Inhibition of peroxidase and catalase activities and modulation of hydrogen peroxide level by inositol phosphoglycan-like compounds

Inositol phosphoglycan-like compounds are produced by the hydrolysis of the membrane bound glycosyl phosphoinositides. Besides being short term mediators of insulin action, they inhibit peroxidases and catalase, increasing the concentration of cellular hydrogen peroxide. Although high concentrations of hydrogen peroxide are toxic, moderate increases of its basal level are signals for different metabolic pathways. The inhibitor, localized in the cytosol of the cell, acts on peroxidases and catalase of the same tissue (homologous action) and of other tissues or organisms (heterologous action). The inositol phosphoglycan-like compound inhibits peroxidases with different prosthetic groups, i.e. containing iron such as: thyroid peroxidase, lactoperoxidase, horseradish peroxidase, soy bean peroxidase; and containing selenium such as glutathione peroxidase and 2-cys peroxiredoxin with no prosthetic group. Besides peroxidases, the inositol phosphoglycan-like compound inhibits catalase, another heme enzyme. The inhibition kinetics demonstrates a noncompetitive effect. The site of action is not the prosthetic group, given that the inhibitor does not produce any effect on the peak in the Soret region in the presence or absence of hydrogen peroxide. In conclusion, the inositol phosphoglycan-like compound is the general inhibitor of peroxidases and catalase involved in the modulation of hydrogen peroxide level that acts in different metabolic pathways as a signal transducer.

Inhibition of Thyroid Peroxidase by Myrcia uniflora Flavonoids

Thyroid peroxidase (TPO), the key enzyme in thyroid hormone biosynthesis, is inhibited by dietary flavonoids; thus, a high consumption of plants containing inhibitory flavonoids may affect thyroid function and lead to hypothyroidism. In this work, TPO inhibition by the aqueous partition of Myrcia uniflora and its isolated compounds has been evaluated. The aqueous partition of the methanolic extract of M. uniflora is able to inhibit TPO activity in vitro. Two known flavonoids were isolated and characterized by mass spectrometry and (1)H NMR from plant extracts: mearnsitrin and myricitrin. The degree of TPO inhibition produced by the aqueous solution of the flavonoids was very high, with a 50% inhibition of the original TPO activity (IC(50)) obtained at 1.97 microM mearnsitrin and at 2.88 microM myricitrin. These results suggest that the indiscriminated consumption of M. uniflora pharmaceutical products allied to the nutritional deficiency of iodine might contribute to the development of hypothyroidism and goiter.

Methylmercury inhibits type II 5'-deiodinase activity in NB41A3 neuroblastoma cells

Methylmercury (MeHg) is a well-known neurotoxicant and prenatal exposure to MeHg results in severe brain damage. Since MeHg has a high affinity for thiol groups, we sought to determine whether MeHg inhibited type II iodothyronine deiodinase (D2) activity, by which prohormone thyroxine (T4) is converted to active thyroid hormone, 3,5,3'-triiodothyronine (T3) in the brain, using NB41A3 mouse neuroblastoma cells. In MeHg-treated cells, D2 activity was inhibited in a dose- and time-dependent manner; relatively low concentrations of MeHg (30 nM) inhibited D2. Kinetic analysis using a double reciplocal plot of D2 activity revealed competitive inhibition by MeHg. DTT protected D2 from MeHg when cells were incubated with both MeHg and DTT or when MeHg was added to the assay buffer containing DTT and cell sonicates from untreated cells. Removal of MeHg from culture medium did not recover D2 activity. These results demonstrate that MeHg inhibited D2 activity in NB41A3 cells and the selenocysteine in the catalytic subunit of D2 may be involved in the inhibitory action of MeHg. Further our results suggest that T3 deficiency due to D2 inhibition in the brain may be involved in the neurotoxicity of MeHg.

