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: We measured serum thyroxine and urine - concentrating ability ( Umax ) in response to desmopressin ( DDAVP ) in 85 patients receiving lithium .
### Output Text:
We measured serum <span class="chemical">thyroxine</span> and urine - concentrating ability ( Umax ) in response to desmopressin ( DDAVP ) in 85 patients receiving <span class="chemical">lithium</span> . 