Ammonium perchlorate

Endpoint summary

Description of key information

Ammonium perchlorate was shown to have antithyroid effects, without any other repeat-dose toxic effects, in a Klimisch 2 GLP rat 90-day study at up to 10 mg/kg/day via drinking water. The overall NOAEL was 1 mg/kg/day. All antithyroid effects were demonstrated to be partly to completely reversible within 30 days.
However, the antithyroid effects from a rat 2-generation study were considered to be more reliable for the classification related to repeated exposure, because of this study's higher sensitivity than the 90-day study (40% longer treatment duration: 113-142 days, more individual thyroid data per sex per dose: 56-60 vs. 9-10, 3-fold higher top-dose, presence of highly sensitive populations: pregnant dams and pups) and of the better biological relevance of the dose-relationship (dose-increase, vs. plateauing of effects in the 90-day study). This resulted in a lower NOAEL of 0.3 mg/kg/day.
Two occupational studies were retained as being the key data sources for risk assessment purposes, based on several quality criteria and also the fact that they involved relatively high exposure levels for chronic exposures: Lamm 1999 and Braverman 2005, both leading to similar repeat-dose toxicity systemic (biomonitoring-based) NOAELs of 0.21 and 0.20 mg Ammonium Perchlorate/kg/day for chronic exposure. These data were retained as the most relevant as they avoid inter-species extrapolations, which are considered difficult and full of uncertainty due to the important inter-species differences in physiology of the thyroid regulation and therefore in sensitivity to antithyroid compounds.

Key value for chemical safety assessment

Repeated dose toxicity: via oral route - systemic effects

Link to relevant study records

Reference

Endpoint:

sub-chronic toxicity: oral

Type of information:

experimental study

Adequacy of study:

key study

Study period:

August 1997 - June 1998

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: see 'Remark'

Remarks:

Sufficiently compliant to GLP and testing guideline; adequate coherence between data, comments and conclusions. The few deviations have no impact on the reliability and completeness of the conclusions. Histopathological examinations did not distinguish between hypertrophy and hyperplasia, but this was done later by peer-review.

Reason / purpose for cross-reference:

reference to same study

Qualifier:

equivalent or similar to guideline

Guideline:

OECD Guideline 408 (Repeated Dose 90-Day Oral Toxicity Study in Rodents)

Deviations:

yes

Remarks:

Temperature (°C): 18-26 instead of 19-25. Highest dose chosen with the aim to induce antithyroid effects, but not based on dose-limiting toxicity. No histological examination of Peyer’s patches.

GLP compliance:

yes

Remarks:

No laboratory compliance certificate. No analytical certificate for test item. Perchlorate concentrations were occasionally outside the range validated for stability.

Limit test:

no

Species:

rat

Strain:

Sprague-Dawley

Sex:

male/female

Details on test animals or test system and environmental conditions:

TEST ANIMALS
- Source: Charles River Laboratories
- Age at study initiation: 6-7 weeks (indicated only in published version of report)
- Weight at study initiation (day -1): 195-197 g (M), 151-153 g (F)
- Fasting period before study: no
- Housing: individual suspended stainless steel wire mesh cages
- Diet (e.g. ad libitum): PMI Certified Rodent Chow, ad libitum
- Water (e.g. ad libitum): reverse osmosis water, ad libitum
- Acclimation period: 11 days

ENVIRONMENTAL CONDITIONS
- Temperature (°C): 18-26
- Humidity (%): 30-70
- Air changes (per hr): 10-15
- Photoperiod (hrs dark / hrs light): 12/12

IN-LIFE DATES: From: 1997-9-9 To: 1998-1-9

Route of administration:

oral: drinking water

Vehicle:

water

Details on oral exposure:

VEHICLE
- Nature: reverse osmosis drinking water
- Justification for use and choice of vehicle: high solubility of the test item in water
- Lot/batch no. (if required): not applicable
- Purity: not applicable
- Analyzed and found to contain no detectable nitrate, a potential interference ion for perchlorate analysis

