Everyone possesses both functions, but according to Jung, one always dominates. If you primarily rely on logic and objective analysis (Thinking),the realm of subjective ethical evaluations (Feeling) is typically less developed and often resides in the unconscious.
Jung believed that the innate type remains stable. However, through the process of "individuation," a person learns to acknowledge and develop their weaker functions. While this may look like a change of type, it is actually the achievement of psychological maturity and balance.
There is no direct link, but long-term suppression of your natural inclinations (e.g., an introvert forced into a hypersocial environment) leads to chronic stress and exhaustion, which can trigger psychosomatic illnesses.