Cowden syndrome is characterized by multiple hamartomatous and neoplastic lesions including Lhermitte-Duclos disease, which is the main criterion for the diagnosis. Herein, we presented a patient with suspected metastatic disease referred to PET/CT, which showed mildly hypermetabolic multinodular thyroid goiter, multiple hamartomatous pulmonary, and breast nodules. Also, intense hypermetabolism was noted on the cerebellar tumor lesion. Lhermitte-Duclos disease was diagnosed based on the characteristic MRI findings, and she was followed up with a diagnosis of Cowden syndrome. Our case indicates that Cowden syndrome should be included as a differential diagnosis of abnormal FDG uptake in the multiple systemic hamartomatous tumors.