Alopecia Areata: This is an autoimmune condition that can cause patchy hair loss on any part of the body, including the eyebrows. It's relatively common and often presents with well-defined bald patches.

Thyroid Disorders: Both hypothyroidism and hyperthyroidism can lead to hair loss, including loss of eyebrow hair. Thyroid disorders are relatively common and can have a wide range of symptoms, making them a plausible cause for eyebrow hair loss.
Nutritional Deficiencies: Deficiencies in vitamins like Biotin, Vitamin D, or minerals such as iron can lead to hair loss. These deficiencies are more common than some of the rarer causes and can easily be tested for.
Androgenetic Alopecia: While more commonly associated with scalp hair, androgenetic alopecia can also affect eyebrow hair, especially in individuals with a family history of the condition.

Systemic Lupus Erythematosus (SLE): SLE is an autoimmune disease that can cause hair loss, including eyebrow hair loss, among many other symptoms. It's crucial to diagnose SLE early due to its potential for serious complications.
Thyroid Eye Disease (TED): Associated with Graves' disease, TED can cause hair loss, including eyebrow hair, along with other eye and facial symptoms. Early diagnosis is critical to prevent serious complications.
Cancer (including skin cancers and lymphoma): Certain types of cancer and their treatments can lead to hair loss. While less common, missing a diagnosis of cancer could have severe consequences.

Frontal Fibrosing Alopecia: A rare condition characterized by hair loss and scarring, primarily affecting the frontal scalp but can also involve the eyebrows.
Discoid Lupus Erythematosus: A chronic skin condition of sores with inflammation and scarring favoring the face, ears, and scalp but can also affect the eyebrows.
Sarcoidosis: A disease in which granulomas (abnormal masses of inflamed tissue) can form in various parts of the body, including the skin, and can cause hair loss, including eyebrow hair loss.