Long-term contraception by steroid releasing implants VI serum concentrations of norethindrone in women bearing a single silastic implant releasing norethindrone acetate

Serum levels of norethindrone were estimated in six women after the insertion of a single silastic implant D releasing norethindrone acetate for long-term contraception. The serum norethindrone levels were estimated by a sensitive radioimmunoassay with high degree of precision and accuracy, involving an antiserum raised against norethindrone 11α-hemisuccinate-bovine serum albumin. The results showed that there was an initial high level of circulating steroid up to a period of about 2 months when it declined to 80 ng/100 ml serum in five subjects. The serum levels of norethindrone were again elevated when the first implant was replaced with the second. It appears that serum concentration of about 80 ng/100 ml of norethindrone is effective in providing contraception.