Although referrals are usually made by a PCP, from what I’ve experienced they can be made by any type of physician. What is important is whether or not your referred doctor is willing to accept you as a patient.

I was once referred by my chiropractor to a physical rehab doctor who refused to accept a referral from a chiropractor.

Another time I was referred to a surgeon who required copies of all my imaging and test results in addition to a written referral from my PCP. After reviewing my file, the surgeon gave me two minutes of his time to say that I needed to go elsewhere as my condition was not that significant for his skills.