thiourea; thiocarbamide

Endpoint summary

Key value for chemical safety assessment

Toxic effect type:

dose-dependent

Effects on fertility

Description of key information

Further investigation on toxicity to reproduction and developmental toxicity will be conducted according to ECHA decision No. CCH-D-2114539794-36-01/F (19th January 2021). This section will be updated after finalization of these studies.

 

 

Supplementary studies:

 

Effects observed in young lambs orally administered thiourea daily for 2, 4, or 6 months include hypothyroidism, reduction of testosterone levels, small and empty seminiferous tubules and “ill developed testes” in males and infantile and stunted external genitalia and an absence of estrus and retarded mammary development in females at 50 mg/kg bw/day. In addition to general signs of toxicity the thyroid gland was moderately to severely enlarged, although there was no direct correlation with length of dosing. Muscular weakness and difficulty standing and walking were noted with increased dosing. Hypoglycaemia, hyperlipidaemia/hypercholesterolaemia, and a significant fall in serum T4 were related to length of treatment. (Sokkar et al., 2000; Nasseri and Prassad, 1987a [7.5.1], Nasseri & Prasad, 1987b [7.8.2]). In addition, effects on reproductive organs observed in female lambs following subchronic oral administration include non-significantly reduced size and weight of ovaries, uterine horn and vagina when administered orally at a dose of 50 mg/kg/d to four female lambs for 80 days. Follicles in the ovaries were atretic and endometrial cells were shorter than the controls, indicating that hypothyroidism probably suppresses the ovarian and other reproductive functions of female lambs (Alavi Shoushtari & Safaii 1993). Oral administration of greater than 35 mg/kg bw/day thiourea to male rats produced a reduction or cessation of spermatogenesis (Fitzhugh & Nelson 1948). Reddy et al. (1998) verified the impairment on male fertility in a one-generation study treating male animals only followed by an artificial insemination of untreated females. The available data suggest that effects on reproductive organs in adult and developing animals are related to the inhibition of thyroid function. Thus, Thiourea is considered to affect fertility as a result of hypothyroidism This conclusion supports the current harmonized classification under CLP (Repro Cat 2).

Short description of key information:
Effects on the male and female reproductive organs have been described in subchronic studies in sheep and rodents. Reddy (1998) reported on a one-generation study with dosing of male goats, only.

Justification for selection of Effect on fertility via oral route:
None of the available studies provide data on full dose response relationship. However, the adverse effects on reproductive organs in developing animals are considered to be related to an impairment of the thyroid function. Thus it is expected that these effects can be extrapolated form repeated dose toxicity studies. Sokkar 2000 has been selected as it provides the lowest LOAEL value for reproductive effects.

Link to relevant study records

Reference

Endpoint:

fertility, other

Remarks:

based on test type

Type of information:

experimental study

Adequacy of study:

supporting study

Study period:

no data

Reliability:

4 (not assignable)

Rationale for reliability incl. deficiencies:

documentation insufficient for assessment

Qualifier:

equivalent or similar to guideline

Guideline:

other: no guideline reported

Principles of method if other than guideline:

Eight male lambs aged 3 to 3.5 months were orally administered 50 mg thiourea/kg body weight daily for 3.5 months. The aim of the study was to induce secondary iodine deficiency in lambs to clarify the biochemical and pathological changes, especially in the male reproductive organs.

GLP compliance:

not specified

Limit test:

yes

Species:

other: Lamb

Strain:

other: Rahmani

Sex:

male

Details on test animals or test system and environmental conditions:

The study was conducted on 12 Rahmani immature growing male lambs 3±3.5 months old. The animals were apparently healthy and free from internal and external parasites. The animals were offered balanced rations and were raised on the National Research Center farm.

Route of administration:

oral: unspecified

Vehicle:

not specified

Details on exposure:

- daily oral administration of 50 mg/kg bw of thiourea for 3.5 month (from June to September).

Details on mating procedure:

- The animals were not mated; the development of the male reproductive organs under the influence of thiourea administration was monitored.

