Registration Dossier

Administrative data

Endpoint:
additional ecotoxicological information
Type of information:
other: literature review
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: Expert review of several literature data on EDC properties of thiourea.

Data source

Reference
Reference Type:
other: literature review
Title:
Discussion of potential endocrine disrupting effects of Thiourea (CAS 62-56-6)
Author:
Deacon, S.
Year:
2013
Bibliographic source:
(unpublished report)
Report Date:
2013

Materials and methods

Principles of method if other than guideline:
Expert review of several literature data on EDC properties of thiourea. Information was reviewed for fish, amphibians and microorganisms.

Test material

Reference
Name:
Unnamed
Type:
Constituent

Results and discussion

Any other information on results incl. tables

It should be noted that the European Commission Staff Working Document on the implementation of the "Community Strategy for Endocrine Disrupters" did not identify thiourea as an endocrine disrupting chemical from a review of a range of substances suspected of interfering with the hormone systems of humans and wildlife (COM (1999) 706), (COM (2001) 262) and (SEC (2004) 1372). The review below presents a short discussion of each paper reviewed by type of organism (e.g. fish, mollusc, etc.) and in chronological order.

1. Potential Effects on Fish

Mackay (1973) reported exposing Hypseleotris galii (the freshwater tropical fish, firetail gudgeon) to 25 mg/l and 200 mg/l of thiourea over a period of three weeks. The author concluded that at the higher dose, thiourea caused thyroid hypertrophy, inhibited vitellogenesis, and caused atresia of yolky oocytes. However, the author considered the antigonadal effects to be part of a generalised non-physiological toxic effect at the 200 mg/l concentration, although further description of toxic effects is not provided. Exposure at the lower thiourea concentration was considered to have caused thyroid hypertrophy without affecting gametogenesis.

McBride and van Overbeeke (1975) studied the effects of thiourea to sexually maturing and gonadectomised Oncorhynchus nerka (sockeye salmon) over a 7-14 week period. Fish were exposed to 0.03 and 0.06% (corresponding to 300 and 600 mg/l) thiourea in water resulting in no effects on spermatogenesis or the development of secondary sexual characteristics (SSC) such as testes, skin colouration, hooked snouts and large premaxillary teeth. Effects of thiourea on the pituitary gland were clearest in the gonadectomised fish causing degranulation of thyrotrophs. The authors concluded that similar, though indirect effects were seen in several cell types in the pituitary gland of sexually maturing fish. However, potential effects on sexually maturing fish are complex and the authors report conflicting studies in papers published between 1968 and 1973, although no information is provided on the study designs (e.g., duration, thiourea concentration). In their review of previous research, the authors decide that definite conclusions cannot be drawn from the previously published studies, although there appears to be some evidence of thiourea effects on degranulation of thyrotropths, correlated with thyroid hypertrophy and, at least in some species, effects on gonadotrophs. This study is considered to be reliable with restriction (Klimisch 2e).

In a study on the freshwater tropical fish Channa punctatus (green or spotted snakehead fish, or spotted murrel), Saxena and Mani (1979) studied the effects of thiourea on sexual maturation in fish. The fish were exposed to 0.033% (corresponding to 330 mg/l) thiourea in water for a total of 145 days with specimens collected at 30, 60, 90, 120 and 145 days. Ovarian weights were significantly lower (P<0.01) in treated fish from 90 days and thereafter, including ruptured tunica albuginea (white covering) and the release of some oocytes into the body cavity, suggest a direct effect of thiourea on the ovaries. However, previous studies, including McBride and van Overbeeke (1975), consider the effect of thiourea to be an indirect one. The authors conclude that further research is required to investigate whether inhibitory effects of thiourea are through the pituitary-thyroid axis or due to its direct toxic action on gonadotrophs or gonads.

Extrathyroidal effects of low concentrations of thiourea on Salmo gairdneri (rainbow trout) were reported by Eales (1981) and assigned Klimisch 2e rating in this review. Immature fish were exposed to 0.0015% (corresponding to 15 mg/l) thiourea for 67 days with the aim of investigating whether thiourea inhibits the thyroid exclusively or whether thiourea exerts confounding extrathyroidal effects. The results showed that thiourea had no effect on body weight or depression of circulating levels of T4 (thyroxine) or tri-iodothyronine (T3), T4 degradation rate, or T4 deiodination rate indicating no significant T4 influence on thyroidal hormone output. However, thiourea caused a small but significant increase in haematocrit and a decrease in distribution spaces for iodide and T4, indicating direct sensitivity of extrathyroidal processes to thiourea. The author concludes that the thiourea concentration of 15 mg/l may be close to the threshold for influencing the thyroid, based on the lowered (but not significantly) thyroid radioiodide uptake. Eales (1981) goes on to state that extrathyroidal effects may have occurred in earlier published studies, but misinterpreted as thyroid inhibition exclusively, although the extent to which results could be mis-represented remains unknown.