PREPARATION OF TREATED DRINKING WATER:
- Dilution from a stock solution at 50 mg/mL
- Concentration adapted weekly (using body weight and water consumption) to reach the target dose-levels in mg/kg/day: 0.04-129 µg/mL
- Stirred for 30 min before storage, and again before delivery
- Frequency of preparation: at least every five weeks (stock solution), and every week (diluted solutions)
- Storage : refrigerated (diluted solutions)

Analytical verification of doses or concentrations:

yes

Details on analytical verification of doses or concentrations:

Methods:
- validated ion chromatography method
- LOD for perchlorate and nitrate (interference ion): 0.005 µg/mL

Results:
- at 0.05 and 200 µg/mL: concentrations within +/- 10% of nominals
- at 0.05 and 200 µg/mL: stable over 109 days at 21-22°C under light (12h/day)

Comment:
- For the first of 5 concentration measurement dates, group 3 was exposed to lower drinking water perchlorate (nominal and achieved) levels than group 2: it was likely a 10-fold too low nominal concentration setting by error (0.04 µg/mL instead of > 0.4 µg/mL at other preparation dates). Furthermore, this low level was below the minimal concentration demonstrated to be stable (0.05 µg/mL).
- This validation otherwise adequately covers the study's test item use conditions.

Duration of treatment / exposure:

- 14 days for satellite groups
- 90 days for terminal and recovery groups

Recovery: for 30 days after the 90-day treatment period

Frequency of treatment:

Continuous (treated water ad libitum)

Remarks:

Doses / Concentrations:
0.01, 0.05, 0.2, 1 and 10 mg/kg/day
Basis:
other: nominal dose; achieved dose within +/- 10% of nominal

No. of animals per sex per dose:

10M+10F (interim group) + 10M+10F (terminal group): at each dose

+10M+10F (recovery groups): at 0, 0.05, 1 and 10 mg/kg/day (there were thus in total 20M+20F treated for 90 days in these groups)

Control animals:

yes, concurrent vehicle

Details on study design:

- Dose selection rationale: panel of toxicology experts, focused on antithyroid effects, to identify doses with frank effects and a possible NOEL
- Rationale for animal assignment: random
- Rationale for selecting satellite groups: random
- Post-exposure recovery period in satellite groups: 30 days, in "recovery" groups treated for 90 days
- Section schedule rationale: random

Positive control:

Only for micronucleus interpretation (additional animals treated with cyclophosphamide i.p.)

Observations and examinations performed and frequency:

CAGE SIDE OBSERVATIONS: Yes
- Time schedule: once daily
- Mortality and morbidity

DETAILED CLINICAL OBSERVATIONS: Yes
- Time schedule: once weekly

BODY WEIGHT: Yes
- Time schedule for examinations: weekly

FOOD CONSUMPTION: Yes
- Food consumption for each animal: weekly

FOOD EFFICIENCY: No

WATER CONSUMPTION AND COMPOUND INTAKE: Yes
- Time schedule for examinations: weekly, individually
- Calculated as mg/kg/day from body weight data

OPHTHALMOSCOPIC EXAMINATION: Yes
- Time schedule for examinations: day -1, day 85 and day 119.
- Dose groups that were examined: all

HAEMATOLOGY: Yes
- Time schedule for collection of blood: sacrifice (14, 90 or 120 days of study)
- Anaesthetic used for blood collection: No data
- Animals fasted: No
- How many animals: 10 per timepoint

CLINICAL CHEMISTRY: Yes
- Time schedule for collection of blood: sacrifice (14, 90 or 120 days of study)
- Animals fasted: No
- How many animals: 10 per timepoint

URINALYSIS: No

NEUROBEHAVIOURAL EXAMINATION: No (not a deviation as it was done in another study)

Sacrifice and pathology:

GROSS PATHOLOGY: Yes
HISTOPATHOLOGY: Yes

All guideline-required organs and tissues were examined, except for histological examination of Peyer's patches.

