Don't understand how antihistamines work for chronic hives that is due to thyroid issues and not allergies?

Histamine: Whatever the cause, the end result is usually from the histamine released from the reaction leading to hives. Thus high dose of antihistamine may work . Do note that not all hives are controlled by antihistamine since they may be mediated by a different kind of mediators.

Chronic Urticaria CU: CU can be associated with various autoimmune disorders. Even in the absence of hypo- or hyperthyroidism, patients with thyroid autoantibodies are often poorly responsive to standard therapies and have more persistent disease. Studies have examined the use of thyroid supplementation in the treatment of CU, with mixed results. Best treatment for this condition is daily antihistamines. See Allergist.