If you're hard of hearing then just like traditional broadcasts, you can turn on subtitles when using the catch-up TV service. Although it's worth noting that all programmes off subtitles. Here's how to turn the function on.
Visit the iPlayer home page and select the programme you want to start streaming.
Scroll down to underneath the programme window. If the show has subtitles available, a small S will appear on the right-hand side once the programme has started playing. Click this to turn subtitles on. To turn them off, simply press it again.