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Diffusely FDG-avid thyroid found on PET scan
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A 55-year-old man was sent for consultation for a hypermetabolic thyroid found on 18F-fluorodeoxyglucose PET imaging.
The patient had the PET/CT scan for the evaluation of a 2-cm pulmonary nodule discovered after a prolonged episode of coughing. The pulmonary mass was resected and revealed a well-defined infarcted pulmonary nodule with acute inflammation and no tumor. He had no history of head and neck radiation. He also had no family history of autoimmune disease or thyroid disease, including thyroid cancer. The patient was unaware of a thyroid problem and had no symptoms of dysphagia, change in voice or anterior neck pressure. On examination, the patient was euthyroid with a mildly enlarged 25-g goiter that was firm and nontender. No thyroid nodules or cervical adenopathy were palpated.
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Laboratory testing showed a normal thyroid-stimulating hormone of 3.34 mIU/L (normal range: 0.35-5.5); free thyroxine of 0.66 ng/dL (normal range: 0.89-1.89); anti-thyroglobulin antibodies of 235 IU/mL (normal range: <40); and anti-thyroid peroxidase antibodies of 195 IU/mL (normal range: <35).
Review of the PET/CT scan demonstrated an enlarged heterogeneous thyroid (Figure 1A). No enlarged nodes in the neck were seen. After injection of 18F-fluorodeoxyglucose (18F-FDG), the entire thyroid was mildly and uniformly hypermetabolic, shown by the accumulation of the isotope with a standardized uptake value (SUV) of 3.7 (Figure 1B). Fusion of the CT and PET scan demonstrated isotope uptake in the entire thyroid (Figure 1C). Sonographic examination of the thyroid showed a mildly enlarged, hypoechoic thyroid (Figure 2A) with increased vascularity throughout the entire gland (Figure 2B). No nodules were seen.
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Photos courtesy of: Stephanie L. Lee, MD, PhD |
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The ultrasound was consistent with a chronic thyroiditis. The entire thyroid was hypoechoic and similar in echogenicity (Figure 2A) as the overlaying strap muscles, the omohyoid and sternohyoid muscles. Also typical of a chronic thyroiditis is the diffuse increased vascularity of the entire thyroid (Figure 2B). Other sonographic characteristics of thyroiditis are echogenic septations with hypoechoic pseudonodules.
Diffuse 18F-FDG uptake is benign and has been reported to occur in 0.6% to 3.3% of the population. This pattern usually occurs in patients with a chronic thyroiditis, such as Hashimoto’s thyroiditis or Graves’ disease, but may occur as a normal variant. One study found 1.8% of 1,626 PET scans showed diffuse or diffuse plus focal thyroidal uptake of 18F–FDG; 100% of these patients had a thyroiditis confirmed by elevated thyroid antibodies. Interestingly, one of the 25 patients with diffuse uptake and two of the four patients with diffuse plus focal uptake had a thyroid nodule on ultrasound with a biopsy consistent with thyroid cancer.
Thus, it is important that any thyroidal uptake of 18F-FDG on a PET scan should be further evaluated by sonography to exclude a concomitant thyroid nodule besides the chronic thyroiditis that must biopsied to exclude thyroid cancer.
Stephanie L. Lee, MD, PhD, is director of the Thyroid Health Clinic at Boston Medical Center and associate professor of medicine at Boston University School of Medicine. She is also an Endocrine Today Editorial Board member.
For more information:
- Anderson L. AJR Am J Roentgenol. 2010;195:216-222.
- Kurata S. Ann Nucl Med. 2007;21:325-330.
- Pedersen OM. Thyroid. 2000;10:251-259.
- Yoshihara A. Thyroid. 2008;18:579-580.
Disclosure: Dr. Lee reports no relevant financial disclosures.