Effects of hexachlorobenzene and its metabolites pentachlorophenol and tetrachlorohydroquinone on serum thyroid hormone levels in rats

Two novel in vitro assays to screen chemicals for their capacity to inhibit thyroid hormone transmembrane transporter proteins OATP1C1 and OAT4

Early brain development depends on adequate transport of thyroid hormones (THs) from the maternal circulation to the fetus. To reach the fetal brain, THs have to cross several physiological barriers, including the placenta, blood-brain-barrier and blood-cerebrospinal fluid-barrier. Transport across these barriers is facilitated by thyroid hormone transmembrane transporters (THTMTs). Some endocrine disrupting chemicals (EDCs) can interfere with the transport of THs by THTMTs. To screen chemicals for their capacity to disrupt THTMT facilitated TH transport, in vitro screening assays are required. In this study, we developed assays for two THTMTs, organic anion transporter polypeptide 1C1 (OATP1C1) and organic anion transporter 4 (OAT4), both known to play a role in the transport of THs across barriers. We used overexpressing cell models for both OATP1C1 and OAT4, which showed an increased uptake of radiolabeled T4 compared to control cell lines. Using these models, we screened various reference and environmental chemicals for their ability to inhibit T4 uptake by OATP1C1 and OAT4. Tetrabromobisphenol A (TBBPA) was identified as an OATP1C1 inhibitor, more potent than any of the reference chemicals tested. Additionally perfluorooctanesulfonic acid (PFOS), perfluoroctanic acid (PFOA), pentachlorophenol and quercetin were identified as OATP1C1 inhibitors in a similar range of potency to the reference chemicals tested. Bromosulfophthalein, TBBPA, PFOA and PFOS were identified as potent OAT4 inhibitors. These results demonstrate that EDCs commonly found in our environment can disrupt TH transport by THTMTs, and contribute to the identification of molecular mechanisms underlying TH system disruption chemicals.

Suspect and non-target screening of contaminants of emerging concern in streams in agricultural watersheds

Surface water runoff is an important source of water contamination affecting nearby rivers and streams. Many rural creeks are documented habitats for important aquatic species and the focus of restoration activities. In this study, we collected creek water samples in watersheds with a range of commercial-to-agricultural land use during rain events, and applied suspect and non-target screening with high-resolution mass spectrometry (HRMS) to characterize the occurrence of contaminants of emerging concern (CECs). In total, 58 CECs were identified, and 36 of them were confirmed and semi-quantified with reference standards. Pesticides were detected in all land use, including urban/commercial areas. Some pesticides were observed at concentrations of >10,000 ng/L demonstrating the strong contamination input during rain events. Five pesticides (azoxystrobin, fludioxonil, 4-hydroxy-chlorothalonil, imidacloprid, 2-methyl-4-chlorophenoxyacetic acid) were prioritized based on their risk quotients. HRMS chemical profiles demonstrated the wide range of chemical exposures in a given stream system and that compounds associated with specific land uses occur across land uses. Temporal trends suggested that some CECs remain present in creek water for months, resulting in chronic exposures across the life stages of aquatic species. These findings highlight the potential for contamination from agricultural runoff and the associated ecological risk to aquatic species. SYNOPSIS: Suspect and non-target screening revealed the chronic occurrence of emerging contaminants in streams in agricultural catchments during rain events.

Occupational exposure to pesticides and other biocides and risk of thyroid cancer

OBJECTIVES

To assess the associations between occupational exposure to biocides and pesticides and risk of thyroid cancer.

METHODS

Using data from a population-based case-control study involving 462 incident thyroid cancer cases and 498 controls in Connecticut collected in 2010-2011, we examined the association with occupational exposure to biocides and pesticides through a job-exposure matrix. We used unconditional logistic regression models to estimate OR and 95% CI, adjusting for potential confounders.

