Dr. Brendan McCarthy

In this episode of the progesterone series, Dr. Brendan McCarthy — Chief Medical Officer of Protea Medical Center in Tempe, Arizona — explores the often misunderstood relationship between progesterone, estrogen, and breast health.

For decades, women have been taught to fear their breasts and fear hormones. While awareness matters, fear is disempowering — and it has left many women confused about what’s actually happening in their bodies.

In this episode, we discuss:

Why breast tissue is dynamic, not static

How estrogen stimulates growth and progesterone restores balance

The role of progesterone in breast tissue maturation and architecture

Why dense or fibrocystic breasts often reflect unopposed estrogen

How restoring ovulation and progesterone can reduce breast pain and density in some women

The difference between natural progesterone vs synthetic progestins

Where the fear around progesterone and breast cancer really came from

Progesterone is not something to fear — it is a hormone of organization, balance, and maturation. Understanding how it works allows women to approach breast health with clarity instead of anxiety.

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💬 Comments are read and appreciated.

Citations: (Provided for educational purposes; this episode discusses biologic frameworks and observational data, not medical guarantees.)

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Korenman SG. Estrogen window hypothesis (1980)

Korenman SG. The etiology of breast cancer: hormone factors.
Cancer. 1980;46(4 Suppl):874–880.

Context:
This paper introduced what later became known as the “estrogen window” hypothesis—the idea that prolonged estrogen-driven proliferation without adequate progesterone signaling may create periods of increased tissue vulnerability. This is a mechanistic framework, not a prevention claim, but it remains foundational in how endocrinologists think about hormonal timing and breast biology.

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Estrogen as a proliferative signal in breast tissue

Key TJ, Pike MC. The role of oestrogens and progestagens in the epidemiology and prevention of breast cancer.
Eur J Cancer Clin Oncol. 1988;24(1):29–43.

Context:
Establishes estrogen’s role as a mitogenic (growth-promoting) signal in breast epithelium and frames cancer risk partly in terms of cumulative proliferative exposure over time.

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Progesterone and breast differentiation biology

Brisken C, O’Malley B. Hormone action in the mammary gland.
Cold Spring Harb Perspect Biol. 2010;2(12):a003178.

Context:
Describes progesterone’s role in lobuloalveolar development, differentiation, and architectural organization in breast tissue. Supports the concept that progesterone signaling is biologically distinct from estrogen-driven proliferation.

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Fibrocystic breast change and hormonal signaling

Sitruk-Ware R. Hormonal replacement therapy and the breast.
Menopause. 2002;9(4):237–251.

Context:
Reviews how different hormonal environments influence benign breast changes, including pain, nodularity, and cystic architecture, and discusses differential tissue effects of estrogen and progesterone signaling.

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Mammographic density and hormonal influence

Boyd NF et al. Mammographic density and the risk and detection of breast cancer.
N Engl J Med. 2007;356:227–236.

Context:
Establishes mammographic density as a biologic and radiographic marker influenced by hormonal, stromal, and epithelial factors. Density reflects tissue composition rather than disease itself.

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Bioidentical progesterone vs synthetic progestins (E3N cohort)

Fournier A et al. Breast cancer risk in relation to different types of hormone replacement therapy.
Int J Cancer. 2005;114(3):448–454.

Context:
Large observational cohort suggesting that estrogen combined with synthetic progestins was associated with higher breast cancer risk, whereas estrogen combined with micronized progesterone did not show the same risk signal. Observational data—not proof of protection.

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Systematic review: progesterone vs progestins

Stute P et al. The impact of micronized progesterone on breast cancer risk.
Climacteric. 2018;21(2):111–122.

Context:
Systematic review concluding that micronized progesterone appears to have a more favorable breast safety profile compared with many synthetic progestins when used in menopausal hormone therapy.

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Endocrine-disrupting compounds and estrogenic signaling

Diamanti-Kandarakis E et al. Endocrine-disrupting chemicals: an Endocrine Society scientific statement.
Endocr Rev. 2009;30(4):293–342.

Context:
Summarizes evidence that environmental compounds can exert estrogen-like signaling and disrupt normal hormonal balance, lending plausibility to concerns about prolonged estrogenic exposure without physiologic counter-regulation.

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Important Clarification

The research above supports discussion of biologic mechanisms, tissue behavior, and relative risk profiles.
It does not establish progesterone as a guarantee against breast cancer, nor does it replace individualized screening, genetics, or oncology care.

 

Dr. Brendan McCarthy is the founder and Chief Medical Officer of Protea Medical Center in Arizona. With over two decades of experience, he’s helped thousands of patients navigate hormonal imbalances using bioidentical HRT, nutrition, and root-cause medicine. He’s also taught and mentored other physicians on integrative approaches to hormone therapy, weight loss, fertility, and more. If you’re ready to take your health seriously, this podcast is a great place to start.

 

👇 Tap Subscribe to learn more about what’s actually happening in your body, and what to do about it.

