Mental illnesses, such as depression or post-traumatic stress disorder (PTSD), often impact an individual's physiological reaction to stress via their cortisol response. We examine the regulation of cortisol within the hypothalamic-pituitary-adrenal (HPA) axis as a dynamic biosystem. We investigate the difference between the cortisol regulation of people with PTSD and healthy controls. Additionally, we investigate models for potential therapies based on proportional-derivative (PD) control to normalize and improve the stress response of those with PTSD. Such a design may be relevant to improving the treatment of PTSD, as current medical interventions are frequently imprecise and may result in unwanted side effects. The use of a PD controller in administration of hormone therapy, in contrast, can be highly measurable and patient specific.Clinical Relevance- This paper offers an approach to treating post-traumatic stress disorder (PTSD) via stabilizing an individual's endocrinal regulation as a potential future alternative to current pharmaceutical practices.