Sexual dimorphism in postnatal gonadotrophin levels in infancy reflects diverse maturation of the ovarian and testicular hormone synthesis

Summary

Background

The postnatal gonadotrophin surge is sexually dimorphic: FSH levels predominate in girls and LH levels in boys. However, in preterm (PT) girls, both gonadotrophin levels are higher than in PT boys.

Objective

To evaluate how gonadal maturation contributes to the sex differences in FSH and LH.

Design

Monthly follow‐up of 58 full‐term (FT, 29 boys) and 67 PT (33 boys) infants from 1 week (D7) to 6 months of age (M1‐M6). Analyses were also carried out according to postmenstrual (PM) age in PT infants.

Methods

Urinary LH, FSH, oestradiol (E2), testosterone (T) and serum inhibin B (InhB) levels.

Results

High gonadotrophin levels in PT girls abruptly decreased (P < .001) by M2, corresponding to a PM age of 38‐42 weeks, and LH levels fell below the levels found in boys. This decrease was parallel to a steep increase in E2 levels (P < .001), and, from M4 to M6, LH and E2 correlated positively in PT girls (P < .01). T levels in PT boys increased earlier than E2 levels in PT girls. In addition, InhB levels were high in PT boys already at D7, in contrast to low InhB in PT girls. InhB and FSH correlated negatively in the whole group (P < .001).

Conclusions

Ovarian hormone synthesis is immature and incapable of responding to gonadotrophin stimulus before 38‐42 PM weeks in PT girls, which may explain their highly elevated FSH and LH levels. The higher InhB levels in boys compared to girls may explain sexual dimorphism in FSH levels.

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