RESULTS

Individuals who were occupationally ever exposed to biocides had an increased risk of thyroid cancer (OR=1.65, 95% CI 1.16 to 2.35), and the highest risk was observed for the high cumulative probability of exposure (OR=2.18, 95% CI 1.28 to 3.73). The observed associations were similar when we restricted to papillary thyroid cancer and well-differentiated thyroid cancer. Stronger associations were observed for thyroid microcarcinomas (tumour size ≤1 cm). No significant association was observed for occupational exposure to pesticides.

CONCLUSIONS

Our study provides the first evidence linking occupational exposure to biocides and risk of thyroid cancer. The results warrant further investigation.

Disruptive effects of persistent organohalogen contaminants on thyroid function in white whales (Delphinapterus leucas) from Svalbard

We analysed levels of 56 organohalogen contaminants (OHCs) including brominated flame retardants, polychlorinated biphenyls (PCBs), and organochlorine pesticides in the blubber of white (beluga) whales (Delphinapterus leucas) from Svalbard, Norway (N=12; 6 adults [5 males and 1 female] and 6 subadults [4 males and 2 females]) collected in 1996-2001. We also measured circulating levels of thyroid hormones (THs) and thyroid stimulating hormone (TSH) in the whales. The results confirm that OHC levels in these white whales are among the highest levels recorded in wildlife from Svalbard, and at the high end of the range when compared to white whales from the North American Arctic. A projection to latent structure (PLS) model (subadults and adult males grouped together) revealed that known or suspected thyroid disruptive contaminants (polybrominated diphenylether [PBDE]-28, -47, -99, -100, and -154, hexachlorobenzene [HCB], and PCB-105) were negatively correlated with circulating levels of total thyroxin (TT4), free T4 (FT4) and free triiodothyronine (FT3). Most of these negative relationships were also confirmed using partial correlations controlling for length (and thus age) of the whales. The positive correlations of TT4, FT4 and FT3 with hexabromocyclododecane (HBCD), α-hexachlorocyclohexane (α-HCH), chlorinated bornanes CHB-40 and CHB-62 revealed by the PLS model were not confirmed by partial correlations. TH levels in the present study appeared to be somewhat lower than levels measured in beluga whales from the Canadian Arctic. However, we were not able to determine if this was caused by different levels of OHCs, or differences in biological factors (e.g. age, sex, moulting status, and season) and analytical methods between the studies. Although the sample sizes were low and statistical models cannot depict the biological cause-effect relationships, this study suggests negative influences of specific OHCs, particularly PBDEs, on thyroid hormone levels in white whales. The impact this might have on individual and population health is unknown.

Exposure to mixtures of organohalogen contaminants and associative interactions with thyroid hormones in East Greenland polar bears (Ursus maritimus)

We investigated the multivariate relationships between adipose tissue residue levels of 48 individual organohalogen contaminants (OHCs) and circulating thyroid hormone (TH) levels in polar bears (Ursus maritimus) from East Greenland (1999-2001, n=62), using projection to latent structure (PLS) regression for four groupings of polar bears; subadults (SubA), adult females with cubs (AdF_N), adult females without cubs (AdF_S) and adult males (AdM). In the resulting significant PLS models for SubA, AdF_N and AdF_S, some OHCs were especially important in explaining variations in circulating TH levels: polybrominated diphenylether (PBDE)-99, PBDE-100, PBDE-153, polychlorinated biphenyl (PCB)-52, PCB-118, cis-nonachlor, trans-nonachlor, trichlorobenzene (TCB) and pentachlorobenzene (QCB), and both negative and positive relationships with THs were found. In addition, the models revealed that DDTs had a positive influence on total 3,5,3'-triiodothyronine (TT3) in AdF_S, and that a group of 17 higher chlorinated ortho-PCBs had a positive influence on total 3,5,3',5'-tetraiodothyronine (thyroxine, TT4) in AdF_N. TH levels in AdM seemed less influenced by OHCs because of non-significant PLS models. TH levels were also influenced by biological factors such as age, sex, body size, lipid content of adipose tissue and sampling date. When controlling for biological variables, the major relationships from the PLS models for SubA, AdF_N and AdF_S were found significant in partial correlations. The most important OHCs that influenced TH levels in the significant PLS models may potentially act through similar mechanisms on the hypothalamic-pituitary-thyroid (HPT) axis, suggesting that both combined effects by dose and response addition and perhaps synergistic potentiation may be a possibility in these polar bears. Statistical associations are not evidence per se of biological cause-effect relationships. Still, the results of the present study indicate that OHCs may affect circulating TH levels in East Greenland polar bears, adding to the "weight of evidence" suggesting that OHCs might interfere with thyroid homeostasis in polar bears.

