Endocrine Disruptors<\/p>\n\n\n\n
Daniel R. Doerge1\n<\/sup>and Daniel M. Sheehan2<\/sup><\/p>\n\n\n\n 1<\/sup>Division of Biochemical Toxicology,\nNational Center for Toxicological Research, Jefferson, Arkansas, USA; 2<\/sup>Daniel\nM. Sheehan and<\/p>\n\n\n\n Associates, Little\nRock, Arkansas, USA<\/p>\n\n\n\n Soy is known to\nproduce estrogenic isoflavones. Here, we briefly review the evidence for\nbinding of isoflavones to the estrogen receptor, in vivo <\/em>estrogenicity and developmental toxicity, and estrogen\ndevelopmental carcinogenesis in rats. Genistein, the major soy isoflavone, also\nhas a frank estrogenic effect in women. We then focus on evidence from animal\nand human studies suggesting a link between soy consumption and goiter, an\nactivity independent of estrogenicity. Iodine deficiency greatly increases soy\nantithyroid effects, whereas iodine supplementation is protective. Thus, soy\neffects on the thyroid involve the critical relationship between iodine status\nand thyroid function. In rats consuming genistein-fortified diets, genistein\nwas measured in the thyroid at levels that produced dose-dependent and\nsignificant inactivation of rat and human thyroid peroxidase (TPO) in vitro. <\/em>Furthermore, rat TPO activity\nwas dose-dependently reduced by up to 80%. Although these effects are clear and\nreproducible, other measures of thyroid function in vivo <\/em>(serum levels of triiodothyronine, thyroxine, and thyroid-stimulating\nhormone; thyroid weight; and thyroid histopathology) were all normal.\nAdditional factors appear necessary for soy to cause overt thyroid toxicity.\nThese clearly include iodine deficiency but may also include additional soy\ncomponents, other defects of hormone synthesis, or additional goitrogenic\ndietary factors. Although safety testing of natural products, including soy\nproducts, is not required, the possibility that widely consumed soy products\nmay cause harm in the human population via either or both estrogenic and\ngoitrogenic activities is of concern. Rigorous, high-quality experimental and\nhuman research into soy toxicity is the best way to address these concerns.\nSimilar studies in wildlife populations are also appropriate. Key words: <\/em>estrogen toxicity,\nestrogenicity, genistein, isoflavones, mass spectrometry, soy, thyroid\nperoxidase, thyroid toxicity. Environ\nHealth Perspect <\/em>110(suppl 3):349\u2013353 (2002).<\/p>\n\n\n\n http:\/\/ehpnet1.niehs.nih.gov\/docs\/2002\/suppl-3\/349-353doerge\/abstract.html<\/p>\n\n\n\n The potential health benefits of soy, and the soy isoflavones in\nparticular, are widely publicized. Although soy and soy isoflavones exhibit\nboth risks and benefits (1,2<\/em>), in this article we focus on the\nthyroid toxicity of genistein. Genistein is the major isoflavone synthesized by\nthe soybean; genistein possesses both estrogenic and goitrogenic activities.\nThe claimed benefits of genistein are being examined in numerous experimental,\nepidemiologic, and clinical studies investigating breast and prostate cancer chemoprevention,\nrelief of postmenopausal symptoms, and prevention or slowing of osteoporosis.\nHere, we first present a brief summary of the estrogenic activity and toxicity\nof genistein and then explore the potential for thyroid toxicity of these\nchemicals, both from a historical perspective and from data reported from\nrecent investigations on the mechanisms of potential toxicity. We also\nintegrate research results on isoflavone thyroid effects in a manner useful for\npredicting and identifying potential risks from soy consumption in various\nhuman populations.