It depends on what the underlying health issue is. If it's an issue like Narcolepsy or Diabetes, or if you've had a stroke, then you would need to notify your insurer. If it's just general tiredness, then you do not need to contact them. If you are seeing a consultant, it must be serious though. Without being specific, it's hard to answer. If you're unsure, speak to your insurer and ask their advice.