We report on a novel pump-probe photothermal system based on the self-mixing effect. Experimental observations using a single mode laser show that the photothermal lens distorts the self-mixing beam propagation, thus modulating the light re-injected in the laser cavity and producing fluctuations in the output power. Moreover, a linear dependence of power amplitude oscillations as a function of the photothermal parameters is observed, enabling determination of Fe(II) concentrations at parts-per-trillion levels. A theoretical model is presented to describe the self-mixing signal as a function of the photothermal parameters, which explains essential features of the experimental results. The proposed thermal-lens self-mixing system provides much higher accuracy and sensitivity over a wide set of measurements as compared to the conventional thermal lens spectroscopy.