<\/p>\n\n\n\n Phytoestrogens comprise a class of several different chemicals\nproduced by a variety of plants (2<\/em>). Of these, the soy isoflavones (particularly genistein) are of greatest\ninterest because of the widespread human consumption of soy, due largely in\nWestern countries to extensive advertising by the soy industry for potential\nhuman health benefits. However, despite the widespread belief that soy\nconsumption is safe, soy isoflavones administered during development can cause\nseveral forms of estrogen toxicity in experimental animals.<\/p>\n\n\n\n As part of a large project to develop a battery of predictive\ncomputational models, we recently assayed 230 chemicals for binding to the estrogen\nreceptor (ER) (3<\/em>).\nOf these, 46 were phytoestrogens from six different chemical structure classes.\nOf the nine isoflavones, seven bound the ER with measurable affinity, ranging\nfrom a relative binding affinity (RBA) of 0.45 for genistein to 0.0013 for\nformononetin, with the RBA for estradiol being 100. Equol, a metabolite of the\nphytoestrogen daidzein, had an RBA for ER that was 33% that of genistein (4<\/em>).\nWhen examined in a battery of rat in vivo <\/em>assays developed to assess\nestrogen activity and toxicity during postnatal development, equol increased\nuterine weight. Also, equol treatment on postnatal days (PNDs) 1\u20135 inhibited\nuterine weight gain on PNDs 20 and 25, as did coumestrol and diethylstilbestrol\n(DES)<\/p>\n\n\n\n (4<\/em>).\nAfter PND 10\u201314 treatment, equol, like coumestrol and DES, inhibited the\ndevelopment of uterine glands. This was a frank toxic response because the\nentire uterine tissue compartment was mostly absent. Recently, genistein was\nfound to cause uterine adenocarcinoma in adult mice, following neonatal\ntreatment (5<\/em>).\nBecause earlier DES studies showed the same effect, equipotent doses of\ngenistein and DES (the positive control) were chosen based on uterotrophic\nactivity. The tumor incidence was statistically the same in both the DES and\ngenistein groups. This finding strongly suggests that the estrogenic activity\nper se, and not the chemical structure, is responsible for this malignant\noutcome.<\/p>\n\n\n\n When assessed in women in a controlled trial, a dose of 30 g soy\nflour\/day had frank estrogenic effects, including lengthening of the menstrual\ncycle (6<\/em>).\nInfants on soy infant formula receive a dose of phytoestrogens that is 5-fold\nhigher than the dose causing estrogenic effects in women (7<\/em>). This level of soy isoflavone\nexposure to approximately 20% of American infants should be of concern, but no\nrobust studies in infants have been conducted.<\/p>\n\n\n\n The evidence outlined here is sufficient to conclude that\ngenistein and equol bind to the ER, are estrogenic in vivo<\/em>, and are estrogenic\ndevelopmental toxicants; that genistein is an estrogenic carcinogen in rodents;\nand that such exposures may be relevant to humans.<\/p>\n\n\n\n It is well described but little known that the soybean and goiter\nhave long been associated in animals and humans. Rodents are useful risk\nassessment models for thyroid toxicants, despite significant differences\nbetween rodent<\/p>\n\n\n\n This\narticle is part of the monograph Impact of\nEndocrine Disruptors on Brain Development and Behavior<\/em>.<\/p>\n\n\n\n Address correspondence\nto D.M. Sheehan, 1422 Scott St., Little Rock, AR 72202 USA. Telephone: (501)\n376-1052. E-mail: dansheeh@swbell.net<\/p>\n\n\n\n We\ngratefully acknowledge helpful discussions with K.B. Delclos of the NCTR. This\nresearch was supported in part by Interagency Agreement 224-<\/p>\n\n\n\n 93-0001\nbetween NCTR\/Food and Drug<\/p>\n\n\n\n Administration\nand the National Institute of Environmental Health Sciences\/National Toxicology\nProgram. Received 8 January 2002; accepted 22 March 2002.<\/p>\n\n\n\n Endocrine Disruptors \u2022 Doerge and\nSheehan<\/p>\n\n\n\n and human thyroid physiology (8<\/em>). In rats the goitrogenic activity\nof soy and its inhibition by dietary iodide supplementation have been defined (9\u201313<\/em>).