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: The literature related to antithyroid drug side effects and the mechanisms for their occurrence are reviewed and the efficacy and complications of thyroidectomy and radioiodine compared to those of antithyroid drugs .
### Output Text:
The literature related to antithyroid drug side effects and the mechanisms for their occurrence are reviewed and the efficacy and complications of thyroidectomy and radioiodine compared to those of antithyroid drugs . 