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: Women over 60 years of age were more often affected by hypothyroidism than women under 60 years of age ( 34 . 6 % versus 31 . 9 % ) .
### Output Text:
Women over 60 years of age were more often affected by hypothyroidism than women under 60 years of age ( 34 . 6 % versus 31 . 9 % ) . 