\nThe negative interaction of low dietary iodine and soy is demonstrated by the\nfinding of Kimura et al. that thyroid carcinoma appeared in rats fed an\niodine-deficient diet consisting of 30% defatted soy (14<\/em>). In humans, goiter has been seen\nin infants fed soy formula; this is usually reversed by changing to cow milk or\niodine-supplemented diets (15\u201322<\/em>). After the 1960s,\nmanufacturers reportedly began adding iodine to formulas to mitigate thyroid\neffects. Fort et al. (23<\/em>) conducted a retrospective\nepidemiologic study on teenage children diagnosed with autoimmune thyroid\ndiseases (Hashimoto\u2019s thyroiditis or Graves\u2019 disease). Those consuming soy\nformula as infants had twice the prevalence of autoimmune disease (18 of 59,\n31%) of healthy siblings (9 of 76, 12%) or controls (7 of 54, 13%). Goiter and\nhigh normal thyroidstimulating hormone (TSH) levels in healthy iodine-sufficient\nadults occurred as early as 1 month (n <\/em>= 37) after commencing a diet that\nincluded 30 g of pickled soybeans per day (24<\/em>). Although it was not measured,\ndietary iodine content may have been insufficient to protect against the\nantithyroid effect of soy. Furthermore, no changes in serum thyroid hormone\n<\/p>\n\n\n\n levels were\nfound. After 1 month off the soy diet, TSH decreased to the pretreatment levels\nand goiters were diminished in size. Lowered T3 <\/sub>levels were seen in\n14 premenopausal but not in 18 postmenopausal women on a soy diet (up to 2 mg\ntotal of soy isoflavones per kilogram body weight per day) for about 3 months (25<\/em>).\nInterestingly, in another study, as little as 1 month of soy supplementation\ndecreased T3<\/sub>\/T4 <\/sub>levels during the luteal phase of the\nmenstrual cycle, but levels increased during the follicular phase (26<\/em>).\n\n<\/p>\n\n\n\n Thyroid peroxidase (TPO) is found in the apical membrane of\nthyroid follicular cells. TPO, a heme-containing enzyme, catalyzes both\nreactions required for thyroid hormone synthesis (see Scheme 1). The first step\nis<\/p>\n\n\n\n iodination of thyroglobulin tyrosyl residues, followed by oxidative coupling to yield T4 <\/sub>and T3<\/sub>. Inhibition of porcine TPO activity is a mechanism common to many classes of synthetic antithyroid compounds (27\u201331<\/em>) and naturally occurring flavonoids (32,33<\/em>). Lactoperoxidase (LPO) is often used as a model for TPO, based on many shared structural and functional properties. For this reason investigations were started on soy isoflavone inhibition of both porcine TPO and LPO activity. Genistein and daidzein were found to be the chemicals in soy that inhibited both TPO-catalyzed iodination and coupling<\/p>\n\n\n\n (33,34<\/em>).\nThe nature of the inhibition of enzymatic activity under various conditions is\nquite interesting, if not startling. First, absent iodide, genistein and\ndaidzein act as suicide substrates for TPO and LPO by covalently binding to the\nactive site. This was shown by the irreversible loss of both iodinating and\ncoupling activities and concomitant changes in the ultraviolet-visible spectrum\nof the enzyme. Second, with adequate iodide, genistein and daidzein are\nalternate substrates; products are mono-, di-, and triiodoisoflavones.<\/p>\n\n\n\n Scheme 2 proposes mechanisms by which genistein can intercept\nreactive enzyme intermediates involved in the iodination and coupling reactions\nrequired for T4 <\/sub>synthesis. Mechanisms include reaction of compound I\nwith isoflavones that could produce a reactive isoflavone radical at the active\nsite, along with a radical form of compound II, which could combine to form\ninactivated enzyme presumably through covalent modification of active site\namino acid residues. Consistent with this hypothesis are the covalent binding\nof approximately 3 mol of radiolabeled genistein per 1 mol of inactivated LPO;\nthe unchanged heme content in inactivated LPO (data not shown); and the\nultraviolet-visible spectral changes observed upon inactivation of LPO and TPO\n(33<\/em>).