Why does CAS 51-26-3 appear in more than one EU cosmetic annex?

A substance identified by CAS 51-26-3 may appear in multiple EU cosmetic regulation annexes when it is subject to different conditions in different contexts — for example, restricted under Annex III with specific conditions, while also appearing in Annex IV as an approved colorant under separate entry conditions. Each entry must be reviewed individually.

How do I check if my formula contains a substance with CAS 51-26-3?

Use the INCI Checker on CosIng Checker with INCI names, CAS numbers or EC numbers. Paste CAS 51-26-3 to screen it directly, then review every linked annex record and supplier document.

CAS 1 record

CAS 51-26-3

Q: How do I use CAS 51-26-3 for compliance screening?

Cross-reference CAS 51-26-3 against your raw material specification sheets and CoA documents. When it appears in Annex II, verify absence. When it appears in Annex III, verify that concentration limits and product type conditions are met. All annex entries sharing this CAS number are listed on this page.

CAS 51-26-3 matches 1 EU annex record across Annex II–VI of Regulation 1223/2009.

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Chemical identity and properties

PubChem 2D chemical structure for CAS 51-26-3

CAS 51-26-3PubChem CID 5804

3,3',5-Triiodothyropropionic acid

Thyropropic acid is an aromatic ether. — ChEBI

IUPAC name: 3-[4-(4-hydroxy-3-iodophenoxy)-3,5-diiodophenyl]propanoic acid

Molecular formula: C15H11I3O4
Molecular weight: 635.96 g/mol
Exact mass: 635.7792 Da
XLogP3: 4.8
Topological polar surface area: 66.8 Å²
Hydrogen-bond donors: 2
Hydrogen-bond acceptors: 4
Covalent units: 1
Defined stereocentres: 0

Also known as

4-(4-Hydroxy-3-iodophenoxy)-3,5-diiodobenzenepropanoic acidRefChem:910529THYROPROPIC ACID3,3',5-Triiodo Thyropropionic AcidAcido tiropropicoTriopron; TripropAcide thyropropiqueThyropropic acid [INN]

16 more reported names

3-(4-(4-hydroxy-3-iodophenoxy)-3,5-diiodophenyl)propanoic acidLevothyroxine Impurity 19OK7481S22O4-(4-Hydroxy-3-iodophenoxy)-3,5-diiodohydrocinnamic acid3-[4-(4-hydroxy-3-iodo-phenoxy)-3,5-diiodo-phenyl]propanoic acidAcidum thyropropicumUNII-OK7481S22OAcido tiropropico [INN-Spanish]Acide thyropropique [INN-French]Acidum thyropropicum [INN-Latin]Benzenepropanoic acid, 4-(4-hydroxy-3-iodophenoxy)-3,5-diiodo-C15H11I3O4cid_5804triiodothyropropionic acid, 5THYROPROPIC ACID [MI]CHEBI:135860

Open full PubChem record PubChem annotations 3D structure image

External database identifiers

ChEBI:CHEBI:135860
ChEMBL:CHEMBL291053
EPA DTXSID:DTXSID20198998
PubMed:18279024
PubMed:9294866

Retrieved from NIH PubChem on 2026-08-31 by checksum-valid CAS matching. Chemical properties do not replace the regulatory decision on this page.

Records for CAS 51-26-3

1 total

Annex

Entry

Thyropropic acid (INN) and its salts

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How CAS 51-26-3 is used on this site

CAS 51-26-3 is the Chemical Abstracts Service identifier used to pin this page to a specific substance. In the current dataset, 1 EU annex record shares this identifier, which helps you cross-check whether the same substance appears under one regulatory context or several.

Use this page when a supplier specification, SDS or raw-material dossier gives you CAS 51-26-3 and you need to see every linked Annex II–VI record in one place. The entry cards above are the quickest route to concentration limits, warnings, annex placement and product-type conditions.

Frequently asked questions

A substance identified by CAS 51-26-3 may appear in multiple EU cosmetic regulation annexes when it is subject to different conditions in different contexts. For example, a substance might be restricted under Annex III with specific conditions, and the same CAS might also appear in Annex IV as an approved colorant under separate entry conditions. Each entry must be reviewed individually.

Cross-reference the CAS number against your raw material specification sheets and CoA documents. When a CAS appears in Annex II, verify absence. When it appears in Annex III, verify that concentration limits and product type conditions are met in your formulation. All annex entries sharing this CAS number are listed above.

A CAS number is assigned by the Chemical Abstracts Service (USA) and is globally recognised. An EC number is a European Commission identifier from inventories such as EINECS, ELINCS or the NLP list. Both identifiers may appear in EU cosmetic annex entries. Some substances have both, some have only one, and some older entries may have neither.

The INCI Checker accepts ingredient names, CAS numbers and EC numbers. Check this CAS number directly, then verify the result against supplier specification sheets and every linked annex condition.

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