Transformer losses have two sources-copper loss and magnetic loss. Copper losses are caused by the resistance of the wire (I2R).
Magnetic losses are caused by eddy currents and hysteresis in the core.
Copper loss is a constant after the coil has been wound and therefore a measurable loss.
Hysteresis loss is constant for a particular voltage and current.
Eddy-current loss, however, is different for each frequency passed through the transformer.