A few days ago, Niall posted on Snapchat that he was getting his vocal chords checked at the doctor. It seems that he's back at the doctors for a new reason, which may or may not be completely unrelated. He posted this on Snapchat:
Given that his solo career is due to begin any time now, it's great that he's getting checked out and fixing any underlying issues that may affect his performance/voice! Health is always a number 1 priority.