<\/p>\n\n\n\n Microsomal rat TPO (rTPO; from untreated animals) incubated with genistein<\/p>\n\n\n\n Scheme 2. <\/strong>Proposed\nmechanisms for inhibition of TPO by soy isoflavones.<\/p>\n\n\n\n and hydrogen peroxide (H2<\/sub>O2<\/sub>) was used to\ncharacterize isoflavone-mediated, timedependent TPO inactivation in vitro <\/em>(35<\/em>)\n(Figure 1). The control experiments demonstrate that neither H2<\/sub>O2\n<\/sub>nor genistein alone altered activity, consistent with the suicide\ninactivation mechanism previously proposed (33,35<\/em>) and Scheme 2. The apparent\ninhibition binding constant, Ki<\/sub><\/em>, and the maximal\ninactivation rate constant, k<\/em>inact<\/sub>, were 50 nM and\n0.28 min-1<\/sup>, respectively. Daidzein likewise inactivated TPO, with\nconstants of 143 nM and 0.31 min-1<\/sup>. These kinetic parameters are\nconsistent with very potent inactivation, unlike other dietary flavonoids\ntested (32<\/em>),\nand suggest a mechanism for low-dose effects of soy. The sensitivity of\nmicrosomal rTPO to inactivation by genistein was compared with other mammalian\nperoxidases (35<\/em>).\nPurified bovine LPO, porcine TPO, human TPO, and microsomal rTPO all showed\n40\u201366% inactivation at 30 min, suggesting that isoflavone-mediated inactivation\nof TPO is a general phenomenon across mammalian species.<\/p>\n\n\n\n The observed isoflavone inhibition of peroxidase\nactivity in vitro <\/em>led\nto treatment of Sprague-Dawley rats with genistein to investigate possible\nendocrine disruption in both dose-range-finding (36<\/em>) and multiple-generation studies\n(in progress). Genistein doses of 0, 5, 100, and 500 ppm were administered in\nsoy-free basal diet (total genistein and daidzein ~0.5 ppm each) to pregnant\nfemale rats 4 weeks before mating through pup weaning at PND 21. This was\nfollowed<\/p>\n\n\n\n by consumption of the same diet until offspring were sacrificed\nat PND 140. Total blood genistein (i.e., both conjugates and aglycone) was\nmeasured using electrospray mass spectrometry (ES\/MS) (Table 1) (37<\/em>).\nThe range of total genistein traversed the range found in several human\npopulations (Table 2) (38,39<\/em>). On the 500 ppm diet, rat\ngenistein levels were similar to those in infants on soy formulas (7<\/em>).\nRats on the 100 ppm diet had genistein levels similar to those found in adults\non typical Asian diets<\/p>\n\n\n\n (38<\/em>)\nor soy isoflavone dietary supplements (39<\/em>). Rats consuming 5 ppm and control\ndiets had low genistein levels typically found with a Western diet (Table 1).\nFemale rats had higher blood genistein levels than males, consistent with the\nsex-specific difference in genistein elimination half-time (3.0 vs. 4.3 hr in\nmales and females, respectively) (37<\/em>). The predominant circulating\nmetabolites (97\u201399%) are genistein glucuronides in both rats (40<\/em>)\nand humans (39<\/em>).\nThe predominance of circulating metabolites (97\u201399%) is consistent with\nextensive first-pass metabolism of genistein in the gut and\/or liver (41<\/em>).<\/p>\n\n\n\n Intrathyroidal\nAccumulation of Genistein<\/strong><\/p>\n\n\n\n Both total and aglycone genistein concentrations were measured in\nthyroids using liquid chromatography (LC)-ES\/MS (35<\/em>). Figure 2 shows the results for\nfemales and males; the higher average thyroidal levels observed in females\nreflected the higher average blood concentrations. Thyroid genistein aglycone\n(i.e., the unconjugated form) was substantially increased relative to that in blood\n(18\u201328% vs. 1\u20133%). Total thyroidal genistein occurs in the range of 0.1\u20131.2\nnmol\/g tissues and for the aglycone, 0.1\u20130.3 nmol\/g. Because water accounts for\nslightly less than two-thirds of thyroid weight, concentrations of genistein\naglycone were up to 350 nM. Elevated tissue aglycone levels, up to 100% of\ntotal, were also measured in several male and female reproductive organs (37<\/em>).