Woman's 39-lb Breasts Won't Stop Growing: Gigantomastia Discovered on Mounjaro
In a striking case of how GLP-1 medications like Mounjaro can uncover hidden health issues, 30-year-old Tianna Moon from East Anglia, eastern England, was diagnosed with gigantomastia in July. This rare condition causes rapid, excessive breast tissue growth, and Moon first noticed abnormalities after starting Mounjaro (tirzepatide) in 2024. While the drug helped her lose weight, her breasts continued to expand, reaching an estimated 39 lbs.—about 20% of her total body weight.
What is Gigantomastia? Understanding the Rare Breast Growth Condition
Gigantomastia is a medical condition characterized by rapid and excessive growth of breast tissue, often leading to breasts that are disproportionately large relative to the body. According to the Cleveland Clinic, it differs from more common macromastia, which involves larger-than-average breasts but without the extreme, uncontrolled growth seen in gigantomastia.
Causes of Gigantomastia
The exact causes remain multifactorial, including hormonal changes, genetics, and autoimmune disorders, as noted by the Cleveland Clinic. There are only about 300 reported cases worldwide, making it exceptionally rare. While puberty can trigger larger-than-average breasts in some women like Moon, who has dealt with this since adolescence, the accelerated growth in gigantomastia sets it apart.
Gigantomastia vs. Macromastia
Moon's doctor initially considered macromastia, a less severe condition with similar symptoms but slower progression. However, after reviewing her medical history, weight loss on Mounjaro, and escalating breast growth with side effects, the diagnosis shifted to gigantomastia. "But when I broke down medical history, the weight loss on Mounjaro and the increased growth and side effects, he was point-blank like, 'This is gigantomastia,'" Moon recalled.
Mounjaro (Tirzepatide): How This GLP-1 Drug Played a Role in Discovery
Mounjaro is an FDA-approved prescription medication for type 2 diabetes, containing tirzepatide—the same active ingredient in Zepbound, approved for weight management. It works by mimicking GLP-1 and GIP hormones, reducing appetite, increasing fullness, and improving sugar and fat metabolism in the body. For many, this leads to significant weight loss, as it did for Moon.
Paradoxically, while Moon shed pounds overall, her breasts grew noticeably larger, prompting medical evaluation. This contrast highlighted the underlying gigantomastia, which might have gone undetected longer without the weight loss revealing the disproportionate growth. GLP-1 agonists like tirzepatide are not known to cause gigantomastia, but they can bring attention to coexisting conditions through bodily changes.
Common Uses and Benefits of Mounjaro
- Primary: Type 2 diabetes management
- Secondary: Weight loss (as Zepbound)
- Mechanism: Dual GLP-1/GIP receptor agonist for better glycemic control and satiety
Tianna Moon's Personal Story: From Puberty to Diagnosis
Tianna Moon, 30, has had larger-than-average breasts since puberty, but the rapid changes began accelerating after starting Mounjaro in 2024. The diagnosis came via SWNS reporting, confirming gigantomastia in July. Now, her breasts weigh approximately 39 lbs., representing roughly 20% of her body weight—a burden she describes as her "normal" but fraught with daily struggles.