 

📘 Read Dr. McCarthy’s Book: Jump Off the Mood Swing – A Sane Woman’s Guide to Her Crazy Hormones https://www.amazon.com/Jump-Off-Mood-Swing-Hormones/dp/0999649604

 

📲 Follow Dr. McCarthy:

Instagram: @drbrendanmccarthy

TikTok: @drbrendanmccarthy

Website: www.protealife.com

 

💬 Got a question or topic for a future episode? Let us know in the comments!

Unexplained infertility, PMS, and low progesterone are often dismissed when labs fall “within range.” In this episode, Dr. Brendan McCarthy explains why prolactin may be the missing piece.

Learn how mildly elevated prolactin can suppress ovulation, lower progesterone, and impact fertility—even when labs appear normal. We also discuss common causes, symptoms, the role of stress and medications, and why diet (including gluten sensitivity) may matter.

This episode focuses on precision medicine, not fear—helping you understand what standard reference ranges often miss.

Citations: Research — Prolactin and Breast Cancer Risk

Below are key epidemiologic and review papers that inform the discussion in this episode regarding prolactin and breast biology. These studies look at associations, not simple cause-and-effect relationships, and help explain why prolactin shows up in breast health conversations.

Meta-analysis: circulating prolactin and breast cancer risk

Wang M, et al. (2016).
Plasma prolactin and breast cancer risk: a meta-analysis.
Cancer Causes & Control.

This meta-analysis pooled data from multiple observational studies comparing women with higher versus lower circulating prolactin levels. Across studies, higher prolactin levels were associated with a modest but statistically significant increase in breast cancer risk. The association was most evident in postmenopausal women and in hormone-receptor–positive tumors.

This helps explain why prolactin is considered a relevant growth signal in breast tissue rather than just a “lactation hormone.”

Systematic review and meta-analysis: prolactin levels across breast cancer cohorts

Aranha AF, et al. (2022).
Impact of prolactin levels in breast cancer: a systematic review and meta-analysis.
Endocrine-Related Cancer.

This more recent systematic review and meta-analysis evaluated circulating prolactin levels across breast cancer populations and control groups. Elevated prolactin levels were associated with higher breast cancer occurrence, with stronger associations seen in invasive cancers and hormone-receptor–positive disease.

This paper adds weight to the idea that prolactin participates in breast biology in ways that matter clinically, even outside of pregnancy and breastfeeding.

Prospective cohort studies: prolactin measured before diagnosis

Tworoger SS, et al. (2004; 2006).
Prospective analyses from large cohorts including the Nurses’ Health Study.

In these studies, prolactin was measured years before any breast cancer diagnosis. Women with higher prolactin levels had a higher likelihood of developing breast cancer later, particularly estrogen-receptor–positive tumors in postmenopausal women.

Because prolactin was measured before cancer developed, these studies help clarify timing and reduce the concern that elevated prolactin is simply a consequence of disease.

Mechanistic context (supportive background)

Experimental and translational studies show that prolactin receptor signaling influences mammary epithelial cell growth, differentiation, and interaction with estrogen signaling pathways.

This provides a biologic backdrop for why epidemiologic associations between prolactin and breast cancer risk keep appearing across different study designs.

How to read this as a clinician or patient

These data do not mean prolactin “causes” breast cancer in a simple or deterministic way. What they do show is that prolactin is an active hormone in breast tissue, and chronically higher levels are consistently associated with changes in breast risk profiles across large populations.

That’s why prolactin deserves attention in conversations about fertility, breast symptoms, and long-term hormonal signaling—not fear, and not dismissal. 

 

Dr. Brendan McCarthy is the founder and Chief Medical Officer of Protea Medical Center in Arizona. With over two decades of experience, he’s helped thousands of patients navigate hormonal imbalances using bioidentical HRT, nutrition, and root-cause medicine. He’s also taught and mentored other physicians on integrative approaches to hormone therapy, weight loss, fertility, and more. If you’re ready to take your health seriously, this podcast is a great place to start.

 

👇 Tap Subscribe to learn more about what’s actually happening in your body, and what to do about it.

 

📘 Read Dr. McCarthy’s Book: Jump Off the Mood Swing – A Sane Woman’s Guide to Her Crazy Hormones https://www.amazon.com/Jump-Off-Mood-Swing-Hormones/dp/0999649604

 

📲 Follow Dr. McCarthy:

Instagram: @drbrendanmccarthy

TikTok: @drbrendanmccarthy

Website: www.protealife.com

 

💬 Got a question or topic for a future episode? Let us know in the comments!

In this episode of our progesterone series (Episode 5), Dr. Brendan McCarthy — Chief Medical Officer of Protea Medical Center in Tempe, Arizona — breaks down the often-misunderstood relationship between stress, ovulation, progesterone, and cortisol.

We explore the concept commonly referred to as the “progesterone steal” and why this term can be misleading. Rather than hormones being “stolen,” Dr. McCarthy explains how the body intelligently reroutes hormone production under stress to prioritize survival over reproduction.

