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: OBJECTIVE : This study was conducted to assess the occurrence of hepatic adverse effects encountered with antithyroid drugs .
### Output Text:
OBJECTIVE : This study was conducted to assess the occurrence of hepatic adverse effects encountered with antithyroid drugs . 