Eyebrow loss can be caused by a number of things, and is not necessarily reason for concern. In many cases, it is just temporary, and can be precipitated by things such as a nutrient deficiency, or even just over-plucking the eyebrows. Over-plucking is one of the most common causes of eyebrow hair loss, in fact, and can also be quite difficult to correct in the future.
Many people attempt to shape and pluck their eyebrows into a pleasing shape, which is perfectly fine. However, in some cases it is possible to damage the hair follicles when repeatedly plucking the eyebrows in the same place, or when plucking too frequently, which can cause the hair to never grow back. For this reason, it is recommended that one only pluck the eyebrows as often as necessary, and to attempt to keep them as close to their natural shape as possible.
Nutrient deficiencies can be a cause of hair loss, including eyebrow loss, so it is important to eat a balanced diet with plenty of fruits, vegetables, and protein. Taking a multivitamin every day can help as well. If a balanced diet does not help with eyebrow loss, it may be necessary to visit a doctor for blood tests to rule out any other conditions; some autoimmune diseases, such as lupus or alopecia areata, can cause hair loss. Certain medications can cause it as well, so recent changes or additions in medication should be considered first.
Eczema or even just very dry skin can cause patchy eyebrow loss as well. Again, if simple daily moisturization doesn't help, a visit to a doctor may be in order to help identify the problem. A dermatologist will be able to treat conditions like eczema. In addition, a condition known as trichotillomania can cause one to pull out one's own hair compulsively, which can obviously lead to eyebrow hair loss as well; this type of condition will require psychological treatment.
In the meantime, it may be possible to cover up eyebrow loss with makeup. Powder, liner, and even a mascara-type product may all be applied to the eyebrows to fill in thin spots. For permanent eyebrow loss, some people get eyebrow tattoos instead; if done properly, these can look fairly natural. In other, more extreme cases, an eyebrow transplant procedure may be performed; these procedures are typically considered to be cosmetic procedures and will not be covered by insurance.