Another noteworthy finding of this study is the trend of premature thelarche (PT). This study identified 1,594 girls with PT whose trajectory differed from that of CPP. The incidence of CPP plateaued after peaking in 2021, with no significant difference between the postpandemic and pandemic periods (IRR, 1.06; 95% confidence interval [CI], 0.89–1.26). In contrast, the incidence of PT did not plateau. The incidence of PT rose from 7.2 per 100,000 person-years in 2018 to 14.6 in 2024. The incidence was higher during the pandemic (IRR, 1.31; 95% CI, 1.09–1.57) than the prepandemic period, and it rose further in the postpandemic period (IRR, 1.74; 95% CI, 1.43–2.11). The postpandemic incidence also remained higher than that during the pandemic (IRR, 1.33; 95% CI, 1.13–1.57). Among the five PT age groups, age 7–8 years was the only group in which all three period comparisons were significant, immediately below the conventional 8-year age cutoff for precocious puberty.

Although PT has traditionally been considered a benign pubertal variant, its increasing incidence in this age group deserves particular attention. Some cases of PT will subsequently progress to CPP, whereas others will remain nonprogressive, and the two cannot always be distinguished by initial clinical presentation. Meta-analytic data indicate that the age at thelarche declined by approximately 3 months per decade between 1977 and 2013 [2]. In this secular trend, breast development at 7 years cannot be dismissed as benign, even when it appears in isolation. In our findings, chronological age, bone age, height, weight, body mass index, and predicted adult height were of limited value in distinguishing PT from CPP. Rather, the discrimination relies primarily on breast maturation and basal and gonadotropin-releasing hormone (GnRH)–stimulated luteinizing hormone (LH) levels [3]. Similarly, a Thai study of girls aged 6–8 years identified a combination of basal LH and basal LH/follicle-stimulating hormone ratio as useful discriminators between CPP and PT [4]. This diagnostic uncertainty may be confounded by childhood obesity. In a previous study from our group, overweight girls with CPP had significantly lower GnRH-stimulated LH peaks than normal-weight girls (8.95 IU/L vs. 11.97 IU/L, respectively; P<0.01) [5]. The 2022 Clinical Practice Guidelines of the Korean Society of Pediatric Endocrinology recommend cautious interpretation of GnRH stimulation test results in children with obesity [6].

This nationwide population-based study provided valuable epidemiological data from a region where such information is limited. The continued increase in PT, particularly among girls aged 7–8 years, underscores the need for longitudinal follow-up rather than reassurance based on a single visit. As these estimates were derived from claims data, changes in healthcare utilization, referral patterns, diagnostic intensity, and coding may also have contributed to the observed trends, and the temporal association with the coronavirus disease 2019 pandemic should not be interpreted as causal.