Analytical verification of doses or concentrations:

not specified

Details on analytical verification of doses or concentrations:

no data

Duration of treatment / exposure:

3.5 month

Frequency of treatment:

daily

Details on study schedule:

3.5 month (14 weeks)

Dose / conc.:

50 mg/kg bw/day (nominal)

Remarks:

Basis:nominal conc.

No. of animals per sex per dose:

8 of 12 male animals were administered thiourea

Control animals:

yes, concurrent no treatment

Positive control:

no

Oestrous cyclicity (parental animals):

- no female lambs were included in the study

Litter observations:

- The animals were not mated; the development of the male reproductive organs under the influence of thiourea administration was monitored.

Postmortem examinations (parental animals):

SACRIFICE
- All animals were sacrified
GROSS NECROPSY
- Gross necropsy consisted of external and internal examinations

HISTOPATHOLOGY / ORGAN WEIGHTS
- histopathological examination of the thyroid gland, testis, liver, renal tubuleskidney, spleen, skin.
- determination of organ weight of testis and the thyroid gland

Postmortem examinations (offspring):

n.a.

Statistics:

Statistical analyses used Student's t-test according to Snedecor and Cochran (1976)

Reproductive indices:

n.a.

Offspring viability indices:

n.a.

Clinical signs:

effects observed, treatment-related

Body weight and weight changes:

effects observed, treatment-related

Food consumption and compound intake (if feeding study):

effects observed, treatment-related

Organ weight findings including organ / body weight ratios:

effects observed, treatment-related

Histopathological findings: non-neoplastic:

effects observed, treatment-related

Other effects:

not specified

Reproductive function: oestrous cycle:

not examined

Description (incidence and severity):

only male animals were administered thiourea

Reproductive function: sperm measures:

not specified

Reproductive performance:

effects observed, treatment-related

Description (incidence and severity):

no sexual desire when introduced to ewes in oestrus compared with control animals

- facial oedema and variable degrees of alopecia
- the wool became rough, lustreless and easily pulled, with thick keratinized skin
- by the 8th week, the appetite of the treated lambs decreased gradually until the lambs were off their food
- gradual decrease in body weight in treated lambs with significant (P<0.01) reduction by weeks 13 and 14 (Table 1)
- no sexual desire when introduced to ewes in oestrus compared with control animals
- palpation of the testes of treated lambs revealed hydrocele with small testes
- significant decrease in the mean values of erythrocyte count, leucocyte count and haemoglobin concentration during the last weeks of treatment (10±12 weeks) as compared with the control group
- the treated lambs showed significant (P<0.05) hypoalbuminemia, significant (P<0.05) hyperlipidemia and significant (P<0.01) hypercholesterolemia at the end of the experiment
- macroscopic examination of the testes revealed hydrocoele in two cases with small testes
- the average weight of the testes of hypothyroid lambs showed a significant reduction in comparison with those of control lambs, weight being 3.2 +/- 0.255 g for treated and 8.9 +/- 1.00 g for control lambs
- microscopically, the seminiferous tubules were underdeveloped, small in size, having an empty lumen with thick basement membrane; degenerative changes; most Sertoli cells were primitive and nonfunctional; infiltration of mononuclear cells, mainly lymphocytes, with oedema was noticed in interstitial tissue
- degeneration of hepatocytes
- hyperplasia of follicular cells of thyroid gland

Key result

Dose descriptor:

LOEL

Effect level:

50 mg/kg bw/day (nominal)

Based on:

test mat.

Sex:

male

Basis for effect level:

other: hypothyroidism; retardation of growth; weight loss; lack of sexual desire; Hydrocoele; small testes; hyperplasia of follicular cells of thyroid gland; degenerative changes of liver; alopecia

Clinical signs:

not examined

Mortality / viability:

not examined

Body weight and weight changes:

not examined

Sexual maturation:

not examined

Organ weight findings including organ / body weight ratios:

not examined

Gross pathological findings:

not examined

Histopathological findings:

not examined

There were only male lambs included in the study. No offspring.

