The interaction between sex hormone binding globulin and levonorgestrel released from vaginal rings in women.

The levels of levonorgestrel (L-NOG), progesterone and estradiol were measured in plasma samples of 17 normally menstruating women during a control cycle and during a subsequent period (90 days) with a L-NOG-releasing vaginal ring. During days 38-66 after the insertion of the vaginal ring the concentrations of sex hormone binding globulin binding sites (hereafter: SHBG levels) were also assayed. Significant correlations were found not only between the corresponding levels of SHBG and L-NOG during exposure to the latter compound (r = 0.44; P less than 0.05), but also between the levels of SHBG in the control cycle and the levels of L-NOG measured during exposure (r = 0.60; P less than 0.01). Furthermore, the decrease in SHBG levels during the vaginal administration of L-NOG was directly proportional to the levels of SHBG in the pretreatment cycle (r = 0.64; P less than 0.01). A significant relationship was found between the levels of L-NOG (and, hence - indirectly - the levels of SHBG) and the degree of suppression of ovarian function. Thus the levels of L-NOG were lower (P less than 0.01) in the subjects (n = 8) with an apparently normal or partially suppressed ovulatory-like pattern of progesterone than in those subjects (n = 9) in whom progesterone levels were completely suppressed.