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GH responsiveness varies during the menstrual cycle.

OBJECTIVE
The GH-IGF-1 axis is affected by oestrogen. Both endogenous and exogenous oestrogen facilitates the central drive of pulsatile GH secretion. However, the effect on IGF-1 levels is more subtle, and a reduction in GH sensitivity has been proposed. The IGF generation test has confirmed reduced GH sensitivity with high doses of exogenous oestrogen. It is not known, however, whether fluctuant levels of endogenous oestrogen modify GH sensitivity. To investigate this further, women were challenged with the IGF-1 generation test at different stages of the menstrual cycle.


METHODS
Nine women (age 38(6) years (mean (s.d.)) with regular menstrual cycles were recruited. An IGF-1 generation test, s.c. injection of 7 mg GH, was performed in the early-follicular (EF), periovulatory (PO) and midluteal (ML) phases. IGF-1, insulin-like growth factor binding protein (IGFBP)-3 and acid-labile subunit (ALS) levels were measured at baseline and 24 h after GH administration.


RESULTS
Oestradiol levels were lower in the EF than PO or ML phases (32.6(7.8) vs 69.6(16.2) vs 66.6(23.6) pg/ml respectively (repeated measures ANOVA, P < 0.001)). Baseline IGF-1 was lower, but increment IGF-1 (peak minus baseline) was higher in the EF than PO or ML phases (baseline: 291.8(56.6) vs 335.0(55.2) vs 346.6(78.2) ng/ml (P = 0.008); increment IGF-1: 234.6(59.2) vs 194.7(37.8) vs 185.2(37.3) ng/ml (P = 0.008)).


CONCLUSIONS
Increased endogenous oestrogen levels are associated with only a modestly elevated baseline IGF-1 from midcycle onward despite a previously reported twofold increase in GH secretion. In parallel with this apparent GH insensitivity, increased endogenous oestrogen levels are associated with reduction in GH sensitivity evidenced by reduction in increment IGF-1. This may have clinical implications for women with isolated GH deficiency with regular menstrual cycles on a fixed dose of GH. This possibility requires further study.