Plasma levels of levonorgestrel and free levonorgestrel index in women using NORPLANT implants or two covered rods (NORPLANT-2)
Olsson, S.E.; Odlind, V.; Johansson, E.D.; Nordström, M.L.
Contraception 35(3): 215-228
1987
ISSN/ISBN: 0010-7824 PMID: 3111784 DOI: 10.1016/0010-7824(87)90024-2Document Number: 332756
The plasma levels of levonorgestrel, sex hormone binding globulin (SHBG), and estradiol were compared over a 4-year period in 283 users of 2 types of Norplant systems--implants and covered rods. Plasma levels of norgestrel decreased throughout the study period in both groups, and there were no significant differences in mean plasma levels of levonorgestrel. 7 of the 8 pregnancies recorded during the study occurred after 35 months of implant use, and all involved women in the covered rods group. No significant differences were found between the women who became pregnant and the rest of the study subjects in terms of plasma levels of levonorgestrel. SHBG capacity was slightly higher in women using the 2 covered rods. The free levonorgestrel index, calculated as a ratio between levonorgestrel and SHBG, was significantly lower in users of 2 covered rods than in users of implant capsules at 1, 12, and 48 months of use. In addition, women who became pregnant had a significantly lower free levonorgestrel index. These findings suggest that the 2 covered rods had a lower release rate of levonorgestrel than Norplant implants, despite almost identical plasma levels. Thus, the free levonorgestrel index is considered a better parameter than levonorgestrel plasma concentrations to describe implant function and to predict the risk of pregnancy.