Participants were primarily female (93%), Caucasian (95%), and non-Hispanic (88%). POTS participants were more likely (OR; 95% CI) to present class I alleles B*15:01 (2.52, 1.66-3.81), B*18:01 (2.03, 1.22-3.39), and B*40:01 (2.91, 1.93-4.41). Similarly, class II alleles DQA1*02:01 (1.94, 1.32-2.86), DQB1*02:01 (2.84, 1.97-4.10), DRB1*03:01 (1.85, 1.30-2.65), and DRB1*13:01 (1.80, 1.11-2.92) were more common. 48% of cases tested positive for GH and showed associations with DQB1*02:01 (2.43, 1.41-4.23) and DRB1*15:01 (1.85, 1.07-3.20), while non-hypermobile subjects carried B*40:01 (3.59, 2.06-6.24), DRB1*04:01 (1.91, 1.05-3.46), and DRB1*13:01 (2.10, 1.09-4.05) alleles more frequently.