Relationship of thyroid hormone levels to levels of polychlorinated biphenyls, lead, p,p'- DDE, and other toxicants in Akwesasne Mohawk youth

BACKGROUND

It is well documented that acute exposure to high levels of persistent organic pollutants, such as polychlorinated biphenyls (PCBs), p,p'-dichlorophenyldichloroethylene (p,p'-DDE), and hexachlorobenzene (HCB), can affect human health including thyroid function. Chronic exposure to multiple toxicants is common but difficult to analyze, and most prior studies have focused on adults or newborns, creating a gap in our understanding of multitoxicant effects among adolescents.

OBJECTIVE

We investigated whether levels of PCBs, p,p'-DDE, HCB, mirex, lead, and mercury reflecting past chronic exposure are associated with alterations in levels of thyroid-stimulating hormone (TSH), triiodothyronine (T(3)), total thyroxine (TT(4)), and free thyroxine (FT(4)) among older children and adolescents.

METHODS

The sample consists of youth from the Akwesasne Mohawk Nation (n=232) who reside in proximity to several industries that have contaminated the local environment. We used multiple regression analysis to examine the effect of PCB groupings, p,p'-DDE, HCB, lead, and mercury on thyroid hormones after adjusting for sociodemographic covariates and controlling for all other toxicants.

RESULTS

Exposure to PCBs affects the thyroid hormone profile in adolescents. The group of persistent PCBs was positively associated with TSH but inversely related to FT(4). Nonpersistent PCBs were significantly and negatively related to FT(4) only. HCB was negatively associated with T(4), and lead was positively associated with T(3). Breast-fed adolescents had higher levels of persistent PCBs and p,p'-DDE but not of nonpersistent PCBs or any other toxicant when compared with non-breast-fed adolescents. Though having lower levels of persistent PCBs and p,p'-DDE, non-breast-fed adolescents exhibited significant relationships between persistent PCBs and TSH and FT(4), but breast-fed adolescents did not. It appears that PCBs from breast milk obscure the relationship between prenatal PCB exposure and thyroid function by adding random variation in PCB levels.

CONCLUSION

Our results demonstrate a reduction in thyroid function in adolescents in relation to their current serum levels of PCBs. These observations are consistent with the hypothesis that pre-natal exposure to PCBs alters thyroid function in a long-lasting manner but does not exclude the possibility that postnatal exposure is influential also.

Developmental exposure to pentachlorophenol affects the expression of thyroid hormone receptor beta1 and synapsin I in brain, resulting in thyroid function vulnerability in rats

Pentachlorophenol (PCP), a component of biocides and a contaminant in diverse tissue samples from humans from various geographic areas, disrupts regulatory effects of thyroid hormones. Here we examined the effects of developmental exposure of rats to PCP on various aspects of brain development, male reproductive function, and adrenal function, all of which are under thyroid hormones regulation. PCP was administered to dams and their offspring via drinking water (6.6 mg l(-1)) during gestation and lactation. Tissue samples were obtained from dams, 3-week-old weanling pups, and 12-week-old pups. Gene expressions of thyroid hormone receptor beta1 and synapsin I, factors that promote brain growth, was increased in the cerebral cortex of PCP-treated weanling females, whereas plasma concentrations of total thyroxine were decreased in dams and weanling pups, and plasma thyroid-stimulating hormone concentrations were higher in PCP-treated weanling males. PCP caused a decrease in plasma corticosterone concentrations in 12-week-old female rats, but not in male rats or weanling females. PCP-treated male pups had significantly increased testis weight at 12 week of age. No overt signs of toxicity were noted throughout this study. Our results show that PCP exposure during development causes thyroid function vulnerability, testicular hypertrophy in adults, and aberrations of brain gene expression.

