Semaglutide Can Make Brown Fat Look Like Cancer on PET Scans: A Diagnostic Warning

What the researchers found

In a 61-year-old female with Class III obesity who had lost significant weight on semaglutide, PET/CT imaging showed FDG uptake in a right level II lymph node consistent with her known neck mass. However, the scan also revealed extensive brown adipose tissue (BAT) activation throughout the cervical and supraclavicular regions and mediastinum, which mimicked diffuse regional metastasis.

Careful fusion of anatomical CT data with functional PET data was required to distinguish metabolically active brown fat from malignant disease. Without this careful interpretation, the brown fat activation could have led to misdiagnosis of widespread cancer and unnecessary aggressive treatment.

Why it matters

With tens of millions of people now using GLP-1 receptor agonists like semaglutide, radiologists and oncologists need to be aware that these drugs can activate brown fat in ways that confound cancer imaging. PET/CT is a cornerstone of cancer staging and surveillance, and false positives from GLP-1-activated brown fat could lead to unnecessary biopsies, delayed treatment, or overtreatment. This case is among the first to formally document this diagnostic interaction.

How the study worked

This is a clinical case report documenting a single patient encounter at a medical center. The case describes a 61-year-old female on semaglutide who presented with a right-sided neck mass. Diagnostic workup included PET/CT imaging, with detailed analysis of FDG uptake patterns to differentiate brown adipose tissue activation from malignant disease.

What this study cannot tell us

This is a single case report, which provides the lowest level of clinical evidence. It describes one patient's experience and cannot establish how frequently this diagnostic confusion occurs across the broader population of GLP-1RA users. The case does not include data on how commonly brown fat activation causes diagnostic challenges in other patients on semaglutide or other GLP-1 drugs.

How to read the evidence

This is a single case report — the lowest level of clinical evidence. While it effectively raises awareness of a novel diagnostic interaction, it cannot determine how common this problem is or establish clinical guidelines.

When this study was published

Published in 2025, this is a very timely case report addressing an emerging clinical challenge as GLP-1 receptor agonist use continues to surge worldwide.

The bigger picture

GLP-1 receptor agonists are known to activate brown adipose tissue as part of their metabolic effects — this is actually one mechanism by which they may promote weight loss. As these drugs become ubiquitous, their interactions with diagnostic imaging will become increasingly important. This case joins a growing literature on unexpected drug-imaging interactions and highlights the need for updated radiology guidelines that account for the metabolic effects of GLP-1 drugs.

Questions still open

• How frequently does GLP-1 receptor agonist-induced brown fat activation cause clinically significant diagnostic confusion on PET/CT scans?
• Should radiology protocols be updated to include GLP-1 drug use as a standard question before PET/CT imaging?
• Would temporarily pausing GLP-1 therapy before diagnostic PET scans reduce brown fat activation and improve imaging accuracy?

Common questions

Read the original research

Brown Adipose Tissue Mimicking Head and Neck Cancer on PET Scan in a Patient on GLP-1 Drug.

The Laryngoscope, 135(2), 741-743

Citation

Harrison, Daron B; Phillips, Alisa L; Tansey, James B; Clarke, Travis J; Wood, C B; Nedzi, Lucien; Schwartz, David L; Makowski, Liza; Hayes, D N; Newman, Grace; Gleysteen, John P. (2025). Brown Adipose Tissue Mimicking Head and Neck Cancer on PET Scan in a Patient on GLP-1 Drug.. The Laryngoscope, 135(2), 741-743. https://doi.org/10.1002/lary.31815