How does an Intrauterine Device (IUD) deliver constant progesterone?

Mechanism of Constant Progesterone Delivery from Hormonal IUDs

The levonorgestrel IUD delivers constant progesterone (technically, the progestin levonorgestrel) through a silastic reservoir attached to the device's vertical arm, which is covered with a rate-limiting silastic membrane that releases approximately 20 micrograms of levonorgestrel per 24 hours directly into the uterine cavity for at least 5 years. 1

Physical Design and Release Mechanism

• The device consists of a T-shaped frame with a silastic (silicone elastomer) reservoir impregnated with levonorgestrel attached to its vertical arm 1

• A rate-limiting silastic membrane covers the reservoir, which controls the steady release of hormone into the uterine cavity 1

• This membrane technology ensures a predictable, continuous daily release rate of approximately 20 micrograms of levonorgestrel over an extended period 2, 1

Local vs. Systemic Delivery

• The intrauterine administration delivers hormone directly into the uterine cavity, where it causes pronounced endometrial suppression with minimal systemic absorption 1

• The dose of hormone is so low that the effect on ovarian function is negligible in most women, though some disruption of follicular development can occur 3

• This local delivery mechanism distinguishes hormonal IUDs from oral or transdermal progesterone, which have higher systemic hormone levels 4

Duration of Constant Release

• The levonorgestrel reservoir maintains its release rate of approximately 20 micrograms per 24 hours for at least 5 years, which is the FDA-approved duration 5, 1

• Clinical evidence demonstrates effectiveness extending to 7-8 years, with cumulative failure rates remaining below 1% during years 6-8 5

Important Clinical Caveat

• Note that hormonal IUDs contain levonorgestrel (a progestin), not natural progesterone—this is a synthetic progestogen that mimics progesterone's effects 5, 6

• The American College of Obstetricians and Gynecologists confirms levonorgestrel is the only hormone used in hormonal IUDs currently available in the United States 5