  In this work, we consider the thermal correction to the hyperfine interaction
in hydrogen, deuterium, and the $^3$He$^+$ ion. This correction is effectively
described by one-loop Feynman graphs in the framework of the quantum
electrodynamics theory for bound states at a finite temperature. A simple
analysis shows the importance of the obtained results for future prospects for
measuring hyperfine splitting. In addition, the application for testing the
time variation of fundamental constants is briefly discussed.
