Premature Graying of hairs

Hair greying is an eventual and irreversible reality of life and is often associated with growing old. But how and why exactly does black turn white? The answer lies in a chemical called melanin. The root of every hair strand under the scalp is surrounded by a tube of hair tissue called hair follicle which contains pigment cells. These pigment cells produce the chemical called melanin that gives the hair strand its colour - black, brown, blonde. When a person grows older, these pigment cells begin to die and the melanin production reduces too, making the strand less colourful and more transparent - grey, silver or white. And that is why hair greying is associated with age.
Even though genetics and hereditary factors play a major role in determining the time of your hair turning grey.

Causes:

A number of medical conditions as well as lifestyle choices have been related to premature greying of hair. 

Medical Conditions:

• Thyroid Disease: An underactive or overactive thyroid can affect the melanin production in the hair which can cause premature greying.

• B12 Deficiency: Deficiency of B12 has also been linked to premature greying of hair.

• Vitiligo: Vitiligo is a disease that is caused when the pigment producing cells in your skin and hair are damaged. Once the cells in your scalp are damaged, melanin production is reduced  and hair begins to turn grey.

It is advisable to see a doctor if you experience premature greying of hair and have a reason to believe that it is not hereditary. This is essential so that you can rule out these serious disorders that have caused early hair greying. 
While some medical conditions can be the cause for premature graying .