Characterization of an iodothyronine 5'-deiodinase in gilthead seabream (Sparus auratus) that is inhibited by dithiothreitol

Iodothyronine deiodinases catalyze the conversion of the thyroid prohormone T(4) to T(3) by outer ring deiodination (ORD) of the iodothyronine molecule. The catalytic cycle of deiodinases is considered to be critically dependent on a reducing thiol cosubstrate that regenerates the selenoenzyme to its native state. The endogenous cosubstrate has still not been firmly identified; in studies in vitro the sulfhydryl reagent dithiothreitol (DTT) is commonly used to activate ORD. We now have characterized an ORD activity in the teleost gilthead seabream (Sparus auratus) that is inhibited by DTT. DTT inhibited reverse T(3) (rT(3)) ORD by 70 and 100% in kidney homogenates (IC(50) 0.4 mmol/liter) and microsomes (IC(50) 0.1 mmol/liter), respectively. The omission of DTT from the incubation medium restored renal ORD Michaelis-Menten kinetics with a Michaelis constant value of 5 mumol/liter rT(3) and unmasked the inhibition by 6-n-propyl-2-thiouracil. A putative seabream deiodinase type 1 (saD1), derived from kidney mRNA, showed high homology (> or = 41% amino acid identity) with vertebrate deiodinases type 1. Features of this putative saD1 include a selenocysteine encoded by an in-frame UGA codon, consensus sequences, and a predicted secondary structure for a selenocysteine insertion sequence and an amino acid composition of the catalytic center that is identical with reported consensus sequences for deiodinase type 1. Remarkably, three of six cysteines that are present in the deduced saD1 protein occur in the predicted amino terminal hydrophobic region. We suggest that the effects of DTT on rT(3) ORD can be explained by interactions with the cysteines unique to the putative saD1 protein.

Studies on the goitrogenic mechanism of action of N,N,N',N'-tetramethylthiourea

N,N,N',N'-Tetramethylthiourea (TMTU) is a rat goitrogen inducing thyroid hyperplasia, hypertrophy, and tumor formation. Little is known about the exact underlying mechanism of action. As thyroid peroxidase (TPO) and type I iodothyronine deiodinase (ID-I) have been established as targets of goitrogenic thiourea derivatives, we investigated interactions of TMTU with target enzymes using a partially purified fraction from hog thyroids or solubilized hog thyroid microsomes and 10,000g supernatant from rat liver homogenate, respectively, as enzyme sources. For comparison, comprehensively characterized goitrogenic thiourea derivatives were studied as well. In contrast to propylthiouracil (PTU), and like ethylenethiourea (ETU), TMTU only marginally affected TPO-catalyzed oxidation of guaiacol. TMTU, like ETU, concentration-dependently suppressed TPO-catalyzed iodine formation with concomitant oxidative metabolism. Suppression ceased upon consumption of thiourea derivatives, the rate of the reappearing iodine formation was similar to that of controls. TMTU, like ETU, also suppressed non-enzymatic and TPO-catalyzed monoiodination of l-tyrosine with a stoichiometry of 2:1, i.e., one molecule of thiourea derivative suppressed two times monoiodination. TMTU and ETU were unable to irreversibly inhibit TPO. In contrast to PTU, TMTU did not inhibit ID-I. These findings provide evidence that TMTU interferes with thyroid hormone synthesis at the level of iodination and demonstrate a metabolic route for the oxidative detoxification of TMTU in the thyroid suggesting that low-level or intermittent exposure to TMTU would have only minimal effects on thyroid hormone synthesis. Finally, it can be concluded that meaningful toxicological studies on TPO inhibition can be performed without a need for highly purified TPO.

Suppression of hypothalamic deiodinase type II activity blunts TRH mRNA decline during fasting

Fasting is characterized by disrupted thyroid feedback, with suppressed levels of thyroid hormones and paraventricular thyrotropin releasing hormone (TRH). We found that third ventricle administration of the deiodinase inhibitor, iopanoic acid, dose-dependently reduced deiodinase type II (DII) activity selectively in the hypothalamus. This suppression of DII by iopanoic acid during fasting prevented elevated DII activity and blunted the decline in hypothalamic TRH mRNA levels. Because fasting-induced elevation in hypothalamic DII activity is paralleled by increased hypothalamic T3 concentration, our study suggests that T3 formation by DII in the hypothalamus is the cause of disrupted thyroid feedback during fasting.

Inhibition of pituitary type 2 deiodinase by reverse triiodothyronine does not alter thyroxine-induced inhibition of thyrotropin secretion in hypothyroid rats

OBJECTIVES

Intrapituitary triiodothyronine (T3) production plays a pivotal role in the control of TSH secretion. Its production is increased in the presence of decreased serum thyroxine (T4) concentrations and the enzyme responsible, deiodinase type 2 (D2), is highest in hypothyroidism. In order to document the role of this enzyme in adult rats we developed an experimental model that inhibited this enzyme using the specific inhibitor, reverse T3 (rT3).

