Progesterone: What It Does and Why It Matters

When people talk about hormones, oestrogen tends to take centre stage. Progesterone is frequently treated as the lesser half of the pair, known mainly for its role in the second half of the menstrual cycle and in pregnancy. But progesterone has a wide range of effects throughout the body, and its decline, which begins earlier than oestrogen's during perimenopause, has consequences that are often attributed to stress, anxiety, or simply getting older.

Understanding what progesterone does, and what happens when levels are lower than optimal, is useful for anyone trying to make sense of changes in mood, sleep, or cycle-related symptoms.

What progesterone does

Progesterone is produced primarily by the corpus luteum, the structure that forms in the ovary after ovulation. Its production is therefore tied to ovulation: if ovulation doesn't occur, progesterone levels are low for that cycle. This matters because anovulatory cycles, cycles where ovulation doesn't happen, become more common in perimenopause, meaning progesterone may decline before oestrogen does in many women.

In the brain, progesterone has a calming, mildly sedating effect. It enhances the activity of GABA, the brain's main inhibitory neurotransmitter, which is why adequate progesterone is associated with better sleep, lower anxiety, and a sense of emotional steadiness. When progesterone is low, many women notice increased anxiety, poorer sleep, and heightened emotional reactivity, even when oestrogen levels are still reasonable.

Progesterone also opposes the effects of oestrogen in the uterine lining, preventing overgrowth. In the context of HRT, women who still have a uterus need to take a progestogen alongside oestrogen for this reason. The form of progestogen matters, as we'll come to shortly.

"Progesterone's effect on the brain is one of its most significant and least discussed roles. Its absence often feels like anxiety or insomnia rather than a hormonal shift."

The difference between progesterone and progestogens

This distinction matters and causes a great deal of confusion. Progesterone refers to the hormone produced naturally by the body, and to bioidentical progesterone, which is chemically identical to the body's own hormone. Progestogens is a broader term that includes both bioidentical progesterone and synthetic progestins, which are chemically similar but not identical to the body's progesterone.

In HRT, the type of progestogen used makes a difference to both the experience and the risk profile. Synthetic progestins, such as medroxyprogesterone acetate, were used in older HRT formulations and are associated with a higher risk of breast cancer and cardiovascular effects. Micronised progesterone, a bioidentical form available as Utrogestan in the UK, has a more favourable risk profile and also carries the calming, sleep-supportive effects of natural progesterone. Many women and clinicians now prefer this form where appropriate.

Progesterone and perimenopause

In early perimenopause, when cycles are still occurring but becoming less regular, progesterone may already be falling due to increasing anovulatory cycles. This can produce a pattern that some practitioners describe as oestrogen dominance: the relative ratio of oestrogen to progesterone shifts, even when oestrogen itself is not dramatically reduced. Symptoms may include heavier periods, worsening PMS, sleep disruption, and increased anxiety.

Some women are prescribed low-dose progesterone in this phase, particularly to support sleep and mood. Others address it through lifestyle measures that support progesterone production, including reducing intense exercise, managing stress, and eating adequately. A GP or women's health specialist can advise on what's appropriate based on individual circumstances.

Progesterone is one of the more nuanced hormones in the female system, and its effects on mood and sleep mean that its decline is often felt before it's understood. Knowing what it does is useful context for any woman paying attention to her own hormonal health.