Progesterone vs progestins: what are they and what’s the difference?

The hormone progesterone, and its human-made cousin progestin, are sex hormones that play an important role in your body, especially your menstrual cycle and all the things that come with it.

Progesterone levels fluctuate with your cycle, and play a big role during pregnancy.¹ Progestin is the key ingredient in all forms of hormonal birth control, acting to interrupt your normal cycle and keep you from getting pregnant. Progestins do come with some side effects, though these can vary depending on your unique biology, and what kind of progestin you’re taking.

Having a better understanding of progesterone can help you understand its role in both periods, pregnancy, and beyond.

What is progesterone?

Progesterone is a sex hormone that is carried through the bloodstream, and has effects throughout the body. Progesterone belongs to a class of sex hormones called progestogens, which are defined as hormones that bind to the progesterone receptor.²

Progesterone is critical for establishing and sustaining the menstrual cycle and pregnancy. The main sources of progesterone in the body are the ovaries and the adrenal gland, as well as the placenta during pregnancy.³

Progesterone and the menstrual cycle

Early in the menstrual cycle, during the follicular phase prior to ovulation, progesterone production in the ovaries is low. Once ovulation occurs and an egg is released from your ovary, the follicle that produced the egg produces increasing amounts of progesterone.⁴

Progesterone signals to the lining of the uterus that it’s time to prepare for pregnancy. If the released egg is not fertilized and implanted in the uterus, progesterone production falls, and menstrual bleeding occurs.

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Progesterone beyond pregnancy

Historically, progesterone has been considered a sex hormone, meaning it’s a hormone primarily involved in reproduction. However, progesterone has many roles beyond just pregnancy and menstrual cycle.

Progesterone plays a key role in breast development and regulation of lactation. Progesterone receptors are also present in your bones, pancreas, brain, and the cardiovascular system.⁵ That means progesterone is also important for your skeleton, mental health, and heart health.

Progesterone has many roles beyond just pregnancy and menstrual cycle.

How much progesterone should I have?

The short answer is: It depends! The amount of progesterone normally in your body will vary from someone else’s. This is because of how progesterone acts in the tissues of your body.

Most tissues in the body have progesterone receptors, which come in two kinds, progesterone receptor A and B (PRA and PRB).⁶ The levels of PRA and PRB in your body’s tissues can vary from person to person,⁷ and there are other potential differences in how progesterone is taken up and used by your body. Altogether, that means that just looking at the level of progesterone in your body can’t tell us exactly what effects it’s having for any one individual person. It also means that there’s no one “right” level of progesterone for you.

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How progesterone is broken down in the body

After progesterone is released by the ovaries, it doesn’t circulate in the bloodstream for very long. Instead, progesterone is broken down into molecules called metabolites. This conversion primarily occurs in the liver but happens in other tissues as well, including your kidneys.⁸ These metabolites are then eliminated from the body.

Some of these metabolites are biologically active, though they don’t always have the same effect as progesterone does in your body. For example, one of the main metabolites of progesterone helps reduce anxiety⁹ and regulate immune cell function.¹⁰

As with progesterone itself, the formation of metabolites and their effect on your body vary from person to person. That adds just another layer of complexity to the multiple roles progesterone plays within your body, and makes predicting “ideal” levels of the hormone hard.

What are progestins?

Is progestin the same as progesterone? 

Progestins are not exactly the same as progesterone, but they do have almost exactly the same effect on your body.¹¹ Progestins are synthetic forms of progestogens — they are man-made compounds that bind to the progesterone receptor. Progestins are used in several forms of hormonal birth control, including:

• Combined oral contraceptives
• Mini pills (progestin only)
• The shot, implants
• Intrauterine devices (IUDs)

These forms of birth control are used not only for pregnancy prevention, but for treatment of polycystic ovarian syndrome (PCOS), endometriosis, and menstrual cycle-related symptoms, as well as cycle regulation.

Why use progestins when we could use progesterone?

Great question! When progestogens are needed simply for hormone replacement, then progesterone is a very good option. A relatively new formulation called micronized progesterone is available and is an excellent option for progesterone hormone replacement in individuals that are near or past menopause.¹² When it comes to birth control, however, the role of progesterone is a bit more complicated.

Progestins were first synthesized in the first half of the 20th century, and were used in the first birth control pill, Enovid, released in 1957.¹³ Progestins are usually made from other hormones, such as progesterone and testosterone.¹⁴

Progestins have longer half-lives than progesterone, which means they stick around in your body for longer, and can also be more bioavailable, meaning it’s easier for your body to use them.¹⁵ Additionally, different progestins can have different side effects, which can be a good thing in some cases (reducing acne, helping with PMDD), but can also cause you grief sometimes. Finding the right progestin for your body is the key.

Not all progestins are the same

There is more than one kind of progestin — many more, in fact. These different kinds of progestin all do broadly the same thing — prevent pregnancy — but they can have different side effects, and some might work better for you than others. These different effects are due in part to the fact that some progestins activate progesterone receptors more strongly than others, as well as their effects on other biological pathways in your body.¹⁶

Progesterone is a steroid hormone, which means it’s made from cholesterol. There are several other steroid hormones in the human body including aldosterone, cortisol, estrogens, and androgens (like testosterone).

These steroid hormones are all similar in their molecular structures.  In fact, most progestins are derived either from progesterone or testosterone. This shared chemical structure means a particular progestin might impact the systems in your body that use these other hormones for communication. Some progestins may also influence these other pathways indirectly, for instance via the formation of specific metabolites that have androgenic or estrogenic activity.

Different steroid hormones made by the human body. Most look quite similar to each other. (Credit: Encyclopædia Britannica, Inc.)

So, what are the “generations” of progestins and how do they differ? Learn all about the different kinds of progestins in Part II!