This episode covers:

Why the body must feel safe to ovulate and produce progesterone

How chronic stress impacts PMS, fertility, and cycle regularity

The truth about cortisol (and why it isn’t the villain it’s often made out to be)

Why low progesterone is not a personal failure or flaw

Why you can’t medicate someone out of stress — and what good medicine actually looks like

This conversation is about biology, not blame. Your body is not broken — it’s responding exactly as designed.

Dr. Brendan McCarthy is the founder and Chief Medical Officer of Protea Medical Center in Arizona. With over two decades of experience, he’s helped thousands of patients navigate hormonal imbalances using bioidentical HRT, nutrition, and root-cause medicine. He’s also taught and mentored other physicians on integrative approaches to hormone therapy, weight loss, fertility, and more. If you’re ready to take your health seriously, this podcast is a great place to start.

 

👇 Tap Subscribe to learn more about what’s actually happening in your body, and what to do about it.

 

📘 Read Dr. McCarthy’s Book: Jump Off the Mood Swing – A Sane Woman’s Guide to Her Crazy Hormones https://www.amazon.com/Jump-Off-Mood-Swing-Hormones/dp/0999649604

 

📲 Follow Dr. McCarthy:

Instagram: @drbrendanmccarthy

TikTok: @drbrendanmccarthy

Website: www.protealife.com

 

💬 Got a question or topic for a future episode? Let us know in the comments!

In this episode, Dr. Brendan McCarthy, Chief Medical Officer of Protea Medical Center, explains why progesterone delivery systems matter—and how different routes change what progesterone actually does in the body.

Part 4 of the progesterone series covers oral, topical, vaginal, rectal, injectable, and sublingual progesterone, breaking down which methods affect the brain, uterus, and breast tissue—and why choosing the right route is critical.

If progesterone hasn’t worked for you in the past, the issue may not be the dose, but how it was delivered.

This episode focuses on education, patient agency, and thoughtful hormone care—no shortcuts, no selling.

Subscribe for more in-depth conversations on hormones and women’s health, and share with someone who may benefit.

 

Dr. Brendan McCarthy is the founder and Chief Medical Officer of Protea Medical Center in Arizona. With over two decades of experience, he’s helped thousands of patients navigate hormonal imbalances using bioidentical HRT, nutrition, and root-cause medicine. He’s also taught and mentored other physicians on integrative approaches to hormone therapy, weight loss, fertility, and more. If you’re ready to take your health seriously, this podcast is a great place to start.

 

👇 Tap Subscribe to learn more about what’s actually happening in your body, and what to do about it.

 

📘 Read Dr. McCarthy’s Book: Jump Off the Mood Swing – A Sane Woman’s Guide to Her Crazy Hormones https://www.amazon.com/Jump-Off-Mood-Swing-Hormones/dp/0999649604

 

📲 Follow Dr. McCarthy:

Instagram: @drbrendanmccarthy

TikTok: @drbrendanmccarthy

Website: www.protealife.com

 

💬 Got a question or topic for a future episode? Let us know in the comments!

If progesterone makes you feel wired, anxious, angry, emotional, or unable to sleep, this episode is for you.

In this deeply important continuation of our reverse responding series, Dr. Brendan McCarthy—Chief Medical Officer of Protea Medical Center—returns to clarify what was missing in Episode 3C and to walk you through the real physiology, compassion, and treatment strategy behind reverse responding.

Reverse responding is not intolerance, weakness, anxiety, noncompliance, or failure. It is an adaptive response rooted in threat-state physiology, chronic stress, and lived experience. Your body is not broken—it is protecting you.

In this episode, Dr. McCarthy covers:

What reverse responding actually is (and what it is not)

The difference between sulfation and 5-alpha pathways

Why labs often miss this entirely

Why “just more progesterone” makes things worse

How trauma, chronic stress, and safety shape hormone response

The importance of earning permission from the nervous system

Practical treatment pillars:

Glycemic stability

Circadian safety and sleep rhythm

Reducing inflammatory load

Gentle nervous system regulation

Slow, low, respectful progesterone onboarding

Supplement strategies used clinically (and what to avoid)

Most importantly, this episode is a reminder:
You are not the problem. Your body is doing something intelligent.

 

Dr. Brendan McCarthy is the founder and Chief Medical Officer of Protea Medical Center in Arizona. With over two decades of experience, he’s helped thousands of patients navigate hormonal imbalances using bioidentical HRT, nutrition, and root-cause medicine. He’s also taught and mentored other physicians on integrative approaches to hormone therapy, weight loss, fertility, and more. If you’re ready to take your health seriously, this podcast is a great place to start.

 

👇 Tap Subscribe to learn more about what’s actually happening in your body, and what to do about it.

 

📘 Read Dr. McCarthy’s Book: Jump Off the Mood Swing – A Sane Woman’s Guide to Her Crazy Hormones https://www.amazon.com/Jump-Off-Mood-Swing-Hormones/dp/0999649604

 

📲 Follow Dr. McCarthy:

Instagram: @drbrendanmccarthy

TikTok: @drbrendanmccarthy

Website: www.protealife.com

 

💬 Got a question or topic for a future episode? Let us know in the comments!