Data obtained during long-term follow-up of 68 girls with premature thelarche were analysed. In 85% onset was before the age of 2 years, in 30.8% being present at birth. In 44.1% there was a regression after 3 2/12 +/- 2 8/12 years (SD). Basal levels of plasma FSH and response to LH-RH were significantly higher than prepubertal controls (1.93 +/- 1.56 vs. 0.8 +/- 0.1 mU/ml and peaks 12.3 +/- 5.4 vs. 7.9 +/- 1.0 mU/ml respectively; p less than 0.001). Twenty-seven of 52 patients tested had increased plasma estradiol and in 27 of 40 patients tested, urocytograms or vaginal smear showed estrogenization. Basal levels of LH and response to LH-RH were prepubertal. The girls with premature thelarche were significantly taller than normal controls of the same age (p less than 0.001). These results suggest that premature thelarche is an incomplete form of precocious sexual development probably due to derangement in the maturation of the hypothalamo-pituitary-gonadal axis which results in a higher than normal secretion of FSH, as well as a defect in the peripheral sensitivity to the sex hormones.

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