METHODS

Hypothyroidism was induced with propylthiouracil (PTU; 0.025 g/l in drinking water) which in addition blocked deiodinase type 1 (D1) activity, responsible for the rapid clearance of rT3 in vivo. During the last 7 days of the experiment, the hypothyroid rats were injected (s.c.) for 4 days with 0.4 or 0.8 nmol T4 per 100 g body weight (bw) per day. For the last 3 days, the same amount of T4 was infused via s.c. minipumps. In additional groups, 25 nmol rT3/100 g bw per day were added to the 3-day infusion of T4.

RESULTS

Infusion of 0.4 nmol T4/100 g bw per day did not affect the high serum TSH levels, 0.8 nmol T4/100 g bw per day decreased them to 57% of the hypothyroid values. The infusions of rT3 inhibited D2 activity in all organs where it was measured: the pituitary, brain cortex and brown adipose tissue (BAT). In the pituitary, the activity was 27%, to less than 15% of the activity in hypothyroidism. Despite that, serum TSH levels did not increase, serum T4 concentrations did not change and the changes in serum T3 were minimal.

CONCLUSIONS

We conclude that in partly hypothyroid rats, a 3-day inhibition of D2 activity, without concomitant change in serum T4 and minimal changes in serum T3 levels, is not able to upregulate TSH secretion and we postulate that this may be a reflection of absent or only minimal changes in circulating T3 concentrations.

L’iode 131 comme traitement des goitres bénins

INTRODUCTION

In order to evaluate the efficacy of 131 Iodine on goitre volume and on thyroid function, we studied a cohort of patients exhibiting a multinodular and toxic or non toxic goitre.

METHODS

This retrospective study was conducted at the Marc Linquette clinic in Lille, in collaboration with the department of nuclear medicine. Thirty-eight patients treated with 131 Iodine were included from 1995 to 2001. Clinical examination and serum analyses including TSH, free T4 and T3, anti-thyroid peroxidase and anti-thyroglobulin antibodies and TSH-receptor antibodies measurements were conducted on inclusion and then at 3, 6, 12 and 72 months. The activity of 131 Iodine corresponded to a standard dose or was calculated according to Marinelli's method. We excluded patients who had not undergone assessment at the above-mentioned time schedules.

RESULTS

The treatment was indicated in 30 patients presenting with a non compressive but toxic goitre, in 5 patients with a toxic compressive goitre and in 3 patients with a compressive but non-toxic goitre. Surgery had been excluded for all these patients because of their age, their cardiac status or because they had refused surgery after failure with prior partial thyroidectomy or medical treatment. Among the toxic goitres, TSH levels were low and T3 and T4 increased in 17 patients. In the 18 others, hyperthyroidism was manifested by an isolated decrease of TSH. The thyroid volume before treatment, assessed in 20 patients, was of 18 to 135 cm3 (mean: 53 cm3). Treatment consisted in administration of radioactivity of 3 to 30 mCi in 30 patients and standard activity of 20 to 25 mCi in 8. Functional efficacy with reduction in hyperthyroidism was noted after 3 months, and corrected in nearly all patients after 1 year, and morphological efficacy, with a mean decrease of 33.5% in the size of the goitres. No supplementary surgery was required, notably for the initially compressed goitres. Immediate and long term tolerance was satisfactory.

CONCLUSION

Metabolic 131Iodine radiotherapy is effective for the functional and morphological treatment of goitres with good tolerance and few side effects. 131 Iodine is a reasonable alternative in cases with absolute or relative contraindication for surgery.

