The present disclosure provides a method for determining whether a patient's immune system can be modified through stimulation of thymus function. In one embodiment, sex steroids are ablated in the patient, and the resulting production of thymic factors is monitored. In particular, the level of these factors in the patient's blood stream is observed. In another embodiment, the level of new T cells is monitored. An early response, such as within hours or days of the ablation, indicates that the patient's thymus is disposed to regeneration through sex steroid ablation.