Toxicité de l'hexachlorobenzène chez Meriones unguiculatus : effets sur la thyroïde et le foie

The effect of in vivo administered hexachlorobenzene (HCB) on liver and thyroid was studied on Meriones unguiculatus. HCB (1.6, 4, and 16 mg/kg of body weight) has been administered orally to meriones for 30 days. At the end of the experiment, the body weight of the animals did not show significant change. However, the higher dose of HCB treatment led to a pronounced hepatic hypertrophy comparatively to controls. Histological observations revealed many cytomorphological alterations. Cellular necrosis, periportal, and centrolobular vein congestion and cytoplasmic vacuolisation were noted and correlated with the administered doses of HCB. The higher dose of HCB induced modifications in the activities of hepatic transaminases and on thyroid hormones levels: ALAT activity level was more pronounced in males (170+/-24.7 U/l vs. 52.66+/-8.29 U/l in controls) than in females (120+/-12.47 U/l vs. 56+/-5 U/l in controls). However, ASAT activity increased significantly only in females (259+/-29 U/l vs. 244.66+/-18 U/l in controls). Plasma total triiodothyronine (TT3) and total thyroxine (TT4) levels seemed to be sex-dependent in intoxicated animals, since TT4 decreased significantly in males (21.95+/-7.46 nmol/l vs. 40.59+/-1.08 nmol/l in controls) and TT3 in females (1.42+/-0.11 nmol/l vs. 3.96+/-0.48 nmol/l in controls).

Pentachlorophenol exposure in women with gynecological and endocrine dysfunction

Exposure to wood preservatives containing pentachlorophenol (PCP) was detected in 65 women who consulted the Endocrinological Department of the University Hospital of Obstetrics and Gynecology, Heidelberg, Germany, because of gynecological problems. Blood PCP levels ranged from 20.7 to 133 microg per liter of serum. One hundred and six women with similar clinical conditions, corresponding age and body weight, no PCP exposure in history, and PCP levels below 20 microg per liter of serum served as control group. Significant associations were found between serum PCP concentrations, age, and different parameters of the endocrine system. PCP may act centrally on a hypothalamic or suprahypothalamic level which may result in mild ovarian and adrenal insufficiency. PCP may, therefore, play a role in the increasing infertility problem.

Effects of environmental synthetic chemicals on thyroid function

Synthetic chemicals are released into the environment by design (pesticides) or as a result of industrial activity. It is well known that natural environmental chemicals can cause goiter or thyroid imbalance. However, the effects of synthetic chemicals on thyroid function have received little attention, and there is much controversy over their potential clinical impact, because few studies have been conducted in humans. This article reviews the literature on possible thyroid disruption in wildlife, humans, and experimental animals and focuses on the most studied chemicals: the pesticides DDT, amitrole, and the thiocarbamate family, including ethylenethiourea, and the industrial chemicals polyhalogenated hydrocarbons, phenol derivatives, and phthalates. Wildlife observations in polluted areas clearly demonstrate a significant incidence of goiter and/or thyroid imbalance in several species. Experimental evidence in rodents, fish, and primates confirms the potentiality for thyroid disruption of several chemicals and illustrates the mechanisms involved. In adult humans, however, exposure to background levels of chemicals does not seem to have a significant negative effect on thyroid function, while exposure at higher levels, occupational or accidental, may produce mild thyroid changes. The impact of transgenerational, background exposure in utero on fetal neurodevelopment and later childhood cognitive function is now under scrutiny. There are several studies linking a lack of optimal neurological function in infants and children with high background levels of exposure to polychlorinated biphenyls (PCBs), dioxins, and/or co-contaminants, but it is unclear if the effects are caused by thyroid disruption in utero or direct neurotoxicity.