Other examinations:

- Estrous cycles, all females daily during 3 weeks before sacrifices at 90 and 120 days*
- Sperm analyses, all males at 90 and 120-day sacrifices*
- Bone marrow micronucleus evaluation, rats at 0 and 10 mg/kg/day and positive controls, at 90 and 120-day sacrifices (NB: does not follow requirements of an in vivo micronucleus study)*
*: low-detail sufficient as specific, more sensitive studies cover the endpoints fertility and mutagenicity

- Serum hormone levels: TSH, T3, T4 in all rats of all groups at 14, 90 and 120 days.

Statistics:

Continuous variables: one-way ANOVA + Tukey-Kramer intergroup comparisons
Chi-square for % females cycled.
All tests at significance level of 5%.

Clinical signs:

no effects observed

Mortality:

no mortality observed

Body weight and weight changes:

no effects observed

Food consumption and compound intake (if feeding study):

no effects observed

Food efficiency:

not examined

Water consumption and compound intake (if drinking water study):

no effects observed

Ophthalmological findings:

no effects observed

Haematological findings:

no effects observed

Clinical biochemistry findings:

no effects observed

Urinalysis findings:

not examined

Behaviour (functional findings):

not examined

Organ weight findings including organ / body weight ratios:

effects observed, treatment-related

Gross pathological findings:

effects observed, treatment-related

Histopathological findings: non-neoplastic:

effects observed, treatment-related

Histopathological findings: neoplastic:

no effects observed

Details on results:

MORTALITY:
One female at 0.05 mg/kg/day died during the recovery period. The cause could not be determined, but was not considered to be related to treatment.

BODY WEIGHT GAIN
Slightly lower in males at 10 mg/kg/day over the first week only. Not considered toxicologically relevant.

FOOD CONSUMPTION
Slightly reduced in all treated groups over first 3 weeks only. Not considered toxicologically relevant.

WATER CONSUMPTION AND COMPOUND INTAKE (if drinking water study)
Consumption reduced in all treated groups at some occasions during first 3 weeks. Not considered toxicologically relevant.

HAEMATOLOGY
Decreased monocytes at 0.05 and 0.2 mg/kg/day in females at 90 days. Not considered toxicologically relevant.

CLINICAL CHEMISTRY
Some slight changes in 0.01 mg/kg/day males at 90 days. Not considered toxicologically relevant.

ORGAN WEIGHTS
Higher absolute and relative thyroid (weighed with parathyroid) weights at 10 mg/kg/day at 14 days (males only) and 90 days (both sexes). Full recovery in the recovery group.
Some changes in absolute but not relative testes and pituitary weights, without lesions. Not considered toxicologically relevant.

GROSS PATHOLOGY
Reddened thyroids in 3/10 males at 10 mg/kg/day at 90 days.

HISTOPATHOLOGY: NON-NEOPLASTIC
Increased incidence of follicular cell hypertrophy and hyperplasia (based on peer-review) in thyroids of both sexes at 14 and 90 days, at 10 mg/kg/day. Generally minimal.

OTHER FINDINGS
- Estrous cycles: no effect.
- Sperm analyses: no effect.
- Bone marrow micronucleus: neither myelotoxic nor genotoxic effects (NB: cyclophosphamide: genotoxicity, without myelotoxicity)
- Thyroid hormone levels: KEY EFFECTS: dose-related reduction in T3 and T4 and increase in TSH, see table below. They occurred at low doses but presented an unusual pattern (plateauing of amplitude + minimal TSH increase for the observed changes in T3/T4), suggesting absence of biological relevance, below 10 mg/kg/day. All effects were reversible.

Dose descriptor:

NOAEL

Effect level:

1 mg/kg bw/day (nominal)

Based on:

test mat.

Sex:

male/female

Basis for effect level:

other: Hormonal effects (T3/T4 reductions, TSH increase) occur at low doses but present an important plateauing of amplitude, suggesting absence of biological relevance, below 10 mg/kg/day. All effects were reversible.

