The tooth is a product of 4 components: enamel, cementum, pulp, and dentin. Enamel is the surface of the tooth which is clear or transparent and designed to protect the tooth from extreme temperatures and abrasions. Cenmentum is essentially the root of the tooth and pulp is the connective tissue which brings the tooth to life by allowing the body to supply it with blood.
But what is dentin?
Dentin is a calcified (calcium carbonate or some other insoluble calcium compounds) bodily tissue that is protected by the tooth's enamel. 70% of dentin is composed of the mineral hydroxylapatite, 20% is organic material and 10% is water. Everyone's dentin is shade of yellow but the intensity varies per person which is why after teeth whitening treatments some never achieve the desired level of whiteness.
Dentin is a bone like structure however is soft in comparison to actual bone. The tooth is composed primary, secondary, and tertiary dentin. The outer layer is the primary, the secondary layer of dentin is produced after the root of the tooth has been fully developed and the tertiary dentin is a response to a stimulus. All three types of dentin are vital to the longevity of the tooth.
Dentin can NOT be whitened
One of several reasons dentists advise consulting them prior to whitening is because there's a small chance you may not achieve the desired level of whiteness due to the color of your dentin. All dentin has a yellowish tint, the question is "how yellow is your dentin.' If you're among the majority the yellow will not be detectable by the naked eye, but if you're among the less fortunate you will be left thinking the teeth whitening product failed you when in fact it was sufficient.
Both at home and in office teeth whitening products are formulated to remove the tough stains that brushing will not combat; which is why teeth whitening is also referenced to teeth bleaching. Tooth whitening restores teeth to their natural color whether it is white, yellow, brown, or grey. If you do not recall ever having a white smile or have had little luck with custom whitening trays and high strength carbamide whitening gel then you need to ask your dentist if you're a candidate for teeth whitening.