Multi-organic endocrine disrupting activity of the UV screen benzophenone 2 (BP2) in ovariectomized adult rats after 5 days treatment

The chemical industry has developed sun protection factor products, which contain a variety of so-called "UV screens", among others, benzophenones (BP). Based on the structure it can be assumed, that the variant BP2 may be a potent estrogenic endocrine disrupter (ED). Only very limited data are available in the literature about such action of BP2, which focussed on the uterotrophic effect in immature rats. However, determination of ED activity in the uterus is only a restricted approach with the potential risk of missing undesirable actions. Therefore, we examined a putative multiple organ ED activity of BP2 by measuring gene expression of marker genes in the uterus, liver, vagina and pituitary after 5 days oral application in adult ovariectomized (ovx) rats. An effect on lipid metabolism was assessed by determination of cholesterol, high- and low-density lipoproteins (HDL and LDL) in the blood. As control compound, estradiol (E2) was included in the study. A dose dependent E2-agonistic activity was observed in the uterus (increased weight), vagina (increased IGF1 expression), pituitary (reduced LH synthesis), liver (increased IGF1 expression) and lipid parameters (reduction). A non-E2-like action of BP2 was observed on T4- and T3-levels, which were significantly reduced. Except for the action of BP2 on thyroid hormone levels where it may inhibit thyroid peroxidase, this UV screen exerts clear E2-agonistic actions. Application of BP2 for 5 days proved to be a sufficient treatment period to unravel a multi-organic endocrine disrupting activity of this UV screen.

Effect of bamboo shoot, Bambusa arundinacea (Retz.) Willd. on thyroid status under conditions of varying iodine intake in rats

Young shoots or sprouts of common bamboos are used as food in third world countries. Evidences suggest the presence of cyanogenic glucoside like anti-thyroidal substance in bamboo shoots (BS) but effect of prolonged BS consumption on thyroid status under conditions of varying iodine nutriture remains unexplored. The study was undertaken to evaluate goitrogenic content, in vitro anti thyroid peroxidase (TPO) activity and in vivo anti thyroid potential of BS with and without extra iodide. Fresh BS contains high cyanogenic glucoside (551 mg/kg), followed by thiocyanate (24mg/kg) and glucosinolate (9.57mg/kg). In vitro inhibition in TPO activity was found with raw, raw boiled and cooked extracts. Inhibition constant (IC50) and PTU equivalence of fresh BS were 27.5+/-0.77 microg and 3.27 respectively. Extra iodide in the incubation media reduced TPO inhibition induced by BS but could not cancel it. Thyroid weight, TPO activity and total serum thyroid hormone levels of BS fed animals for 45 and 90 days respectively were determined and compared with controls. Significant increase in thyroid weight as well as higher excretion of thiocyanate and iodine along with marked decrease in thyroid peroxidase activity, T4 and T3 levels were observed in BS fed group. Chronic BS consumption gradually developed a state of hypothyroidism. Extra iodide had reduced the anti-thyroidal effect of BS to an extent but could not cancel it because of excessive cyanogenic glucoside, glucosinolate and thiocyanate present in it.

Thyroid hormone-dependent seasonality in American tree sparrows (Spizella arborea): effects of GC-1, a thyroid receptor beta-selective agonist, and of iopanoic acid, a deiodinase inhibitor

To explore the role of TH in the control of seasonality [i.e., photoperiodic testicular growth, photorefractoriness, and postnuptial (prebasic) molt] in American tree sparrows (Spizella arborea), we performed experiments in which THX males were simultaneously photostimulated and given TH replacement therapy. In the first experiment, equimolar concentrations (1X = 1.3 nmol) of T4, T3, or GC-1, an iodine-free TRbeta agonist, were administered s.c. daily during the first 21 days of photostimulation. Two additional THX groups received GC-1 at 0.1X or 10X, and THX and THI control groups received vehicle. In the second experiment, T4 or T3, alone or in combination with the deiodinase inhibitor IOP, was injected i.m. twice daily during the first 14 days of photostimulation. In both experiments, end points were testis length and molt score. In the first experiment, THI birds given vehicle and THX birds given T4 replacement therapy exhibited all three components of seasonality. THX birds given T3 or GC-1 (1X or 10X) showed a subdued photoperiodic testicular response, but they did not become photorefractory or initiate molt. THX birds that received 0.1X GC-1 or vehicle exhibited none of the components of seasonality. These data are consistent with the hypothesis that photoperiodic testicular growth, a vernal component of seasonality, is a TRbeta-mediated response and suggest that T4 may activate TRbeta more efficiently than does T3 or GC-1. By contrast, the failure both of T3 and of GC-1, but not of T4, to program photostimulated THX males for photorefractoriness and postnuptial molt suggests that autumnal components of seasonality may be TRalpha-mediated responses solely to T4. In the second experiment, IOP administered alone had no significant impact on seasonality. THX birds that received T4 with or without IOP showed all components of seasonality, whereas birds that received T3 with or without IOP showed only photoperiodic testicular growth. These results challenge the widely held view that T4 is merely a prohormone for T3 and support the emerging view that T4 has intrinsic hormonal activity. Because IOP augmented the photoperiodic testicular response in T3-treated THX birds, T3 may act either independently or co-dependently with T4 in programming vernal seasonal events.

