Low estrogen oral contraceptives and the hypothalamo-pituitary axis.

The hypothalamo-pituitary axis in a group of 10 women (17-25 years of age) taking low-dose-estrogen oral contraceptives (20 micrograms ethinylestradiol (EE) + 150 micrograms desogestrel) for at least 6 months was investigated. The subjects underwent the GnRH (50 micrograms) and TRH (200 micrograms) stimulation tests for the evaluation of LH, FSH, Pr1, TSH and GH secretion. In the basal blood sample, E2, P, free T, A and SHBG levels were also determined. An elevation in LH response to GnRH was found, whereas FSH was inhibited. After TRH administration, the Pr1 response was the same in the oral contraceptive cycle as in the pretreatment cycle. Similar results were observed after TSH. Significantly, GH responded to TRH stimulation in the oral contraceptive cycles, but not in the pretreatment cycles. These results show that the new combination of 20 micrograms EE and desogestrel blocks hypothalamic production of GnRH. The different response to GnRH of LH and FSH reveals a differentiated sensitivity to the oral contraceptive which seems to inhibit the pituitary FSH-secreting gonadotrope cells in particular.