Just as they have suggested IHC(markers ) should be done to confirm the diagnosis . Obviously the post operative report should be considered , what ever if the report of post operative IHC we have to treat accordingly . It’s not due to any mistake in the pre operative report , the few cells taken in pre operative FNAC may not be enough to identify th features . In addition to the IHC , if the patient is confirmed to have medullary thyroid disease further investigation including RET mutation, SR Calcitonin and CEA should be checked . Also please mention any family history of cancer or the patient itself had any other cancer before .