It is found much more commonly in women than in men. Light brown skin types from regions of the world with intense sun exposure are much more prone to the development of melasma. Identical twins have been reported to develop melasma, while other siblings under similar conditions did not. Another major factor is exposure to sunlight. Ultraviolet radiation can cause peroxidation of lipids in cellular membranes, leading to generation of free radicals, which could stimulate melanocytes to produce excess melanin.
Hormonal influences play a role in some individuals.
The exact mechanism by which pregnancy affects melasma is unknown. One study found a four-fold increase in thyroid disease in patients with melasma when compared to matched controls. A case report of two women who developed melasma after sudden and profound emotional stress implicated the release of MSH by the hypothalamus as a cause. Exactly which hormones and what mechanisms are involved in the development of melasma are yet to be determined. Genetic and hormonal influences in combination with ultraviolet radiation are the two most important causes of melasma, yet phototoxic and photoallergic medications and certain cosmetics have been reported to cause melasma in rare instances.