Stress Is Associated with Inhibition of Type I Iodothyronine 5′-Deiodinase Activity in Rat Liver

Type I iodothyronine deiodinase (5'-DI) generates the thyromimetically active hormone 3,5,3'-triiodothyronine (T(3)) by reductive monodeiodination of the phenolic ring of L-thyroxine (T(4)). The present study was undertaken to compare the effects of cold stress (4 degrees C) alone or in combination with immobilization stress (IMO), insulin treatment (5 IU/kg, i.p.) or 2-deoxy-D-glucose (2DG)-induced intracellular glucopenia on the activity of 5'-DI in rat liver. Cold stress either for 24 h or 28 days when compared to that in the nonstressed group of rats significantly reduced (P < 0.001) the activity of 5'-DI in liver. In comparison with cold stressed rats for 28 days, an additional decrease in 5'-DI activity was observed when those rats underwent 1 x IMO in cold for 2 h (P < 0.001) or insulin treatment (P < 0.05). A significant decrease in 5'-DI activity was found also in rats maintained at room temperature that underwent a single 1 x IMO for 2 h (P < 0.001) or insulin treatment (P < 0.01) when compared to nonstressed animals. In comparison with nonstressed rats, no significant change of the 5'-DI activity was observed after 2DG application (500 mg/kg, i.p.) at room temperature. In conclusion, cold stress and/or 1 x IMO, insulin treatment, or 1 x IMO at room temperature markedly affects reductive monodeiodination of T(4), and thus reduces the concentration of biologically active T(3) in liver.

Photoperiodic regulation of type 2 deiodinase gene in Djungarian hamster: possible homologies between avian and mammalian photoperiodic regulation of reproduction

The molecular mechanisms responsible for seasonal time measurement have yet to be fully described. Recently, we used differential analysis to identify that the type 2 iodothyronine deiodinase (Dio2) gene is responsible for the photoperiodic response of gonads in Japanese quail. It was found that expression of Dio2 in the mediobasal hypothalamus is induced by light and that T(3) content in the mediobasal hypothalamus increased under long day conditions. In addition, we showed that intracerebroventricular infusion of T(3) mimics photoperiodically induced testicular growth. Because it is well known that thyroid hormone is also essential for the maintenance of the seasonal reproductive changes in a number of mammals, we examined expression of Dio2 in Djungarian hamsters and found expression in the ependymal cell layer lining the infralateral walls of the third ventricle and the cell-clear zone overlying the tuberoinfundibular sulcus. Signal intensity was high under long days and weak under short days. Although light pulse did not affect Dio2 expression, melatonin injections decreased Dio2 expression under long days. These results indicate that Dio2 may be involved in the regulation of seasonal reproduction in mammals in the same way as observed in birds.

Biomimetic Studies on Anti-Thyroid Drugs and Thyroid Hormone Synthesis

The selenium analogues of anti-thyroid drugs exhibit their anti-thyroid action by a mechanism different from that of MMI. The selenium analogue of MMI and related selenium compounds exhibit high GPx activity, providing a novel method for the reversible inhibition of thyroid hormone biosynthesis.

Expression of type II iodothyronine deiodinase marks the time that a tissue responds to thyroid hormone-induced metamorphosis in Xenopus laevis

The thyroid gland synthesizes thyroxine (T4), which passes through the larval tadpole's circulatory system. The enzyme type II iodothyronine deiodinase (D2) converts thyroxine (T4) to the active hormone 3,5,3'-triiodothyronine (T3) in peripheral tissues. An early response to thyroid hormone (TH) in the Xenopus laevis tadpole is the stimulation of cell division in cells that line the brain ventricles, the lumen of the spinal cord, and the limb buds. These cells express constitutively high levels of D2 mRNA. Exogenous T4 induces early DNA synthesis in brain, spinal cord, and limb buds as efficiently as T3. The deiodinase inhibitor iopanoic acid blocks T4- but not T3-induced cell division. At metamorphic climax, both TH-induced cell division and D2 expression decrease in the brain. Then D2 expression appears in late-responding tissues including the anterior pituitary, the intestine, and the tail where cell division is reduced or absent. Therefore, constitutive expression of D2 occurs in the earliest target tissues of TH that will grow and differentiate, while TH-induced expression of D2 takes place in late-responding tissues that will remodel or die. This pattern of constitutive and induced D2 expression contributes to the timing of metamorphic changes in these tissues.

