You do not usually need medical clearance to travel. You also do not need to discuss the details of your disability, but there are exceptions. For example, if you use a wheelchair or need oxygen, this can involve more discussion about your disability. An airline may want you or your doctor to talk to its medical department about travelling with a personal attendant, using oxygen or requests for additional seating due to a disability which may include obesity. Be clear on exactly what your service needs are and ask about the services you can get.
Note: Airlines have rules about how they handle oxygen supplies, and may not allow yours on board.