Biotransformation and toxicity of halogenated benzenes

1. Multiple potentially harmful metabolites can be distinguished in the metabolic activation of halogenated benzenes: epoxides, phenols, benzoquinones and benzoquinone-derived glutathione conjugates. 2. The role of these (re-) active metabolites in the toxic effects induced by halogenated benzenes such as hepatotoxicity, nephrotoxicity, porphyria and thyroid toxicity is discussed. 3. Evidence is presented suggesting that the formation of reactive benzoquinone metabolites rather than the traditional epoxides is linked to halogenated benzene-induced hepatotoxicity. 4. A crucial role for the benzoquinone-derived glutathione adducts in halogenated benzene-induced nephrotoxicity is clearly established. 5. Although metabolic activation appears to be involved in porphyria, the nature of the ultimate porphyrinogenic metabolite has not been elucidated yet. 6. Disturbances in thyroid hormone (and retinoid) homeostasis can be (at least partially) explained by the formation of halogenated phenol metabolites. 7. In conclusion, for a relevant prediction of the ultimate fate of a compound in a living organism, one should know the chemical characteristics and reactivity of the parent compound and its metabolites, together with insight into the formation mechanism of each of the suspected metabolites, and an understanding of the interaction between a specific chemical (reactive) structure and its target molecule.

Reduction of thyroxine uptake into cerebrospinal fluid and rat brain by hexachlorobenzene and pentachlorophenol

In the present study the effects of hexachlorobenzene (HCB) and the metabolite pentachlorophenol (PCP) were investigated with respect to uptake of thyroxine (T4) into cerebrospinal fluid (CSF) and brain structures of rats. [125I]T4 was taken up into CSF of control rats by a relatively slow process, reaching a steady state after about 3 h. Both repeated dosing of HCB and single doses of PCP caused decreased uptake of [125I]T4 into CSF, total brain tissue as well as specific brain structures, such as occipital cortex, thalamus, and hippocampus. Although HCB-treatment caused a build-up of HCB and PCP levels in serum in brain only HCB was present in significant amounts (16% of the serum level). In CSF, both HCB and PCP concentrations were below detection levels. Separate experiments with PCP showed, however, a dose- and time-dependent uptake of PCP into CSF. The present results indicate that PCP and the parent compound HCB are able to affect brain supply of T4. This may have consequences for an adequate development of the brain or proper brain function in adults. The exact mechanisms of interference of PCP and/or HCB in brain uptake of T4 remain to be established.

Human pentachlorophenol poisoning

Pentachlorophenol (PCP) was, and still is, one of the most frequently used fungicides and pesticides. Its toxicity is due to interference with oxidative phosphorylation. Acute and chronic poisoning may occur by dermal absorption, inhalation or ingestion. Chronic poisoning occurs mainly in sawmill workers or people living in log homes treated with PCP-containing wood protecting formulations. Quantitative determination of PCP in urine and serum is useful to detect occupational or subclinical exposure. The clinical features of acute and chronic PCP poisoning can be classified systematically into effects on the skin, metabolism (fever), the haematopoietic tissue, the respiratory system, the central and peripheral nervous system, the kidney and the gastrointestinal tract. Although PCP is not classified as a human carcinogen, some epidemiological observations suggest that exposure to chlorophenols in general and PCP solutions in particular may result in an increased risk for certain malignant disorders such as nasal carcinoma and soft tissue sarcoma. There is concern that contamination of PCP-solutions with products such as chlorodibenzo-p-dioxins is the real cause of this suspected carcinogenicity. No specific antidote exists for the treatment of (acute) PCP poisoning. The basis of the treatment of acute poisoning is intensive supportive care with prevention of dangerous rise in temperature. Use of PCP-based products as indoor wood preservatives poses an unacceptable risk to human health.