Selenenyl iodide: a new substrate for mammalian thioredoxin reductase

Areneselenenyl iodide stabilised by internal chelation has been synthesized and evaluated as a substrate of thioredoxin reductase (TrxR). The reactivity of TrxR obtained from human placenta towards selenenyl iodide was found to be much higher than that of the E. coli enzyme, indicating the essential nature of a selenocysteine residue in the active site of the human enzyme. The addition of thioredoxin (Trx) significantly enhanced the TrxR-catalysed reduction of selenenyl iodide 1. These studies on the reduction of a selenenyl iodide by the thioredoxin system suggest that stable selenenyl iodides could be new substrates for human TrxR. The Trx system could act as a cofactor for iodothyronine deiodinase by reducing the selenenyl iodide intermediate in the second-half of the deiodinase catalytic cycle to regenerate the active site. The TrxR-catalysed reduction of 1 was not inhibited by the anti-thyroid drug, PTU, suggesting that the involvement of the Trx system in the deiodinase cycle may be responsible for the insensitivity of certain deiodinases towards clinically useful thiourea drugs.

Serotonin effect on deiodinating activity in the rat

Serotonin (5-HT) and thyroid hormones are part of a complex system modulating eating behaviour and energy expenditure. 5-Deiodinase (5-D) converts the relatively inactive thyroxine (T4) to triiodothyronine (T3), and its activity is an indirect measure of T3 production in peripheral tissues, particularly in the brain, intrascapular brown adipose tissue (IBAT), heart, liver, and kidney. We evaluated the effect of 5-HT on 5'-D activity during basal conditions and after short (30 min) cold exposure (thyroid stimulating hormone stimulation test, TST). 5'-D activity was assessed in the liver, heart, brain, kidney, and IBAT. TST increases 5'-D activity in the brain, heart, and IBAT and decreases it in kidney, leaving it unchanged in the liver. 5-HT alone did not modify 5'-D activity in the organs under study but decreased it in the IBAT, heart, and brain when injected before the TST was administered. Our results confirm the important role of 5-HT in thermoregulation, given its peripheral site of action, in modulating heat production controlling intracellular T3 production. These effects are more evident when heat production is upregulated during cold exposure in organs containing type II 5'-D, such as the brain, heart, and IBAT, which are able to modify their function during conditions that alter energy balance. In conclusion, 5-HT may also act peripherally directly on the thyroid and organs containing type II 5'-D, thus controlling energy expenditure through heat production.

Treatment of type II amiodarone-induced thyrotoxicosis by either iopanoic acid or glucocorticoids: a prospective, randomized study

Amiodarone-induced thyrotoxicosis (AIT) may occur either in the presence of underlying thyroid disease (type I AIT) or in apparently normal thyroid glands (type II AIT). Type II AIT, a destructive thyroiditis, often favorably responds to glucocorticoids. Iopanoic acid (IopAc) is an iodinated cholecystographic agent that inhibits deiodinase activity and reduces the conversion of T(4) toT(3). It has recently been reported that cholecystographic agents restore euthyroidism in patients with type II AIT. We describe the results of a prospective randomized study conducted in 12 patients with type II AIT treated with either iopanoic acid (group A, n = 6) or glucocorticoids (group B, n = 6). Serum free T(3) levels normalized rapidly in both groups after 7 d, from 0.75 +/- 0.20 ng/dl (11.5 +/- 3.1 pmol/liter) to 0.46 +/- 0.10 ng/d (7.1 +/- 1.7 pmol/liter), P < 0.01, and from 0.58 +/- 0.10 ng/dl (9.0 +/- 1.2 pmol/liter) to 0.34 +/- 0.03 ng/dl (5.2 +/- 0.5 pmol/liter), P < 0.003, in groups A and B, respectively (P = NS). Serum free T(4) levels reduced at 6 months in group B [from 2.70 +/- 0.32 ng/dl (35.1 +/- 4.1 pmol/liter) to 1.0 +/- 0.04 ng/dl (13.4 +/- 0.6 pmol/liter), P < 0.0001] but not in group A (from 2.90 +/- 0.6 ng/dl (38.0 +/- 7.5 pmol/liter) to 2.30 +/- 0.4 ng/dl (35.6 +/- 6.1 pmol/liter, P = 0.39; P = 0.005 group B vs. group A). All patients in both groups became euthyroid and had their amiodarone-induced destructive thyroiditis cured as defined by normalization of both serum free T(4) and free T(3) levels, during both drugs therapy. However, patients in group B were cured more rapidly than patients in group A (43 +/- 34 d vs. 221 +/- 111 d, respectively, P < 0.002). This study shows that, albeit both drugs are effective, glucocorticoids are probably the drug of choice for more rapidly curing type II AIT.