Critical effects observed:

not specified

Mean thyroid hormone level values in adult rats

	At 90 days of treatment						At 120 days (30-day recovery)
Dose-level (mg/kg/day)	0	0.01	0.05	0.2	1	10	0	0.05	1	10
T3 (ng/dL) - males	180	158*	125***	123***	122***	117***	203	214	212	185
T3 (ng/dL) - females	170	143**	143**	137***	123***	122***	224	216	215	197*
T4 (µg/dL) - males	5.1	4.4**	3.7***	3.5***	3.5***	2.9***	4.9	4.0***	4.0***	3.4***
T4 (µg/dL) - females	4.4	3.5***	3.4***	3.4***	3.2***	3.0***	3.5	3.4	3.3	3.1
TSH (ng/mL) - males	16.2	16.9	17.8	19.0**	19.1**	19.1**	20.9	21.3	22.2	22.8
TSH (ng/mL) - females	16.5	16.8	16.8	17.4	17.7	20.0***	13.1	15.5**	15.8**	16.0***

*: p<0.05; **: p<0.01; ***: p<0.001; N= 9-10 rats/sex/dose/timepoint

 

Incidence of histopathological thyroid follicle lesions in adult rats (peer-reviewed data)

	At 90 days of treatment						At 120 days (30-day recovery)
Dose-level (mg/kg/day)	0	0.01	0.05	0.2	1	10	0	0.05	1	10
Hypertrophy - males	1/10	2/10	0/10	2/10	3/10	8/10	2/10	4/10	2/10	0/10
Hypertrophy - females	0/10	0/10	3/10	2/10	1/10	5/10	0/10	0/10	1/10	0/10
Hyperplasia - males	0/10	0/10	0/10	0/10	0/10	4/10	1/10	3/10	1/10	0/10
Hyperplasia - females	0/10	0/10	0/10	0/10	0/10	3/10	0/10	0/10	0/10	0/10

no statistical analysis performed

Conclusions:

After 90-day oral (drinking water) treatment of rats with ammonium perchlorate, thyroid hormone levels were affected at low doses (T3/T4 decreases, TSH increases) but presented an unusual pattern (plateauing of amplitude + minimal TSH increase for the observed changes in T3/T4), suggesting absence of biological relevance, below 10 mg/kg/day. Non-hormonal adverse effects were only observed at 10 mg/kg/day: thyroid redness, increased thyroid weight and thyroid follicle hypertrophy and minimal hyperplasia.

Executive summary:

Ammonium perchlorate was provided to groups of 10 male and 10 female rats via drinking water, at dose-levels of 0 (control), 0.01, 0.05, 0.2, 1 and 10 mg/kg/day for 14 and 90 days. Additional 10 rats per sex were treated in the same way at 0, 0.05, 1 and 10 mg/kg/day and kept for a 30-day treatment-free period. Data for the 14-day treatment are ignored here.

• At 10 mg/kg/day, the only toxicologically relevant effects were related to thyroids. Thyroids were reddened at 90 days (3/10 males) and of higher absolute and relative weights than in controls, with follicle hypertrophy and hyperplasia (generally minimal). All these changes were completely reversible in 30 days. Marked thyroid hormone changes were noted in both sexes. Serum levels of T3 and T4 were significantly lower than in controls (-28% to -43%), respectively with almost complete (-9% to -12%) and incomplete (-11% to -31%) recoveries. Serum levels of TSH were significantly higher than in controls (+18% to +21%), with no clear recovery (+9% to +22%).

• At 0.05 to 1 mg/kg/day, poorly dose-related (plateauing) effects on thyroid hormones occurred: T3 and T4 reductions (-16% to -32%) and minimal TSH increases (at most +18%), which seems not coherent with the usual high compensation by TSH for moderate changes in T3/T4. The biological relevance seemed therefore low. Recovery was evidenced for T3, T4 and TSH.

• At 0.01 mg/kg/day, T3 and T4 were still significantly lower (-12% to -20%) while TSH was unaffected. Recovery was not assessed.

