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: OUTCOME MEASURES : Laboratory analyses of calcium , magnesium and thyroid - stimulating hormone levels performed before beginning lithium therapy and at biannual follow - up .
### Output Text:
OUTCOME MEASURES : Laboratory analyses of <span class="chemical">calcium</span> , <span class="chemical">magnesium</span> and thyroid - stimulating hormone levels performed before beginning <span class="chemical">lithium</span> therapy and at biannual follow - up . 