Dry hair lacks the oil and moisture it needs to give it sheen and a soft texture. As a result, hair that's dry will be brittle and dull and have a straw-like texture. Most of us will get dry hair at one point or another--either due to over-processing or exposing it to sun, wind and chlorinated swimming pools.

Dry hair that comes and goes from these external causes is an annoyance. Chronic dry hair that comes from an internal source, however, can be a sign of an underlying health problem. That's why, if your hair is dry, it's important to take a look through these six top causes and try to pinpoint yours. If conditioning treatments do not improve your hair's moisture level, it may be time to contact a health care provider.

1. Excessive Washing and Blow-Drying, Harsh Detergents

Washing your hair too often, especially with a harsh shampoo, is a surefire way to strip moisture away. Heat from blow dryers, curling irons and electric curlers will also contribute to dryness.

"It's in vogue these days to shampoo every day, but shampooing doesn't only wash away dirt, it washes out the hair's protective oils," says Thomas Goodman, Jr., M.D., a dermatologist from Memphis, Tennessee, and assistant professor at the University of Tennessee Center for Health Sciences.

If your hair is dry, try washing it just two or three times a week using a mild shampoo and a quality conditioner like Giovanni Nutrafix Hair Reconstructor, which uses vegetable proteins to reconstruct extremely dry and damaged hair.

Like home remedies? "Mayonnaise makes an excellent conditioner," says Steven Docherty, senior art director at New York City's Vidal Sassoon Salon. Leave it on for five minutes to an hour before washing out.

2. Environmental Dryness

The climate you live in can also dry out your hair. Areas with lots of sun, dry heat and little humidity, for instance, will definitely make your hair drier than tropical, humid locales. Likewise, if you're an outdoorsy person who likes to spend time in the sun, wind, ocean or pool, your hair also risks being dry. You can cut down on the damage to your hair from the elements by wearing a hat while outdoors and always using a swim cap when swimming in chlorinated water.

3. Anorexia

Because people with anorexia engage in self-starving to stay dangerously thin, their bodies are denied the nutrients they need to function. This includes the nutrients necessary to maintain luster, shine and softness in their hair. Dry hair (along with dry skin and hair loss) is a common side effect of anorexia, and one that may manifest early on.

4. Malnutrition

Similar to anorexia, a person who is malnourished does not take in the nutrients necessary for the body to maintain healthy hair. As a result, the hair becomes dry, brittle and damaged. In particular, dry hair can be a sign that your diet is lacking in omega-3 essential fatty acids, which can be found in salmon and fish oil, walnuts and flax seeds.

5. Hypothyroidism

This is a condition in which the body produces too little of the thyroid hormone. Dry, brittle and thin hair is an early symptom of hypothyroidism, along with weakness, fatigue, depression and joint or muscle pain. If left untreated, the condition causes the body to slow its functions, leading to mental and physical sluggishness and other symptoms that can range from mild to severe.

6. Hypoparathyroidism

Hypoparthyroidism is having too little parathyroid hormone, which causes blood levels of calcium to fall and phosphorus to rise. This can lead to dry hair, scaly skin, cataracts, muscles cramps and spasms, seizures and more. The most common cause of hypoparathyroidism is injury to the parathyroid glands during head and neck surgery.