I will present a theory of fluctuation interactions between charge-regulated surfaces, i.e. surface or colloidal particles that have a charge responding to the local electrostatic potential. Proteins would be a prime example of such systems. I will show that the well known Kirkwood-Schumaker interaction corresponds to the thermal Casimir (zero frequency van der Waals) interaction for this kind of surfaces. While usually fluctuation interactions are subdominant to the mean field interaction, here this need not be the case, because close to the isoelectric point the mean field interaction vanishes and fluctuations dominate.