What Causes Brittle Fingernails?
Rinse, Dry, Repeat
Although iron deficiency can in some cases cause brittle fingernails, the most common causes for the ailment are usually external rather than internal. Repeatedly wetting and drying nails can lead the cuticles to become brittle. So too can performing household chores and handling related chemicals without wearing rubber gloves or some other sort of proper hand protection.
For women, the process of applying and removing nail polish can also be a culprit. Especially if the nail polish removing product contains acetone. Dermatologists recommend using acetone-free nail polish removal products, and ideally using them no more than once a week. Lotions containing the chemical lanolin or alpha-hydroxy acids can be helpful to prevent brittle nails, as can - in more extreme symptom cases - the vitamin biotin, taken usually by mouth.
Thyroid, Other Diseases
Patients suffering from hyperthyroidism, wherein the thyroid gland produces an excess of the thyroid hormone, may also experience brittle fingernails. In some cases, this condition can even lead to the splitting of the nail, entirely, from its base (a.k.a. Onycholysis).
It's a commonly stated truism that the state of a person's nails generally reflects their overall, body health. Beyond thyroid disease, brittle nails can often be a symptom of other much more serious situations than lack of moisture. Diseases that can cause brittle nails include: kidney problems and the related build-up of nitrogen; liver disease; psoriasis; malnutrition and vitamin deficiencies; systematic amyloidosis; and the skin disease Lichen Planus.