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: Over the following 9 days , the symptoms improved and plasma bilirubin levels were normal after 12 weeks without methimazole .
### Output Text:
Over the following 9 days , the symptoms improved and plasma <span class="chemical">bilirubin</span> levels were normal after 12 weeks without <span class="chemical">methimazole</span> . 