Key result

Remarks on result:

not measured/tested

Reproductive effects observed:

not specified

Table 1: The mean values SEM of body weights (kg) of control and hypothyroid lambs

Time	Control group (kg)	Treated group (kg)
Day 2	16.00±0.10	17.00±1.15
Week 3	16.00±0.20	17.33± 1.45
Week 4	15.25± 1.25	17.33± 0.88
Week 5	16.25± 1.25	18.00± 0.88
Week 6	16.25± 1.25	17.66± 1.20
Week 8	17.00± 0.99	17.60± 1.20
Week 9	17.50± 1.50	17.83±1.16
Week 11	20.00± 1.00	18.33± 0.88
Week 13	22.00 ± 0.20	17.00± 0.10**
Week 14	23.25± 0.25	16.50± 0.29**

** Highly significant at P < 0.01

 

Conclusions:

Secondary iodine deficiency induced by thiourea administration caused clinical and pathological lesions in different organs, especially in the male gonad of growing male lambs.

Executive summary:

Eight male lambs aged 3 to 3.5 months were orally administered 50 mg thiourea/kg body weight daily for 3.5 months. There were four control lambs. Hypothyroidism was caused by thiourea adminsitration and which led to retardation of growth and interfered with the sexual maturity of the growing male lambs. The treated males did not show any sexual desire when introduced to ewes in estrus compared with control animals. Palpation of the testes of treated lambs revealed hydrocoele with small testes. The average weight of the testes of the hypothyroid lambs was significantly reduced (3.2 ± 0.255 g) compared with that of control lambs (8.9 ± 1.00 g). The testes showed underdeveloped, small, empty seminiferous tubules with thick basement membranes. The Sertoli cells were primitive and nonfunctional. The level of testosterone in the plasma of these hypothyroid lambs was not detectable. The results of this publication are summarized in the " Concise International Chemical assessment Document 49 - Thiourea" (WHO, 2003).

Effect on fertility: via oral route

Endpoint conclusion:

adverse effect observed

Dose descriptor:

LOAEL

50 mg/kg bw/day

Study duration:

subchronic

Species:

sheep

Quality of whole database:

The quality of the available data is considered to be moderate. It is impaired by the fact that none of the available studies establish a dose-response relationship. However, the adverse effects are considered to be related to an impairment of the thyroid function. Therefore, the dose-response data from repeated dose toxicity studies can be used to predict toxicity to reproduction.

Effect on fertility: via inhalation route

Endpoint conclusion:

no study available

Effect on fertility: via dermal route

Endpoint conclusion:

no study available

Effects on developmental toxicity

Description of key information

Developmental effects have been observed in young lambs orally administered thiourea at one dose, only (Nasseri and Prassad, 1987b), and in rodents (Kern, 1980; Teramoto, 1981; Ruddick, 1976). Effects on the thyroid and the passage of the blood-placenta barrier are summarized in section 7.1.

 

Administering 0.2% Thiourea in drinking water to pregnant rats on gestation days 1–14 results in malformation of the foetuses, i.e. of the central nervous system and skeleton . Thyroid hyperplasia was found in the rat foetuses and newborn animals. Data on maternal toxicity were not provided in this study (Kern 1980). Hypothyroidism caused by the administration of 50 mg thiourea/kg bw to 66 female sheep for 2, 4, or 6 months adversely influences somatic development, reproductive/gestational performance of animals, and growth of developing fetuses in utero. The growing lambs did not show signs of estrus and mammary development was retarded. Thiourea administered to pregnant ewes induced abortion, stillbirth, birth of weak/low-weight lambs, dystokia, and retention of placenta. The severity of changes was dependent upon the stage of gestation when hypothyroidism was induced. (Nasseri and Prassad, 1987b). Studies with 35S-thiourea in rats and mice demonstrate that the substance crosses the placenta and, depending on the stage of development of the thyroid gland, is stored in particular in this organ where it affects iodine metabolism (Shepard, 1963). Maternally toxic oral doses of 1000 mg/kg given to mice on day 10 or to rats on day 12 or day 14 of gestation were embryotoxic. Absorption frequency was increased without malformations in the foetuses living on day 18 (mice) or 20 (rats) (Teramoto, 1981). Thiourea had neither a maternally toxic nor a teratogenic effect when administered to rats on the 12th or 13th day of gestation as a single oral dose of 480 mg/kg body weight (Ruddick et al., 1976).