\nThis demonstrates preferential tissue accumulation of unconjugated genistein,\nwhich is the biologically active form. Intrathyroidal Genistein aglycone levels\nof 50\u2013300 nM measured in this study are above concentrations that inactivate\nrTPO in vitro <\/em>(Figure\n1) (35<\/em>).<\/p>\n\n\n\n Inactivation of rTPO by Dietary Genistein in Vivo<\/em><\/strong><\/p>\n\n\n\n Given that the thyroid\nconcentration of genistein was sufficient to inactivate TPO in vitro<\/em>,\nTPO activity was assayed in rats fed the genistein-fortified diets. In both\nmale and female rats, dose-dependent and significant decreases in rTPO activity\nwere observed (Figure 3). Although 80% loss of rTPO activity was observed in\nhigh dose<\/p>\n\n\n\n (500 ppm) females, a dose 100-fold lower (5\nppm) also inactivated significant amounts of rTPO. That suggests that TPO\ninactivation can occur, even at very low dietary isoflavone levels.\nFurthermore, because thyroid genistein concentrations were sufficient to\ninactivate rTPO in\nvitro<\/em>, the reductions seen in TPO activity could have been due to\nenzyme inactivation in vivo<\/em>.<\/p>\n\n\n\n We found no evidence from LC-ES\/MS\nanalysis of serum or thyroids suggesting minimal formation of any of the\niodinated genistein species previously characterized in vitro <\/em>(33<\/em>). This finding, coupled with the\nextensive TPO inactivation in vivo<\/em>, suggests that TPO\ninactivation by covalent binding of isoflavones predominates in the rat thyroid\ngland over competitive substrate iodination. This study with genistein appears\nto be the first in which chemically induced loss of TPO activity has been\ndemonstrated both in\nvitro <\/em>and in vivo<\/em>.<\/p>\n\n\n\n Inactivation of rTPO by Dietary Soy in Vivo<\/em><\/strong><\/p>\n\n\n\n A diet comparison study was also\nconducted in which rats were fed either a standard rodent diet (NIH 31)\ncontaining 5% soy meal and approximately 60 ppm total isoflavones or a soy-free\nbasal diet (5K96) containing approximately 1 ppm total isoflavones (35<\/em>).\nIn soy, isoflavones occur as various glucoside conjugates (42<\/em>), so this study addressed the\nissue of whether conjugation of dietary isoflavones produced effects different\nfrom genistein aglycone. TPO activity was reduced approximately 50% in male and\nfemale rats consuming the standard soy diet compared with rats fed the control\ndiet (Figure 4), and the extent of reduction was consistent with the total\nisoflavone blood levels. The average serum concentrations of total genistein\nand daidzein from the NIH 31 diet were 0.35 \u00b1 0.03 and 0.20 \u00b1 0.02 \u00b5M,\nrespectively, in males and 0.62 \u00b1 0.05 and 0.25 \u00b1 0.02 \u00b5M, respectively, in\nfemales (43<\/em>).\nThese results clearly showed that whether the dietary isoflavone is an aglycone\nor a glucoside conjugate has no effect on total serum isoflavones or on TPO\ninactivation. This result is consistent with previous studies that showed\nadministration of genistein aglycone or conjugates had a minimal effect on the\npharmacokinetics of total isoflavone absorption and elimination in rats,\nalthough small differences in the peak concentrations were found (44<\/em>).\nThe isoflavone content of typical open-formula rodent diets can range from\n<5 ppm up to 500 ppm (45<\/em>). These findings suggest that\nadditive effects on TPO activity between soy in rodent diets and exogenous\nchemicals being tested for carcinogenicity or other toxicities may confound the\nconclusions.