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: CONCLUSION : Toxic hepatitis is a potential adverse effect of antithyroid drugs which warrants , as for haematological disturbances , a pre - therapeutic determination and a careful follow - up of relevant biological markers .
### Output Text:
CONCLUSION : Toxic hepatitis is a potential adverse effect of antithyroid drugs which warrants , as for haematological disturbances , a pre - therapeutic determination and a careful follow - up of relevant biological markers . 