In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: RESULTS : Four patients ( 1 . 7 % ) were identified with toxic hepatitis which could reasonably be attributed to the use of antithyroid agent .
### Output Text:
RESULTS : Four patients ( 1 . 7 % ) were identified with toxic hepatitis which could reasonably be attributed to the use of antithyroid agent . 