Brown patches on the lower leg are often caused by conditions related to hyperpigmentation, such as excess levels of melanin in the skin, according to the American Osteopathic College of Dermatology. Also known as liver spots, brown patches can also be caused by sun damage, acne or freckles.
Many women in early stages of pregnancy also experience brown patches of skin due to hormonal changes within the body, according to the American Osteopathic College of Dermatology. Pregnancy triggers overproduction of melanin in the skin and darkens the appearance of skin on the legs, abdomen and even face. Women who take birth control pills may also notice brown patches on the skin due to the body's inability to adjust to increased hormone levels.
Outside environmental conditions may also cause skin discoloration, according to the American Osteopathic College of Dermatology. Sun exposure often darkens age spots or discolored skin patches, especially when sunburns have not healed. When the skin is oily and prone to breakouts due to sweating and heat exposure, acne may leave dark spots once the condition clears. In addition, freckles on the body create small brown spots. Injuries to the skin or surgucal scars may leave dark spots on the legs.