Hexachlorobenzene-induced hypothyroidism. Involvement of different mechanisms by parent compound and metabolite

Rats received repeated oral treatment with different doses of hexachlorobenzene (HCB) (0-3.5 mmol/kg) for 2 or 4 weeks. Measurements of thyroid hormone status after 2 weeks showed a dose-dependent decrease of total thyroxine (TT4) levels, decreased free thyroxine (FT4) levels and little change of total triiodothyronine (TT3) levels. The effects on thyroid hormone status were more pronounced after 4 weeks and also included increased thyroid stimulating hormone (TSH) levels. These conditions suggest that HCB had induced hypothyroidism in these animals. Indications for occupation of thyroid hormone binding proteins were found in serum of exposed animals. The major metabolite pentachlorophenol (PCP) also caused, by competitive interactions with thyroid hormone binding proteins in serum, a rapid and dose-dependent decrease of TT4 and FT4 levels, but not of TT3 levels in serum. The decrease of serum TT4 levels by repeated dosing with 3.5 mmol HCB/kg for 4 weeks could be attributed to competitive interactions of PCP with hormone serum binding proteins and to increased metabolism induced by HCB to an equal degree. At lower dose levels or with shorter dosing periods, increased metabolism of T4 is the main cause of decreased TT4 serum levels. This is the first indication that a similar effect is caused simultaneously by the parent compound and its metabolite through different and independent mechanisms.

Effects on thyroid hormone metabolism and depletion of lung vitamin A in rats by airborne particulate matter

Thyroxine (T4) and vitamin A are important regulators of normal epithelial differentiation and proliferation and might act in the promotion phase of carcinogenesis. Thyroid hormone and vitamin A metabolism are linked by a common plasma carrier protein, transthyretin (TTR). Polychlorinated biphenyls (PCBs) and related organochlorine compounds deplete vitamin A and thyroxine by interaction with TTR and alteration of their metabolism in hepatic and other organs. In the present report an outdoor airborne particulate matter (APM) extract was tested for both interaction with thyroid hormone and vitamin A metabolism, in order to address the question of whether APM has the potency to deplete vitamin A and thyroid hormones. Furthermore, studies were performed to characterize compounds present in APM that interact with TTR. A third aim was to compare the interaction of APM extracts with TTR and thyroxine-binding globulin (TBG), the major carrier protein for thyroxine in humans. Results showed that a single treatment of rats with an outdoor APM extract depleted plasma thyroxine and triiodothyronine levels and increased plasma retinol levels gradually over the time period studied, while liver retinol, lung retinol, and retinyl palmitate levels were depleted by 30-50%. As outdoor APM was able to inhibit T4-TTR binding in vitro, this suggests that the reduction in thyroxine levels in vivo is caused by the same phenomenon. Experiments showed that the neutral fraction of the APM extract accounted for most of the inhibitory activity on T4-TTR binding. Polycyclic aromatic hydrocarbons and nitrated derivatives are not likely to be responsible for the activity of the neutral fraction, because several representatives of these compounds showed no or very little interaction with TTR. Pentachlorophenol, a compound with known inhibitory activity on T4-TTR binding, was detected in the organic acid fraction of both a cigarette smoke sample and an outdoor APM sample. Finally, it was shown that several indoor and outdoor APM extracts only interact with TTR, but not with TBG. As APM has the potency to deplete lung vitamin A in vivo and vitamin A might have a protective effect in the process of lung carcinogenesis, APM might increase the susceptibility for the development of lung cancer.