Endpoint conclusion

Endpoint conclusion:

adverse effect observed

Dose descriptor:

NOAEL

0.3 mg/kg bw/day

Study duration:

subchronic

Species:

rat

Quality of whole database:

The most reliable study is a 90-day repeated dose toxicity study in rats via drinking water, sufficiently compliant to GLP and OECD guideline 408. A non GLP 14-day preliminary study is also available but not sufficient to complete repeated dose toxicity endpoint. A 30 to 160-day repeated dose toxicity study in mice is used as supporting study but was performed prior to GLP and guidelines and it suffers from many limitations (unclear methods and results). A 2-generation study contains key (most relevant) and reliable repeat-dose toxicity data for oral route. Two key studies on workers (see section 7.10) were retained as most reliable for DNEL derivation.

Repeated dose toxicity: inhalation - systemic effects

Endpoint conclusion

Endpoint conclusion:

no study available

Repeated dose toxicity: inhalation - local effects

Endpoint conclusion

Endpoint conclusion:

no study available

Repeated dose toxicity: dermal - systemic effects

Endpoint conclusion

Endpoint conclusion:

no study available

Repeated dose toxicity: dermal - local effects

Endpoint conclusion

Endpoint conclusion:

no study available

Additional information

Supportive, Klimisch 4, non-GLP and non-guideline studies:

• In a mouse study (Gauss 1972) involving 30 to 160 days of oral treatment with potassium perchlorate at a high dose-level, 1420 mg KClO4/kg/day, equivalent to 1200 mg ammonium perchlorate/kg/day (dose calculated over 30 days), induced progressive clinical impairment of general condition, neuromotricity, visible goiter due to severe antithyroid activity (notably follicle hypertrophy, hyperplasia, adenoma without malignancy, solid degeneration of the thyroid) and ultimately death. A progressive recovery potential was demonstrated for short treatment (35 days). No lower dose was tested (no NOAEL). No read-across was performed due to the important limitations of this study. In a preliminary study (Caldwell 1995), ammonium perchlorate was provided to rats via drinking water, at dose-levels of 0 (control), 0.11/0.12, 0.44/0.47, 1.1/1.2, 2.3/3.1, 4.3/4.9, 11/11 and 22/25 (M/F) mg/kg/day for 14 days. The unit of dose-levels (ammonium perchlorate or perchlorate ion) was unclear. There was no effect on body weight gain or water consumption; clinical signs were not investigated; a whole range of antithyroid effects defined NOAELs of 0.44/0.12 mg/kg/day (M/F). The latter were not revised by CSR author so they should not be compared with NOAELs from longer studies.

Reliable guideline repeat-dose study:

• Ammonium perchlorate was provided to groups of 10 male and 10 female rats via drinking water, at dose-levels of 0 (control), 0.01, 0.05, 0.2, 1 and 10 mg/kg/day for 14 and 90 days. Additional 10 rats per sex were treated in the same way at 0, 0.05, 1 and 10 mg/kg/day and kept for a 30-day treatment-free period. Data for the 14-day treatment are ignored. At 10 mg/kg/day, the only toxicologically relevant effects were related to thyroids. Thyroids were reddened at 90 days (3/10 males) and of higher absolute and relative weights than in controls, with follicle hypertrophy and hyperplasia (generally minimal). All these changes were completely reversible in 30 days. Marked thyroid hormone changes were noted in both sexes. Serum levels of T3 and T4 were significantly lower than in controls (-28% to -43%), respectively with almost complete (-9% to -12%) and incomplete (-11% to -31%) recoveries. Serum levels of TSH were significantly higher than in controls (+18% to +21%), with no clear recovery (+9% to +22%). At 0.05 to 1 mg/kg/day, poorly dose-related (plateauing) effects on thyroid hormones occurred: T3 and T4 reductions (-16% to -32%) and minimal TSH increases (at most +18%), which seems not coherent with the usual high compensation by TSH for moderate changes in T3/T4. The biological relevance seemed therefore low. Recovery was evidenced for T3, T4 and TSH. At 0.01 mg/kg/day, T3 and T4 were still significantly lower (-12% to -20%) while TSH was unaffected. Recovery was not assessed. Conclusion: thyroid hormone levels were affected at low doses (T3/T4 decreases, TSH increases) but presented an unusual pattern (plateauing of amplitude + minimal TSH increase for the observed changes in T3/T4), suggesting absence of biological relevance, below 10 mg/kg/day. Non-hormonal adverse effects were only observed at 10 mg/kg/day: thyroid redness, increased thyroid weight and thyroid follicle hypertrophy and minimal hyperplasia. Therefore the NOAEL was the next lower dose of 1 mg/kg bw/day.

