, the antithyroid effects of SMM were less severe than those of PTU. No change was produced in monkey thyroids by 5-week treatments with 30 mg/kg of PTU or with 270 mg/kg of SMM. The molar concentration of PTU required for the in vitro inhibition of thyroid peroxidase was markedly lower in rats than in monkeys. SMM showed a similar species difference in the inhibition of thyroid peroxidase in vitro, but the enzyme inhibition of SMM was weaker than that of PTU. These findings suggest that rats were more sensitive to the antithyroid effects of PTU and SMM than monkeys, and that inhibition of thyroid peroxidase may play an important role in species difference in the antithyroid effects of the two drugs.

Antithyroid drugs

Over the past four decades, a great deal has been learned about the pharmacology and mechanisms of action of antithyroid drugs. Their ability to inhibit hormone biosynthesis involves complex interactions with thyroid peroxidase and thyroglobulin, many of which are still poorly understood. Their spectrum of activity is much wider than previously thought, and a number of clinically important extrathyroidal actions have been identified. Despite a greater appreciation for the intricacies of antithyroid-drug pharmacology, controversies still surround the use of these agents in the treatment of thyrotoxicosis. These controversies are apt to continue until the pathophysiology of Graves' disease is fully elucidated.