A medical source statement is an opinion from your doctor regarding the nature and severity of your limitations due to your medically-determinable impairments. A medical source statement is often simply a form that is completed and signed by your doctor setting forth the doctor’s opinion regarding your functional limitations.
The SSA must give controlling weight to a your treating doctor’s opinion, if it is “well-supported by medically acceptable clinical and laboratory diagnostic techniques and is not inconsistent with the other substantial evidence in your case record.” See 20 CFR 404.1527(d)(2). This deference due to a medical source statement makes it very important for a claim.
Here are the forms I use. The first form is used to describe physical limitations. The second form is used to describe mental limitations.