The problem of validity of active dynamic thermography (ADT) imaging in medical diagnostics is discussed. Practical applications of ADT in medicine are known where synthetic pictures are defined and calculated for objective, quantitative data visualization as well as first attempts of thermal tomography based on ADT are published. In both cases the same sources of errors are influencing quality of measurements and reconstruction data. The main limitations of the methods are here highlighted. The ways, how to avoid misunderstanding of the measurement data are commented. Practical examples of using SW and LW cameras in ADT diagnostics are discussed.