Body distribution and endocrine toxicity of hexachlorobenzene (HCB) in the female rat

Hexachlorobenzene (HCB) residue levels in dosed rats (50.0 mg kg-1 body wt.day-1, n = 9) were significantly (P < 0.05) greater in the periovarian fat compared to the thyroid gland. Hexachlorobenzene residue levels were significantly (P < 0.05) greater in the thyroid versus the adrenal and ovary. Ovarian HCB residue levels were greater than those found in the thymus, liver and lung. Serum thyroxin (T4) and the free T4 index (FTI) were significantly (P < 0.05) suppressed in HCB-treated rats compared to the control group (n = 8). In contrast, no significant differences in serum concentrations of oestradiol (E2), progesterone (P4) or percentage triiodothyronine uptake (%T3) were observed, thus suggesting an HCB-induced hypothyroid-like state. In a second experiment, adult female Sprague Dawley rats (n = 16) were dosed as above and superovulated with pregnant mare serum gonadotrophin (PMSG, 10 IU s.c.) and human chorionic gonadotrophin (hCG, 20 IU s.c.). Circulating levels of P4 were significantly (P < 0.05) elevated compared to the control group (n = 8). The %T3 uptake and serum T4 levels were significantly (P = 0.05) suppressed compared to controls. Hexachlorobenzene treatment had no effect on circulating levels of E2 or on the FTI. These results suggest that HCB-induced changes found in the spontaneously cycling rat are augmented by ovulation induction strategies. We also conclude that HCB concentrates in the endocrine tissues in addition to the fat.

Increased glucuronidation of thyroid hormone in hexachlorobenzene-treated rats

Metabolism of thyroid hormones was investigated in WAG/MBL rats that had been exposed to hexachlorobenzene (HCB). Serum thyroxine (T4) levels were lowered by 35.5%, whereas triiodothyronine (T3) levels were not changed. Bile flow, as well as T4 excretion in bile were increased by HCB treatment. Analysis of bile by HPLC revealed a more than 3-fold increase of T4 glucuronide (T4G) and a concomitant reduction of non-conjugated T4. T4 UDP-glucuronyltransferase activity (T4 UDPGT) activity in hepatic microsomes was increased more than 4.5-fold in animals exposed to HCB. p-Nitrophenol (PNP) UDPGT showed a comparable increase by HCB. Both T3 and androsterone UDPGT activities were low in WAG/MBL rats compared with normal Wistar rats. T3 UDPGT activity was increased 2.5-fold by HCB, but androsterone UDPGT activity was unchanged. These results suggest that T4 is a substrate for HCB-inducible PNP UDPGT and T3 for androsterone UDPGT. In the absence of the latter, T3 is also glucuronidated to some extent by PNP UDPGT. Type 1 iodothyronine deiodinase activity was decreased by HCB treatment. It is concluded that decreased T4 levels in serum of animals after exposure to HCB may be due to a combined effect of displacement of T4 from carriers, an increased glucuronidation of T4 and enhanced bile flow.

Hexachlorobenzene and its metabolites pentachlorophenol and tetrachlorohydroquinone: interaction with thyroxine binding sites of rat thyroid hormone carriers ex vivo and in vitro

Previous results have indicated that hexachlorobenzene (HCB)-induced hypothyroidism may be caused by its main metabolite pentachlorophenol (PCP), and by tetrachlorohydroquinone (TCHQ), rather than by the parent compound. In the present experiments it was investigated whether hormone displacement from serum carriers could be a factor in the development of this hypothyroidism. In an in vitro competition assay PCP was an effective competitor for the thyroxine (T4)-binding sites of serum carriers, whereas HCB was ineffective. Ex vivo experimental results demonstrated occupation of T4-binding sites in sera from PCP-exposed animals but not in sera from HCB- or TCHQ-treated animals. Competing ability for T4-binding sites was still present in sera of PCP-exposed animals but was absent in HCB- or TCHQ-exposed animals. The results suggest that thyroid hormone displacement by the major metabolite PCP may play a role in HCB-induced hypothyroidism.