No, asexuality is not caused by a hormone imbalance. Just because a person doesn't express sexual desire a traditional doesn't mean it's attributable to an imbalance in their hormones.
Hormones do impact the sexual desires of many individuals, but a lack of sexual desire does not mean that there is a hormone imbalance.
Need to find a therapist near you? Check out the Monarch Directory by SimplePractice to find licensed mental health therapists with availability and online booking.