A 21-year-old woman with consumptive hypothyroidism due to a vascular tumor expressing type 3 iodothyronine deiodinase

We present a 21-yr-old female with a large hepatic vascular tumor and subclinical hypothyroidism. A high level of the thyroid hormone inactivating enzyme type 3 iodothyronine deiodinase (D3) was detected in her tumor, and the TSH of 26.2 mU/liter returned to normal after surgical resection of the mass. This indicates that the vascular tumor caused this adult's hypothyroidism as has now been documented in nine infants with this syndrome. This first example of consumptive hypothyroidism in an adult indicates that the inactivation rate of thyroid hormone by D3 in a vascular tumor can stress the secretory capacity even of the TSH-stimulated normal adult thyroid gland.

Inactivation of thyroid peroxidase by soy isoflavones, in vitro and in vivo

Soy-containing foods and dietary supplements are widely consumed for putative health benefits (e.g. cancer chemoprevention, beneficial effects on serum lipids associated with cardiovascular health, reduction of osteoporosis, relief of menopausal symptoms). However, studies of soy isoflavones in experimental animals suggest possible adverse effects as well (e.g. enhancement of reproductive organ cancer, modulation of endocrine function, anti-thyroid effects). This paper reviews the evidence in humans and animals for anti-thyroid effects of soy and its principal isoflavones, genistein and daidzein.

Inhibition of thyroid type 1 deiodinase activity by flavonoids

Some dietary flavonoids inhibit thyroperoxidase and hepatic deiodinase activity, indicating that these compounds could be classified as anti-thyroid agents. In this study, we evaluated the in vitro effect of various flavonoids on thyroid type 1 iodothyronine deiodinase activity (D1). D1 activity was measured in murine thyroid microsome fractions by the release of 125I from 125I-reverse T3. D1 activity was significantly inhibited by all the flavonoids tested; however, the inhibitory potencies on thyroid D1 activity differed greatly among them. A 50% inhibition of D1 activity (IC(50)) was obtained at 11 microM baicalein, 13 microM quercetin, 17 microM catechin, 55 microM morin, 68 microM rutin, 70 microM fisetin, 72 microM kaempferol and 77 microM biochanin A. Our data reinforce the concept that dietary flavonoids might behave as antithyroid agents, and possibly their chronic consumption could alter thyroid function.

Biomimetic studies on iodothyronine deiodinase intermediates: modeling the reduction of selenenyl iodide by thiols

Enzyme mimetic studies on the crucial intermediate (E-SeI) of the iodothyronine deiodinase cycle have been carried out by using an areneselenenyl iodide stabilized by intramolecular Se.N interactions. Treatment of this compound with aromatic thiols and thiobenzoxazole in the presence of NEt(3) affords areneselenenyl sulfides that are stable towards disproportionation reactions. The structures of three of the areneselenenyl sulfides were determined by X-ray crystallography. In one case, in the absence of NEt(3), a diselenide can be formed rather than the selenenyl sulfide. The areneselenenyl iodide also reacts with a related selenol to produce the corresponding diselenide, and this reaction is found to be much faster than that with thiols. The high reactivity of the selenenyl iodide with the selenol suggests that a reduced selenol group (R'-SeH) may react with the E-SeI intermediate to produce a diselenide (E-Se-Se-R') without any thiol cosubstrate. The intermediacy of selenenyl sulfides during the reduction of selenenyl iodide by thiols and its possible relevance to the iodothyronine deiodinase catalytic cycle is also described.