Human medicine distinguishes between hypertrichosis and hirsutism. Both terms refer to a condition associated with excessive hair growth. Hirsutism is characterized by excessive hair growth in those parts of the body where hair normally is absent or minimal. In humans, hirsutism is usually caused by excess male hormones (androgens) or hair follicles that are oversensitive to these hormones. Hypertrichosis is characterized by increased hair growth in areas of skin that are normally already covered with hair. The condition is usually hereditary or caused by certain types of drugs.
Hirsutism is a clinical sign of Cushing’s disease in humans. For a long time veterinary science did not distinguish between Cushing’s disease and PPID so the term hirsutism was used for the symptom of excessive hair growth in horses.
The cause of excessive and abnormal hair growth in PPID horses is also hormonal, however not by problems with androgens but with cortisol. The typical discoloration of the hair that occurs in hypertrichosis is absent in hirsutism.
In recent years, excessive hair growth in PPID horses is preferably referred to as hypertrichosis.