Dr. Brendan McCarthy

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!