Context: Little information exists regarding FSH values in very young girls with Turner syndrome (TS).

Objectives: To evaluate the pattern, natural progression and karyotype-related differences in FSH secretion in young, prepubertal girls with TS.

Study Design: FSH was measured at study entry and annually for 2 yr.

Setting: "Toddler Turner" study conducted at 11 US pediatric endocrine centers.

Study Participants: 88 girls with karyotype-proven TS aged 9 months to 4 yr.

Main Outcome Measures: By-karyotype differences in FSH concentration; age-related changes in FSH.

Results: Mean (±SD) FSH was markedly elevated in the 45,X (n = 56: 68.3 ± 36.0 IU/L) and Other groups (n = 15 [excluding 3 subjects with Y-containing karyotypes]: 52.7 ± 50.8 IU/L) but was minimally elevated in girls with 45,X/46,XX mosaicism (n = 14: 10.1 ± 13.5 IU/L, P < 0.005 both comparisons). Over the 2-year period, FSH declined in the 45,X group (-13.4 IU/L/yr, P < 0.0001). Nonetheless, only 3 of 159 FSH values fell within normal range for age at any time during the 2-year study. FSH decline was similar in the Other group (-14.3 IU/L/yr, P = 0.0032). In contrast, no significant decrease in FSH with age was observed in the 45,X/46,XX group.

Conclusions: In contrast to the original report of FSH concentrations in individuals with TS (Conte FA et al. 1975, J Clin Endocrinol Metab 40:670-674), this study demonstrates distinct differences in patterns of FSH secretion between young girls with monosomy TS, who have persistent elevation of FSH to age 6, and those with 45,X/46,XX mosaicism, whose FSH values suggest retained ovarian function in the majority. These findings have implications for patient management and family counseling.