In the US, can I refinance my FHA loan so I don't have to pay PMI (Private Mortgage Insurance)?

I purchased a house 4 years ago using an FHA loan. I have very good-excellent credit and I'm at a very low risk. Is it possible to refinance so I no longer have to pay PMI?
Update: MORE DETAILS: My home has gone up significantly in value, about 33%. If I got it appraised, would that help? Thanks.
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