Object of this paper is to emphasize some dissipative aspects of steels in high cycle fatigue from the thermographic point of view by relating anelastic and microplastic behaviours to loading levels respectively below and above the fatigue limit. To this aim, the Two Curves Thermographic Method has been modified by using both a parabola and a power curve as regression laws instead of the traditional linear ones. Two new thermal parameters have also been considered, the thermal increment and the subtended area by the thermal profile at a very low number of cycles. Experimental data referred to C45 standard and 25CrMo4 notched steel specimens have been processed using both original and modified Two Curves methodologies and both traditional and new thermographic parameters. All changes introduced in the present paper provide good results both in terms of fatigue limit values and for as concerns the rapidity of the fatigue tests.