What is the most common cause of hirsutism and menstrual irregularity in women under 50?

Polycystic ovarian syndrome (PCOS) is the most prevalent cause of hirsutism and menstrual irregularity in women under 50. This condition is characterized by a combination of symptoms, including excess hair growth in areas typical of male-pattern hair distribution (hirsutism), irregular menstrual cycles, and often, polycystic ovaries visible on ultrasound.

The hormonal imbalance seen in PCOS typically involves elevated levels of androgens, which directly contribute to increased hair growth and can also impact the regularity of menstrual cycles. Women with PCOS may also experience other symptoms such as weight gain and acne due to this hormonal disruption.

By understanding PCOS as the primary factor leading to both hirsutism and menstrual irregularities, it's clear why it stands out among other conditions such as thyroid dysfunction, adrenal insufficiency, and hormonal therapy, which are less frequently the root causes of these specific symptoms in this demographic.