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: Thyroid - stimulating hormone was found to be 168 uIU / mL ( nl . 0 . 3 - 5 uIU / mL ) and free thyroxine ( FT4 ) was < 0 . 2 ng / dL ( nl . 0 . 8 - 1 . 8 ng / dL ) .
### Output Text:
Thyroid - stimulating hormone was found to be 168 uIU / mL ( nl . 0 . 3 - 5 uIU / mL ) and free <span class="chemical">thyroxine</span> ( FT4 ) was < 0 . 2 ng / dL ( nl . 0 . 8 - 1 . 8 ng / dL ) . 