A medical certificate is a written document from a doctor with a medical opinion on what a patient can or cannot do, for example, in terms of work.
Your GP may not issue a medical certificate about your fitness to work, or the kind of work you could or could not do. Nor may the GP give a note for the employer. Only an occupational physician is authorised to give an opinion on this. Do you need a note for your employer or want to know whether you can work (temporarily)? Then contact the company doctor via your employer or the occupational health and safety service.