<\/p>\n\n\n\n Absence of Hypothyroid Indicators in Rats Fed\nGenistein<\/strong><\/p>\n\n\n\n Given the decreased TPO activity in rats on\ngenistein-fortified or soy diets, a resultant hypothyroid state with decreased\nT3<\/sub>\/T4 <\/sub>and increased TSH levels seemed likely. This was\nfurther reinforced by the known susceptibility of rats (particularly males) to\nantithyroid chemicals (8<\/em>). However, analysis of T3<\/sub>\/T4\n<\/sub>and TSH in sera from all rats in both these studies (35<\/em>) showed that no treated group was\ndifferent from the untreated controls (data not shown). Consistently, gland\nweights and histopathology of thyroid sections were not different between the\ncontrol and 500-ppm genistein groups in a parallel study (not shown). A recent\nstudy showed no significant thyroid histopathology in rats fed a 20% soy diet (46<\/em>).\nThese findings appear paradoxical given the prominent losses of TPO activity\n(Figures 3, 4). Unlike these results, humans consuming soy developed goiter and\nelevated TSH, albeit without changes in T3<\/sub>\/T4 <\/sub>(24<\/em>).<\/p>\n\n\n\n Synergism of Soy with Iodine<\/strong><\/p>\n\n\n\n Deficiency in Producing Hypothyroid<\/strong><\/p>\n\n\n\n Effects in Rats<\/strong><\/p>\n\n\n\n The synergism in antithyroid\nproperties of soy combined with iodine deficiency has been further elucidated\nby Ikeda et al. (46<\/em>).\nThey showed, as did Kimura et al. (14<\/em>), that rats fed an\niodine-deficient diet containing 20% defatted soy bean diet developed a severe\nhypothyroid state characterized by decreased T4<\/sub>, increased TSH,\nincreased thyroid weight, increased cell proliferation, and marked\nhistopathological changes. Histopathology of the anterior pituitary showed that\nan unknown component of soy had a direct action on the pituitary gland.<\/p>\n\n\n\n To further investigate the ability\nof soy isoflavones to interact with iodine deficiency to produce\nhypothyroidism, effects on rats fed soy-free diets (with or without iodine)\nwere compared with effects on rats fed the 20% defatted soy diet (47<\/em>).\nIn other groups, soy isoflavones (12\u201318% genistein aglycone, 12\u201318% daidzein\naglycone, 2\u20134% glycitein aglycone) were added to soy-free diets at 0.2 and\n0.04%. The 0.04% isoflavone diet had isoflavones at the same level as the 20%\ndefatted soy diets. As before, the hypothyroid state in rats on the 20%\ndefatted soy diet required iodine deficiency. However, neither group of\nisoflavone-supplemented rats showed hypothyroidism, whether iodine deficient or\niodine replete. These results suggest that whole soy, but not isoflavones\nalone, is required to produce hypothyroidism in iodine-deficient rats. This is\nconsistent with isoflavone inhibition of TPO synergistically interacting with\nan unidentified component(s) of soy to produce hypothyroidism in iodine-deficient\nrats. Additionally, other components of human or rat diets, or chemical\nexposures, may cause goiter in association with soy consumption.<\/p>\n\n\n\n Possible Effects of Soy on Human Thyroid Health<\/strong><\/p>\n\n\n\n The total genistein\nconcentrations in rat serum (Table 1) are similar to those in humans (Table 2),\nsuggesting a similar tissue exposure. It reasonable to conclude that human\nisoflavone consumption could produce isoflavone levels in the thyroid (Figure\n2) sufficient to inactivate human TPO, as seen in rats.<\/p>\n\n\n\n The failure to\nfind hypothyroidism caused by genistein in rats, despite extensive inactivation\nof TPO (35<\/em>),\nor by mixed isoflavone consumption (47<\/em>) points to additional risk\nfactor(s) necessary to induce hypothyroidism. In particular, iodine deficiency\nis necessary for soy to cause antithyroid effects in rats. Although the\nmechanism of this iodide effect is unknown, a significant literature supports\nthis concept (10,14,\n46,47<\/em>). In humans, early findings showed that goiter in infants fed\nsoy formula was reversed upon supplementation with iodine (17<\/em>). Progression to a hypothyroid\nstate may also be aided by biochemical impairment of hormone synthesis and\nmetabolism, and\/or exposure to environmental goitrogens, for example,\nsulfonamides, glucosinolates, cyanogenic glycosides, flavonoids (48<\/em>),\nand persistent halogenated aromatic compounds. Nonetheless, we should be alert\nto the finding that soy-induced goiter and other hypothyroid indicators have\nbeen reported in humans in the absence of evidence for iodine deficiency (24<\/em>).