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Key value for chemical safety assessment

Effects on fertility

Link to relevant study records
Reference
Endpoint:
fertility, other
Remarks:
based on test type (migrated information)
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:
other: Meets generally accepted scientific standards; well documented and acceptable for assessment.
Qualifier:
no guideline followed
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 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
Type of inhalation exposure (if applicable):
other: not applicable
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)
Remarks:
Doses / Concentrations:
50 mg/kg bw/day
Basis:
nominal conc.
No. of animals per sex per dose:
8 of 12 male animals were administered thiourea
Control animals:
yes, concurrent no treatment
Details on study design:
- Dose selection rationale: no data
- Rationale for animal assignment (if not random): no data
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
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.
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
Additional information

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.

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.
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:
other: 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 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
Remarks:
Doses / Concentrations:
50 mg/kg bw/day
Basis:
nominal conc.
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
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
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
Additional information

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.

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).