A 2-generation study contained key (most relevant) and reliable repeat-dose toxicity data for oral route:

• A two-generation study was conducted in rats exposed to ammonium perchlorate at 0, 0.3, 3.0 and 30 mg/kg/day. At 30 mg/kg/day, there was a clear antithyroid effect in P1 and F1 adults and F1 and F2 pups: lower serum T3 (female pups) or T4 levels (male adults), higher serum TSH level (adult P1 males, adult F1 males and females), higher absolute and (when determined) relative thyroid weights (in all conditions), minimal to marked thyroid follicle hypertrophy and hyperplasia (in all conditions), thyroid follicle adenomas in two males. There was no general parental toxicity: the maximal tolerated dose was not reached, limiting sensitivity of the study according to summary author. No effects on reproduction and in particular fertility and pup growth were noted. At 3 mg/kg/day, no relevant hormonal effects were noted; thyroid weight was affected in adults only; however, incidence and severity of thyroid follicle lesions was increased in all conditions (except possibly in F1 adult females were it was not obvious). At 0.3 mg/kg/day, no relevant hormonal effects were noted; thyroid weight was minimally increased in F1 adult females but this was considered not biologically relevant; no relevant effect on thyroid lesions. Conclusion: when taking into account antithyroid effects, the parental NOAEL was 0.3 mg/kg/day. The offspring NOAEL was revised to 0.3 mg/kg/day, as opposed to original report conclusion (3 mg/kg/day), based on anti-thyroid effects at higher doses.

Two key studies on workers (see section 7.10), retained as most reliable for DNEL derivation due to inter-species differences :

Two occupational studies were run in the same ammonium perchlorate plant in the US, with chronic (mainly respiratory) exposure at 42h/week. In both studies, controls were imperfect (suspected tendency to hypothyroidism) so worker data were compared with normal limits, enabling robust conclusion. Both studies lacked histological data, impossible to gather in worker studies.

In Braverman 2005, among 29 male workers, no effect on T3, T4 and TSH, thyroid volume (as a surrogate for histological investigations), thyroglobulin and urinary iodine excretion occurred at a median absorbed dose of 0.167 mg perchlorate ion/kg/day (based on serum and urine perchlorate concentrations), over 5.9 years (median). Free T4 index was above normal limits in all groups but this is the opposite to the expected effect of perchlorate. Decreased radioiodine uptake and increased serum and urinary perchlorate levels were noted (non-adverse effects). Median exposure corresponded to 0.20 mg ammonium perchlorate/kg/day and was the NOAEL for 42h/week chronic exposure.

In Lamm 1999, among 35 male and 2 female workers, T3, T4, red and white blood cell and platelet counts, urine iodine, were all within normal limits at up to individual absorbed doses of 69 mg perchlorate/shift, for a 60th exposure percentile of 5 years (chronic exposure). TSH was above normal limits (up to 8.2 mUI/L) in about half of the 15 high-dose workers, but as the same observation (TSH up to 8.4 mUI/L) was made in controls, a causal relationship with perchlorate exposure seemed unlikely. Some parameters were not perfectly within normal limits in exposed groups but lacked dose-relationship (thyroid hormone binding ratio, free T4 index, thyroid peroxidase antibody titers). Non-adverse increases in blood and urine perchlorate levels were noted. The high-dose group's 25%tile of absorbed dose was 24.68 mg perchlorate/shift, equivalent to 0.21 mg Ammonium perchlorate/kg/day, retained as NOAEL for 42h/week chronic exposure.

The corresponding most relevant NOAEL value, 0.20 mg/kg/day as long-term systemic value, can not be entered in IUCLID due to software limitations.