<\/p>\n\n\n\n Soy products\nare heavily marketed to postmenopausal women for relief of menopausal symptoms,\ndespite the absence of consistent clinical data demonstrating any such benefit\nin human trials (25<\/em>).\nHowever, of concern is that this is the same subgroup in which frank\nhypothyroidism and a subclinical hypothyroid state (49<\/em>) are most likely to occur (up to 4\nand 10%, respectively) (50,51<\/em>). Further, the incidence of\nchronic autoimmune thyroiditis, the major risk factor for the development of\nhypothyroid disease in women (50<\/em>), increases with age in women.<\/p>\n\n\n\n Dietary\ngenistein causes a potent stimulation of T-cell\u2013 and B-cell\u2013mediated immunity\nin rats (36,52<\/em>),\nan effect found with other estrogenic compounds (e.g.,<\/p>\n\n\n\n 352 VOLUME 110 | SUPPLEMENT <\/sub>3\n| JUNE <\/sub>2002 \u2022 Environmental\nHealth Perspectives p<\/em>-nonylphenol, ethinylestradiol). In\naddition, suicide inactivation of TPO by dietary genistein in rats likely\nproduces covalent binding of genistein to TPO. This modification could well be\nfollowed by TPO structural changes, especially in the three-dimensional shape\nand charge distribution. This could lead to a new antigenic form of TPO (a\nneoantigen) that could stimulate recognition by the immune system (53<\/em>).\nFurther support for this notion comes from the fact that anti-TPO is the major\nthyroid autoantigen circulating in human serum (54<\/em>). Although the etiology of thyroid\nautoimmunity is unknown, the multitude of genistein effects in rats suggests\nthat soy consumption could cause or exacerbate this illness.<\/p>\n\n\n\n Iodine\ndeficiency is an emerging concern in elderly Americans. Consumption of iodized\nsalt, the primary source of dietary iodine, may decrease with the desire or\nneed to reduce the possible hypertensive effects of high salt intake. The data\npresented here suggest that elderly women need to be aware of, and monitored\nfor, possible thyroid problems resulting from consumption of soy products.\nThose postmenopausal women who consume large amounts of soy products may be at\nhigher risk.<\/p>\n\n\n\n Finally, information presented\nhere also allows a new understanding of the increased incidence of autoimmune\nthyroiditis in children fed soy formulas reported by Fort et al. (23<\/em>).\nThat study has been criticized, and even dismissed, because children put on soy\nformula are thought to be more likely to have autoimmune disorders (e.g., food\nallergies, presence of viral gastroenteritis). This assumption, however, can be\nseriously questioned, because only 0.5\u20131.0% of American infants have true\nallergies, but about 20% are on soy formula. The alternative explanation, based\non the findings here, involves stimulation of immune function by genistein,\npossible neoantigen formation through covalent binding of genistein to TPO, and\nincreasing autoimmune disease prevalence. This hypothesis provides a plausible\nexplanation for the observations of Fort et al. (23<\/em>) and we hope will encourage\nfurther study of autoimmune thyroiditis in children consuming soy formula.<\/p>\n\n\n\n Endocrine\nDisruptors \u2022 Soy estrogenic and goitrogenic activities<\/p>\n\n\n\n 60:333-340 (1994). 34.