We utilized an ultrasensitive recombinant cell bioassay to study estrogen levels in 20 girls with premature thelarche and 42 normal prepubertal girls. Estrogen levels were slightly lower in the girls with premature thelarche(1.59 ± 0.73 pg/mL)(mean ± SE) compared to the normal prepubertal girls (2.2 ± 0.50 pg/mL). This difference did not achieve statistical significance. However, the girls with premature thelarche were significantly younger than the normal girls studied (4.5 ± 0.44 vs. 7.1± 0.30 y respectively, p <0.01). In this group of girls estrogen level was not significantly correlated with age, height, weight or BMI. When 14 of the girls with premature thelarche were matched for age with 14 normal prepubertal girls, estrogen levels still did not differ between the two groups, 1.7 ± 0.9 pg/mL for girls with premature thelarche vs. 1.8± 0.9 pg/mL for normal prepubertal girls. Girls with premature thelarche were slightly taller and slightly heavier than the normal girls but these differences did not achieve statistical significance (113.7 ± 4.5 vs. 105.8 ± 5.3 cm, and 26.8 ± 4.8 vs. 21.4 ±5.2 kg, respectively). The detection limit of the assay was 0.02 pg/mL.

In conclusion, girls with premature thelarche do not have higher levels of estrogen than age matched normal prepubertal girls as determined by an ultrasensitive estrogen bioassay. This may indicate that the mechanism of premature thelarche involves increased sensitivity of breast tissue to estrogen level rather than increased estrogen level.