Justification for selection of repeated dose toxicity via oral route - systemic effects endpoint:
This is the most reliable and the longest study available to assess repeated dose toxicity in laboratory animals

Repeated dose toxicity: via oral route - systemic effects (target organ) glandular: thyroids

Justification for classification or non-classification

Because of the higher sensitivity (see above) of the 2 -generation study when compared to the 90 -day study, the classification criteria were applied to the 2 -generation study results.

1) Ammonium perchlorate effects qualitatively meet two of the criteria warranting T/Xn R48 or STOT RE classifications:

Type of effect (CLP, § 3.9.2.7.3)	Associated with AP?	Comments
(b) significant functional change in organ systems	Depends on dose and study	The toxicologically significant functional effect (in animals) is the impairment of thyroid function defined by hormonal changes in TSH, T3 and T4. They occurred in valid 90-day (showing reversibility) and 2-generation studies in rats. All underlying mechanistic steps (e.g. inhibition of iodine uptake) do not have,per se, any consequence of health before progression into hypothyroidism along increasing exposure.
(c) consistent significant adverse effects at clinical biochemistry	Depends on dose and study

2) The two aforementioned effects did not meet the criteria for non-classification which are described in CLP, § 3.9.2.8.1. Notably, TSH increase can be seen as an adaptative effect, but it only occurs in case of significant effect on T3 and/or T4 synthesis. Therefore, as is the case in therapeutic approach of human subclinical hypothyroidism, it must be considered as adverse.

3) Comparison with repeat-dose toxicity classification thresholds:

- The critical effect occurred at 30 mg/kg/day in a 2-generation study, in particular in F1 male adult rats treated for 125-142 days: a 106% increase (doubling) of TSH with a 26% reduction in T4.

- For this treatment duration of 125-142 days, the mean of 133.5 days was used with Haber’s rule to recalculate (i.e. reduce) the thresholds dose-levels:

Dose-level (mg/kg/day) at which the critical effect occurs	[0-3.4]	]3.4-6.8]	]6.8-34]	]34-68]	>68
Directive 67/548/EEC	T, R48/23/25	Xn, R48/20/22		NC
Regulation (EC) No 1272/2008	STOT RE 1		STOT RE 2		NC

- It was therefore concluded that these effects warranted the following classifications:

- for Directive 67/548/EEC: Xn, R48/20/22

- for Regulation (EC) No 1272/2008: STOT RE 2, H373(thyroid)

The relevant exposure routes for these classifications are discussed below:

-      The oral route is relevant since these effects were evidenced in oral studies in animals.

-      The respiratory route is considered to be relevant, in a worst-case and weight-of-evidence approach, because sufficiently fine granulometries may be enough absorbed to cause the same effects as by oral route (in the absence of metabolization, route-specificity should be limited to differences in extent of absorption and in distribution kinetics).

-      The dermal route is clearly not a relevant route for this classification, since the minor dermal absorption (at most 1.85%, based on a read-across from sodium chlorate) enables to exclude a probability of long-term effects after repeated dermal exposure. However this route can not be ignored for exposure assessment.

-      According to Directive 67/548/EEC, the critical effect considered must be « likely to be caused by repeated or prolonged exposure by an appropriate route »: oral and inhalative routes are considered to be routes likely to elicit this effect so R48/20/22 is chosen.

-      Oppositely, Regulation (EC) No 1272/2008 requires to « state route of exposure if it is conclusively proven that no other routes of exposure cause the hazard ». As two exposure routes may elicit the effects warranting classification, and as there is no experimental « proof » that the effect occurs by inhalation in animals or humans, the route shall not be detailed for the classification STOT RE 2.

This conclusion to classify for repeated-dose toxic effects can be balanced by the fact that perchlorate has been used for years as a therapy in hyperthyroidism in patients with Grave’s disease: perchlorate’s effect on the thyroid can be seen either as an adverse (presently, in eu- or hypothyroid persons) or as desirable effect (in hyperthyroid persons) depending on the exposed person and the regulatory frame. It can also be recalled that the effect of perchlorate on the thyroid is easy to monitor (follow-up of thyroid hormones), quickly reversible (with even a reversibility during week-ends: see Endpoint summary on human data) and antagonizable (by iodine supplementation).