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### Input Text: When the androgens were withdrawn , the patient ' s physical changes , symptoms , sleep study , and supraglottic resistance all returned to normal .
### Output Text:
When the <span class="chemical">androgens</span> were withdrawn , the patient ' s physical changes , symptoms , sleep study , and supraglottic resistance all returned to normal . 