The embryotoxic effects of thiourea can be attributed to the depressing action of thiourea on thyroid activity. The effect of Thiourea on the thyroid is well known and has been used in the 1940ies to treat hyperthyroidism in human patients (IUCLID 7.10.3). It is not to be expected that such effects would occur at levels of thiourea that do not result in an inhibition of thyroid function. This conclusion is in line with the risk assessment conducted by the MAK commission (1990) and WHO (IPCS, 2003). Therefore, the dose-response data from repeated dose toxicity studies can be used to predict maternal and foetal toxicity.

Justification for selection of Effect on developmental toxicity: via oral route:
Teratogenic effects have been reported at dose levels eliciting maternal toxicity. None of the available studies provide data on full dose-response relationship. However, the adverse effect is considered to be related to an impairment of the thyroid function. Thus it is expected that teratogenicity occurs at dose levels that also cause maternal toxicity.

 

Further investigations

Dose Range Finding

The effects of thiourea were investigated after oral administration (by drinking water) in female rats during pregnancy and on embryo-foetal development. The study design was as follows: 6 females per group were treated either with the vehicle (water) in the control group or 10, 40 and 80 mg/kg bw/day in the test item groups. All females were treated during the gestation period from Day 3 through Day 20 post coitum.

Mortality check, clinical signs, body weight and water consumption were recorded during the in vivo phase. At term (post coitum Day 20), females were caesarean-sectioned and subjected to detailed post mortem examination.
The number of corpora lutea, implantations, early and late intrauterine deaths, live and dead foetuses, gravid uterus weight, foetal weight and sex were recorded. All foetuses were examined for external abnormalities.

Two premature deaths occurred in the study. On Day 4 of the gestation period (after one day of administration of treated water), one low dose female was found dead and one high dose female was sacrificed for humane reason.
The signs observed prior to death or sacrifice and findings observed at post mortem examination could be related to the death of the animals and therefore are considered treatmentrelated.
The number of females with live foetuses on gestation Day 20 was: 5 in each of the control, low and high dose groups and 6 in the mid-dose group.

Piloerection was the only clinical sign noted in the few first days of administration in females of the mid-dose groups. Tachypnoea and piloerection were the clinical signs seen in the high dose females for approximately one week.

Lower body weights were seen in all treated groups. Body weight gain was reduced in all treated females from gestation day 3.

Reduction in water consumption was recorded in all groups receiving treated water for the first 4 days of treatment. Afterwards, water consumption in the low dose group was generally comparable to control while in the mid- and high dose groups the reduction remained evident throughout the treatment period.

The mean achieved dosages for the dosing period were found to be comparable with the expected ones.

A decrease in gravid uterus weight was observed in females of the high dose group. Theb decrease is attributable to a single females which had only 3 live foetuses in the uterus.

High dose females showed increased incidence in early uterine deaths and post-implantationb loss. Dose-related reduction in mean fetal weight of both sexes of foetuses was seen in all treated groups and was considered related to the maternal toxicity.

There were no treatment-related macroscopic observations in the animals sacrificed at the end of treatment period.

The presence of small foetuses in the treated groups was considered related to the maternal toxicity.

On the basis of the results obtained in this study, the high dose level for the subsequent main study should be below nominal 80 mg/kg/day.

 

For further investigation on toxicity to reproduction and developmental toxicity a study according to OECD 414 in rat will be conducted according to ECHA decision No. CCH-D-2114539794-36-01/F (19th January 2021). This section will be updated after finalization of these studies.

Link to relevant study records

Reference

Endpoint:

developmental toxicity

Type of information:

experimental study

Adequacy of study:

weight of evidence

Study period:

no data

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

data from handbook or collection of data

Remarks:

secondary source; the original reference was not available but is cited in the "Concise International Chemical Assessment Document 49-Thiourea" (WHO, 2003); data sufficient for assessment

Qualifier:

no guideline followed

Principles of method if other than guideline:

In a study in which 66 female sheep (18 growing lambs, 18 maiden ewes, 9 pregnant ewes; controls: 9 growing lambs, 9 maiden ewes, 3 pregnant ewes) were orally administered 0 or 50 mg thiourea/kg body weight daily for 2, 4, or 6 months (six treated and three controls per group)

