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: All patients having first - degree relatives affected by thyroid illness had accelerated onset of hypothyroidism ( 3 . 7 years after onset of lithium therapy ) compared with patients without a family history ( 8 . 6 years after onset of lithium therapy ) .
### Output Text:
All patients having first - degree relatives affected by thyroid illness had accelerated onset of hypothyroidism ( 3 . 7 years after onset of <span class="chemical">lithium</span> therapy ) compared with patients without a family history ( 8 . 6 years after onset of <span class="chemical">lithium</span> therapy ) . 