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Amy B. Killen։ 3 Things You May Not Know about Progesterone

Amy B. Killen/LinkedIn

Aug 20, 2026, 17:10

Amy B. Killen։ 3 Things You May Not Know about Progesterone

Amy B. Killen, Co-Founder and Chief Medical Officer of Humanaut Health, shared on LinkedIn։

“Progesterone is the most misunderstood hormone in the menopause conversation. Everyone talks about estrogen. Progesterone gets treated like the chaperone estrogen has to bring to the party. That framing is wrong, and it’s costing women sleep, sanity, and in some cases the confidence to stay on hormone therapy at all.

Three things you may not know…

1. Oral progesterone makes most people drowsy, and that’s the design, not the side effect.

In your gut and liver, it converts to allopregnanolone, which lands on the same brain receptors as anti-anxiety and sleep medications. Take it at night and use the sleepiness. Take it with fat, and you roughly double how much you absorb, which is why the product guidance says bedtime, on a fairly empty stomach, a couple of hours after eating.

And nobody has ever run the study asking whether that extra absorption buys you any extra protection for your lining, so don’t eat a plate of cheese with your capsule hoping for a safety bonus. If oral leaves you flattened in the morning, moving it earlier and further from dinner is the first thing to try.

2. There is no serum progesterone blood level to chase.

Several of you asked what serum number you need for endometrial protection. There isn’t one. No study has established a blood progesterone level that predicts a protected lining, which is precisely why every guideline on earth is written in milligrams and days instead of ng/mL.

The half-life of oral progesterone is short, so if you take it at night before bed and draw your blood 10 hours later, you won’t see much progesterone still circulating. And oral progesterone is heavily converted to other compounds on its way through your liver, so a single draw is a lousy snapshot regardless. Adequate dose and adequate days protect you. If someone wants to know whether your lining is okay, that answer comes from an ultrasound or a biopsy, not a hormone panel.

3. The study everyone fears used a synthetic progestin, and the other arm of it went the other direction.

When you hear ‘hormones cause breast cancer,’ that fear traces largely to one arm of the Women’s Health Initiative, which used a synthetic progestin. In a large French observational study following 80,000 women (E3N), estrogen paired with body-identical progesterone showed no increase in breast cancer risk over about five years, while estrogen paired with a synthetic progestin raised it by close to 70%. The molecule matters.

What’s funny about this is that if you go to your favorite AI and ask whether progesterone increases breast cancer risk, it will likely say, ‘yes.’ Why? Because even the smartest AIs (and some research papers) confuse ‘progesterone’ with ‘progestin.’ Try it. It’s a fun game I play sometimes to see if I can outsmart my AI friends.

For 13 more Things You May Not Know About Progesterone, check out this week’s Substack.”

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