Misra and Pandy (1985) exposed the tropical freshwater fish Mystus vittatus (catfish) to 0.03% (or 300 mg/l) thiourea over a four week period. The study conclusions indicate a significant suppression of steriodogenesis in the testes of M. vittatus resulting in small changes to secondary sexual characteristics such as a reduction in genital papilla and caudal thickening in the male fish.

Madsen (1989) exposed yearling Salmo gairdneri (rainbow trout) to 3,000 and 5,000 mg/l (or 3 and 5 g/l) thiourea to assess potential effects on osmoregulatory function following exposure to thiourea over a 14-day period (Klimisch 2e rating). The conclusions state that the exposure resulted in depression of gill Na+/K+-ATPase activity in seawater at the higher thiourea concentration or delayed adaptive increase in ATPase activity at the lower thiourea concentration. Plasma Na+ and Cl- levels decreased in freshwater fish and increased in seawater-acclimated fish when exposed to thiourea. These results indicate an impairment of the osmoregulatory capacity of fish by thiourea exposure. Although circulating levels of T4 (thyroxine) tended to offset this effect, the authors stated that the hormone probably acts through a different mechanism associated with thiourea toxicity in extrathyroidal sites. This study, and similar studies, demonstrated the importance of simultaneously investigating thyroid histology and thyroid hormone concentrations when working with supposed goitrogens, such as thiourea.

Tagawa and Hirano (1991) assessed the effect of thyroid hormone deficiency in eggs on the early development of Oryzias latipes (medaka). To achieve thyroid hormone deficiency in eggs, spawning female fish were exposed to 0, 0.001, 0.003, 0.01 and 0.03% (or 0, 10, 30, 100 and 300 mg/l) thiourea for 1-2 months. Several regimes were assessed during the study; (i) changes in thyroid hormone concentrations in the eggs were examined until hatching, (ii) the optimum conditions of thiourea exposure of mother fish to obtain thyroid hormone-deficient eggs, and (iii) hatchability, survival, and growth rate were compared between normal and thyroid hormone-deficient eggs and larvae. The results showed no differences in hatchability, time of hatching and survival or body weight, length, condition factor or survival rate with and without food between control and thyroid hormone-deficient eggs.

Studies that were reviewed but considered to be invalid for use in this assessment include the following papers; Pfeiffer et al., (1985) reported the effects of a range of hormones and other substances on alarm substance cells and mucous cells in the epidermis of Phoxinus phoxinus (European minnow). Fish were injected three times per week with Thiourea concentrations ranging from 0.25, 2.5 and 25 mg for 10-42 days (typically 4 weeks). Thiourea had a reducing effect on alarm substance cells, but, unexpectedly, the size and number of mucous cells increased. These effects are contradictory as an increase in alarm cells is expected to stimulate mucous cells. It should be noted that a dose-response relationship is not reported and nor are any statistical analyses. This study is considered to be a Klimisch 3 due to the method of thiourea exposure (injection), limited methodological information and lack of interpretation of the contradictory results.

Exposure to 30 mg/l thiourea resulted in skin pigmentation changes in Paralichthys olivaceus (flounder) larvae during metamorphosis, although the consequences of these changes are unknown (Sugiyama and Yano, 1989). Only the abstract was reviewed (Klimisch 4a).

In summary, the fish studies reviewed reported test concentrations of thiourea within a broad range of 0.25 to 5,000 mg/l. Several studies reported changes to secondary sexual characteristics at concentrations of 300 mg/l or more, such as McBride & van Overbeeke (1975), Saxena (1975) and Misra & Pandy (1985), although Tagawa & Hirano (1991) reported no effects on hatchability, survival or body weight of young medaka following 300 mg/l thiourea exposure to the mother medaka. General toxicity (non-physiological response) was considered to be responsible for effects seen at 200 mg/l in the Mackay (1973) study. Thyroid hypertrophy, inhibited vitellogenesis, and atresia of yolky oocytes in the firetail gudgeon were observed by Mackay (1973) at 25 mg/l exposure, which may be considered to be extrathyroidal effects by later authors, such as Eales (1981) at 15 mg/l. Importantly, Eales (1981) considered 15 mg/l to be a threshold for influencing the thyroid. Little weight is given to the Sugiyama and Yano (1989) and Pfeiffer et al., (1985) studies, due to the reliability of the studies and the lack of evidence for dose-response related data (Pfeiffer study), though potential effects on skin pigmentation and alarm cells are reported, respectively. It would appear from the literature reviewed that a threshold for influencing the thyroid may be approximately 15 mg/l (Eales, 1981), however, changes to reproductive organs and secondary sexual characteristics, growth or survival appear to occur at 300 mg/l exposure or more.