GLP compliance:

not specified

Limit test:

yes

Species:

other: lamb

Strain:

not specified

Details on test animals or test system and environmental conditions:

no data

Route of administration:

oral: unspecified

Type of inhalation exposure (if applicable):

other: not applicable

Vehicle:

not specified

Details on exposure:

no data

Analytical verification of doses or concentrations:

not specified

Details on analytical verification of doses or concentrations:

no data

Details on mating procedure:

no data

Duration of treatment / exposure:

2, 4, or 6 month

Frequency of treatment:

daily

Duration of test:

no data

Dose / conc.:

0 mg/kg bw/day

Dose / conc.:

25 mg/kg bw/day

Dose / conc.:

50 mg/kg bw/day

No. of animals per sex per dose:

6 animals per dose and treated group; 3 animals per control group

Control animals:

yes, concurrent no treatment

Details on study design:

66 female sheep (18 growing lambs, 18 maiden ewes, 9 pregnant ewes; controls: 9 growing lambs, 9 maiden ewes, 3 pregnant ewes) were orally administered 0 or 50 mg/kg bw daily for 2, 4, or 6 month

Maternal examinations:

no data

Ovaries and uterine content:

no data

Fetal examinations:

no data

Statistics:

no data

Indices:

no data

Historical control data:

no data

Details on maternal toxic effects:

Maternal toxic effects:yes

Details on maternal toxic effects:
- abortion, stillbirth, birth of weak/low-weight lambs, dystokia, and retention of placenta were common features
- external genitalia were infantile and stunted in growing lambs, while they were pale anaemic and dry in maiden ewes
- none of the growing lambs showed signs of estrus
- mammary development was retarded
- the severity of changes was dependent upon the stage of gestation when hypothyroidism was induced

Key result

Dose descriptor:

LOEL

Effect level:

50 mg/kg bw/day (nominal)

Based on:

test mat.

Basis for effect level:

other: maternal toxicity

Details on embryotoxic / teratogenic effects:

Embryotoxic / teratogenic effects:yes

Details on embryotoxic / teratogenic effects:
birth of weak/low-weight lambs

Key result

Remarks on result:

not measured/tested

Abnormalities:

not specified

Developmental effects observed:

not specified

Conclusions:

Thourea was shown to developmental toxicity in females at 50 mg/kg bw/day. The severity of changes was dependent upon the stage of gestation when hypothyroidism was induced.

Executive summary:

66 female sheep (18 growing lambs, 18 maiden ewes, 9 pregnant ewes; controls: 9 growing lambs, 9 maiden ewes, 3 pregnant ewes) were administered orally 0 and 50 mg thiourea/kg body weight daily for 2, 4, or 6 month. External genitalia were infantile and stunted in growing lambs and pale anaemic and dry in maiden ewes. The growing lambs did not show signs of estrus and mammary development was retarded. Thiourea administered to pregnant ewes induced abortion, stillbirth, birth of weak/low-weight lambs, dystokia, and retention of placenta. The severity of changes was dependent upon the stage of gestation when hypothyroidism was induced.

Effect on developmental toxicity: via oral route

Endpoint conclusion:

adverse effect observed

Dose descriptor:

LOAEL

50 mg/kg bw/day

Study duration:

subchronic

Species:

sheep

Quality of whole database:

The quality of the available data is impaired by the fact that none of the available studies establish a dose-response relationship. However, the adverse effects are considered to be related to an impairment of the thyroid function. Therefore, the dose-response data from repeated dose toxicity studies can be used to predict maternal and foetal toxicity.

Effect on developmental toxicity: via inhalation route

Endpoint conclusion:

no study available

Effect on developmental toxicity: via dermal route

Endpoint conclusion:

no study available

Justification for classification or non-classification

The effects on reproductive organs in adult and developing animals are considered to be related to inhibition of thyroid function. Thus, Thiourea is considered to affect fertility and development as a result of hypothyroidism. This conclusion supports the current harmonized classification as toxic to reproduction (Repro Cat 2, CLP), but further investigations are on-going (see above).