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: Thyroid - stimulating hormone , magnesium , and potassium levels were within normal limits , urine drug screen was negative , and alcohol use was denied .
### Output Text:
Thyroid - stimulating hormone , <span class="chemical">magnesium</span> , and <span class="chemical">potassium</span> levels were within normal limits , urine drug screen was negative , and <span class="chemical">alcohol</span> use was denied . 