Consider thyroid. Hypothyroidism can cause dry, brittle hair, as well as brittle nails. It is easy to get tested for this. Other symptoms might include fatigue, depression, temperature intolerance and loss of eyebrow hair.
Many causes/remedies. There are numerous causes of dry hair, such as harsh shampoos, shampooing too often, hair dye, perms, overuse of blow dryers or curling irons, overexposure to sun and wind, minerals in local water supplies, etc. So switch hair products, cover your head when you're outside, and generally cut back on activities that cause it to become dry.