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: PATIENTS : We investigated 102 untreated patients with hyperthyroidism due to Graves ' disease for the presence of MPO - ANCA , and for the development vasculitis after starting PTU therapy .
### Output Text:
PATIENTS : We investigated 102 untreated patients with hyperthyroidism due to Graves ' disease for the presence of MPO - ANCA , and for the development vasculitis after starting <span class="chemical">PTU</span> therapy . 