Puberty menorrhagia is one of the urgent problems of modern reproductive medicine. The study aimed to investigate the relationship between polymorphism of the GP IIIa (PLA1/PLA2) gene and improve the diagnosis of puberty menorrhagia in girls with thyroid gland pathology. Ninety-seven girls at puberty age were divided into three groups: group 1 (main) – girls with puberty menorrhagia and thyroid gland pathology (30 individuals), group 2 (comparison) – 40 girls with puberty menorrhagia, group 3 (control) – 27 practically healthy girls. Polymorphism of the GP IIIa (PLA1/PLA2) gene was studied by isolating genomic DNA from peripheral blood leukocytes, followed by amplification with a polymerase chain reaction. Results showed that mutation in the 17^th chromosome of q21.32 of the GP IIIa gene occurred in 8.6% of cases among adolescents with menorrhagia, in contrast to the control group, where it was not observed at all. The A1A1-genotype occurred by 11.7% (X²=4.01, p=0.041) more often in adolescents with menorrhagia than in girls with concomitant thyroid gland pathology and by 15.0% (X²=4.54, p=0.033) more often than in the control group. It was also found that the presence of the A1A2-genotype unreliably reduced the chances of uterine bleeding in adolescent girls by 1.45 times (OR=2.12) and was a protective factor in the puberty menorrhagia occurrence (OR=0.47). It may be concluded that the identification of a hereditary factor of the reproductive system diseases of adolescent girls fundamentally changes the point of view on the tactics of disease management and subsequent therapy.