I have read some stuff that seems to think these qualities seem to come hand in hand with each other, More being an NT leads to increased chance of certain problems, here is what i think is the best explaination i have found reguarding the definitions of introverts, shyness and schizoid
To further delineate the temperament type and clear up misconceptions about it, Laney discusses the difference between introversion, shyness, schizoid disorder, and highly sensitive temperaments:
Introversion, according to Laney, is a healthy capacity to tune into one’s inner world. Introverts have social skills, they like people, and they enjoy some types of socializing—usually one-on-one.
Shyness, on the other hand, is an extreme self-consciousness experienced around other people. It may have some genetic roots (in the form of a highly reactive fear center), but generally, it’s learned behavior, from experiences at school and with friends and families. It’s not an issue of energy; it’s a lack of confidence in social situations.
The personality disorder known as “schizoid” describes people who need relationships, yet fear close involvement with other people. In most cases, these individuals have grown up in traumatizing or neglectful home environments and have withdrawn or detached to avoid any more pain from human contact.
The highly sensitive temperament, which I discussed in an earlier post (“Are You or a Loved One Highly Sensitive?”), refers to individuals born with a certain cluster of traits that heighten their senses, perceptiveness, and intuition. They may stay away from social engagements because they can’t handle the agonizing flooding of their hyper-developed senses. There can be overlap between the highly sensitive and the introverted temperament, although you can be extroverted and highly sensitive, and introverted and not highly sensitive. What introversion and the highly sensitive temperament have most in common is that they can both become easily over-stimulated. You can imagine what a highly sensitive introvert might be dealing with in terms of overload in our highly stimulating contemporary environment