2. Amphibians

In a study by Takigami et al., (2002) the potential effects of thiourea (and methimazole) were assessed on survival, growth, and metamorphosis (i.e., limb development and tail-length resorption) of the Xenopus laevis (South African clawed frog) were investigated using static-renewal tests. In the frog embryo teratogenesis assay, Xenopus (FETAX) mortality and malformation were not observed for the two chemicals at the highest concentration exposed (100 mg/l), which suggests that FETAX is not an appropriate screening method for thyroid action. Two bioassays for evaluating limb development and tail regression were simplified to a specific assay for thyroid hormone activity. Thiourea did not induce mal-development of the limbs, although at concentrations > 100 mg/l thiourea resulted in a decreased rate of tail resorption.

Rana tigerina (Indian bull frog) were injected with human chorionic gonadotropin (hCG) or hCG with 25 µg of thiourea in 0.1ml of distilled water (equivalent to 25 mg/l) on alternate days for 30 days. Kurian and Saidapur (1985) concluded that thiourea affects spermatogenesis so that stages III (secondary spermatocytes) to V (sperm bundles attached to Sertoli cells) fail to develop despite hCG stimulation.

In summary, under REACH Regulations, amphibians are not considered to be a standard test organism. These studies are provided as supporting information to the data for aquatic organisms and show that effects on reproductive endpoints (e.g., spermatogenesis) can be seen at thiourea concentrations of 25 mg/l or greater. These two studies suggest amphibians are similar in sensitivity to fish.

3. Microorganisms

A study on the inhibition of nitrification in the activated sludge process of sewage disposal was reported by Tomlinson et al., (1966) using a laboratory scale activated sludge unit. Thiourea was shown to be a powerful inhibitor of nitrification in sewage sludge (0.076 mg/l inhibiting oxidation by 75%), however, acclimatisation quickly occurs in the microorganisms and nitrification continues in the activated sludge. It was concluded that short-term effects of thiourea may be unrepresentative of activated sludge plants operating under steady state conditions as microorganisms become adapted with some microorganisms actually decomposing thiourea.

In a short publication by Pandey et al., (1975), 750 mg/l of thiourea was reported to prevent growth in Helminthosporium sativum (fungi) and 1,000 mg/l was stated to be fatal. The authors considered that gradual retardation in the growth of fungal mycelia with increasing concentrations of thiourea may be due to its possible effects on the fungal metabolism and enzyme system.

Kubota and Asami (1985) investigated the source of nitrous acid volatilisation from upland soils and applied a 100 and 200 mg/l thiourea solution, plus nutrient solution, to two air-dried upland soils. After 20 days of incubation, the amounts of thiourea-N were 5 or 10% of the total-N volatilised in the Taki and Matsuzawa soils, respectively. In both soils, thiourea-N concentrations of 200 mg/l almost completely supressed the oxidation of ammonium-N to nitrite-N and nitrate-N.

Zacherl and Amberger (1990) studied the effect of 100 mg/l thiourea on soil microorganisms by measuring growth inhibition and respiration of N-fixing bacteria. Only minor growth inhibition of Azotobacter chroococcum occurred; no effects were seen on Rhizobium leguminosarum. It was concluded that thiourea did not have a toxic effect on N-fixing bacteria in pure culture.

In summary, microorganisms also appear to be similar in sensitivity to thiourea as fish and amphibians. Potential effects from laboratory studies range from 100 mg/l to 1,000 mg/l. The study by Tomlinson et al., (1966) showed thiourea to be a nitrification inhibitor but under real world conditions, microorganisms are likely to be acclimated to thiourea and some microbes will break down thiourea during sewage treatment.

Applicant's summary and conclusion

Conclusions:
Based on the ecotoxicity information in the thiourea dossier, the most sensitive key value of 0.1 mg/l will be protective of long-term effects on aquatic organisms and potential effects on the endocrine system of aquatic organisms.
Executive summary:

The European Commission Staff Working Document on the implementation of the "Community Strategy for Endocrine Disrupters" did not identify thiourea as an endocrine disrupting chemical from a review of a range of substances suspected of interfering with the hormone systems of humans and wildlife (COM (1999) 706), (COM (2001) 262) and (SEC (2004) 1372). In the review of published papers relating to fish, amphibian and microbial studies it can be concluded that thiourea can display endocrine active properties that can result in changes in reproductive organs but the mode of action is complex and varied, the reversibility of these changes and the influence on future generations is unproven. If a precautionary approach is taken, the fish study by Eales (1981) considered a threshold for influencing the thyroid to be 15 mg/l. When these studies are compared with traditional ecotoxicity REACH endpoints, then aquatic invertebrates appear to be the most sensitive organisms with a NOEC/EC10 value of 0.1 mg/l (Boje and Rudolph, 1985). Therefore it can be concluded that, based on the ecotoxicity information in the thiourea dossier, the most sensitive key value of 0.1 mg/l will be protective of long-term effects on aquatic organisms and potential effects on the endocrine system of aquatic organisms.