Easy question, complicated answer.
Common pattern hair loss is known to be triggered by three factors:
1) Genetics
2) Age
3) Circulating hormones
A number of investigators, including those in our lab and elsewhere, believe that stress constitutes a fourth less well-appreciated factor. Here, aside from the well-described androgen hormone 5 alpha-dihydrotestosterone (DHT) other biochemical triggers may negatively influence the health, vitality and growth rate of susceptible scalp hair.
These include the stress hormone cortisol. Cortisol is produced by the adrenal gland in the zona fasciculata, the second of three layers comprising the outer adrenal cortex. This release is controlled by the hypothalamus, a part of the brain. The secretion of corticotropin-releasing hormone (CRH) by the anterior hypothalamus triggers pituitary secretion of adrenocorticotropic hormone (ACTH). ACTH is carried by the blood to the adrenal cortex, where it triggers glucocorticoid secretion.
Additional biochemical agents which could negatively impact hair growth include reactive oxygen species (ROS) as well as reactive nitrogen species (RNS). In the skin, several pathologic disorders have been strongly linked to oxidative stress, a homeostatic dysfunction mediated by ROS and RNS. These include actinic keratosis ---> which may ultimately lead to skin cancer.
In the hair follicle oxidative stress has been shown to influence programmed cell death, otherwise known as apoptosis. Apoptosis constitutes a strong negative determinant of hair production, inasmuch as susceptible substructures of the follicle, including dermal papilla cells, are the key precursors of hair shaft formation.
More work will be required to fully draw out the specific genetic and epigenetic hair loss factors which contribute to phenotype. At the present time however, a growing body of evidence shows that stress certainly plays a role in pattern hair loss.
