Plasma levels of levonorgestrel and free levonorgestrel index in women using NORPLANT implants or two covered rods (NORPLANT-2).
Plasma levels of levonorgestrel, sex hormone binding globulin (SHBG) and estradiol were studied during four years in 283 healthy women using either NORPLANT implants or two covered rods (NORPLANT-2). The women were randomized to use either type of implant. Both implant systems have previously been shown to have similar release rates of levonorgestrel. In both groups plasma levels of levonorgestrel decreased throughout the study, and there were no statistically significant differences between the two groups in mean plasma levels of levonorgestrel. During the study 8 women became pregnant. All pregnancies but one occurred after 35 months of implant use and only in women using the covered rods. No significant differences were seen between the women who became pregnant and the rest of the group using two covered rods with respect to plasma levels of levonorgestrel. SHBG capacity tended to be somewhat higher in women using the two covered rods. As levonorgestrel is to a great extent bound to SHBG, and in that form not biologically active, a "free levonorgestrel index" was calculated as a ratio between levonorgestrel and SHBG. This index was significantly lower in users of two covered rods than in users of NORPLANT implants at 1, 12 and 48 months of use. Women who became pregnant had significantly lower "free levonorgestrel index" than had the rest of the group. It is postulated that the difference in "free levonorgestrel index" between users of the two implant systems reflect differences in release rate, the covered rods having a lower release rate of levonorgestrel than NORPLANT throughout the observation period. It is concluded that "free levonorgestrel index" is a better parameter than levonorgestrel plasma levels to describe implant function, and to discriminate women who are at risk of pregnancy.
D000328:Adult; D003271:Contraceptive Agents, Female / Q000008:administration & dosage / Q000097:blood*; D003692:Delayed-Action Preparations; D004958:Estradiol / Q000097:blood; D005260:Female; D006207:Half-Life; D006801:Humans; D007700:Kinetics; D016912:Levonorgestrel; D009644:Norgestrel / Q000008:administration & dosage / Q000097:blood*; D011247:Pregnancy; D012738:Sex Hormone-Binding Globulin / Q000032:analysis
D003271:Contraceptive Agents, Female; D003692:Delayed-Action Preparations; D012738:Sex Hormone-Binding Globulin; D009644:Norgestrel; D004958:Estradiol; D016912:Levonorgestrel