\nTaurog A. Thyroid hormone synthesis and secretion. In:<\/p>\n\n\n\n Pharmacol 30:217\u2013222 (1999). Warbritton\nA, Weis CC, Newbold RR . Effects of dietary<\/p>\n\n\n\n Anderson DW. The curative action of iodine on soybean 37. Chang HC,\nChurchwell MI, Delclos KB, Newbold RR, goiter and the changes in the\ndistribution of iodoamino Doerge DR. Mass spectrometric determination of genisacids\nin the serum and in thyroid gland digests. Arch tein tissue distribution in\ndiet-exposed Sprague-Dawley<\/p>\n\n\n\n Biochem Biophys 93:15\u201324 (1961). rats.\nJ Nutr 130:1963\u20131970 (2000).<\/p>\n\n\n\n Exp Biol Med 110:417\u2013420 (1962). isoflavonoids\nin plasma by isotope dilution gas chro-<\/p>\n\n\n\n J Sci Food Agric 23:549\u2013555 (1972). 39.\nDoerge DR, Chang HC, Holder CL, Churchwell MI.<\/p>\n\n\n\n nant goiter by defatted soybean with iodine-free diet in 40. Holder\nCL, Churchwell MI, Doerge DR. Quantification of rats. Gann 67:763\u2013765 (1976).\nsoy isoflavones, genistein and daidzein, and conjugates<\/p>\n\n\n\n humans. Pediatrics 24:752\u2013760 (1959). 41.\nSfakianos J, Coward L, Kirk M, Falany CN, Barnes S.<\/p>\n\n\n\n Soybean goiter. N Engl J Med 262:1099\u20131103 (1960). conjugates in soy foods:\nextraction conditions and analy-<\/p>\n\n\n\n 102:106\u2013109 (1961). 42:2466\u20132474\n(1994).<\/p>\n\n\n\n stature. Br Med J 293:232\u2013233 (1989). 44.\nKing RA, Broadbent JL, Head RJ. Absorption and excre-<\/p>\n\n\n\n receiving a soy formula: case report and review of the 45. Thigpen JE,\nSetchell KDR, Ahlmark KB, Locklear J, Spahr literature. Pediatrics 96:148\u2013150\n(1995). T, Caviness GF, Goelz MF, Haseman JK, Newbold RR,<\/p>\n\n\n\n 16:280\u2013282 (1997). 46.\nIkeda T, Nishikawa A, Imazawa T, Kimura S, Hirose M.<\/p>\n\n\n\n Coll Nutr 9:164\u2013167 (1990). 47.\nSon HY, Nishikawa A, Ikeda T, Kimura S, Hirose M. Lack<\/p>\n\n\n\n 67:622\u2013629 (1991). 48.\nGaitan E, Lindsay RH, Cooksey RC. Millet and the thyroid.<\/p>\n\n\n\n effects in premenopausal women. J Clin Endocrinol 49. Woeber KA. Subclinical\nthyroid dysfunction. Arch Intern<\/p>\n\n\n\n Metab 84:192\u2013197 (1999). Med\n157:1065\u20131068 (1997).<\/p>\n\n\n\n BioFactors 12:233\u2013241 (2001). (Braverman LE,\nUtiger RD, eds). Philadelphia:Lippincott27. Doerge DR, Takazawa RS. Mechanism\nof thyroid peroxi- Raven, 1996;474\u2013482.<\/p>\n\n\n\n dase inhibition by ethylenethiourea. Chem Res Toxicol 51. Canaris GJ, Manowitz NR, Mayor\nG, Ridgeway EC. The<\/p>\n\n\n\n 3:98\u2013103 (1990). Colorado\nthyroid disease prevalence study. Arch Intern<\/p>\n\n\n\n alyzed reactions by arylamines: mechanism for the anti- 52. Zhang Y,\nSong TT, Cunnick JE, Murphy PA, Hendrich S. thyroid action of sulfamethazine.\nChem Res Toxicol Daidzein and genistein glucuronides in vitro <\/em>are weakly<\/p>\n\n\n\n 7:164\u2013169 (1994). estrogenic\nand activate human natural killer cells at<\/p>\n\n\n\n 10:49\u201358 (1997). 54.\nMcKenzie JM, Zakarija M. Thyroid autoimmunity. In:<\/p>\n\n\n\n 31. Doerge DR, Chang HC,\nDivi RL, Churchwell MI. Werner and Ingbar\u2019s The Thyroid (Braverman LE, Utiger\nMechanism for inhibition of thyroid peroxidase by leuco- RD, eds).\nPhiladelphia:Lippincott-Raven, 1996;416\u2013432.<\/p>\n","protected":false},"excerpt":{"rendered":" Endocrine Disruptors Daniel R. Doerge1 and Daniel M. Sheehan2 1Division of Biochemical Toxicology, National Center for Toxicological Research, Jefferson, Arkansas, USA; 2Daniel M. Sheehan and Associates, Little Rock, Arkansas, USA … <\/p>\nSoy and Isoflavone Estrogenic<\/h2>\n\n\n\n
Activity<\/h3>\n\n\n\n
Goitrogenic Activity of Soy\nIsoflavones <\/h3>\n\n\n\n
Literature Review<\/h4>\n\n\n\n
Biosynthesis of Thyroid\nHormones and Inhibition by Antithyroid Chemicals<\/h2>\n\n\n\n
Inactivation of TPO by\nIsoflavones in Vitro<\/h2>\n\n\n\n
Dietary Exposure of Sprague-Dawley\nRats to Genistein<\/h3>\n\n\n\n
<\/p>\n\n\n\nReferences and Notes<\/h2>\n\n\n\n
\n
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