Serum norethindrone levels and menstrual pattern after the use of norethindrone implants in rhesus monkeys

Abstract Subdermal Silastic implants releasing norethindrone (NET) were inserted in five rhesus monkeys for a period of seven months. The total length of the implants used was 160 mm (eight implants of 20 mm each) containing 160 mg of NET (1 mg NET/mm). The in vivo release rate of NET from the Silastic implants containing 160 mg NET was 3.12 μg/day/mm. Menstrual bleeding records were maintained and serum NET levels were estimated from blood samples collected during the period implants remained in situ . There was an initial rise in NET levels, followed by fluctuating levels of NET in three out of five monkeys. The menstrual cycle or bleeding episodes showed a pattern commensurate with the circulating NET levels. None of the monkeys showed intermenstrual bleeding during the treatment period. High levels of NET seem advantageous for an undisturbed menstrual cycle.