From the Guidelines
When a PET scan shows increased uptake in the thyroid gland without a palpable mass, a dedicated thyroid ultrasound, thyroid function tests (TSH, free T4, free T3), and fine needle aspiration (FNA) biopsy of any suspicious areas identified on ultrasound are recommended for further evaluation. The ultrasound can detect nodules as small as 2-3mm that may not be palpable on physical examination 1. If the ultrasound reveals nodules, they should be characterized according to size, echogenicity, margins, and vascularity to determine risk of malignancy. For suspicious nodules, FNA biopsy should be performed under ultrasound guidance. Additionally, thyroid antibody tests (anti-TPO, anti-thyroglobulin) may be useful to evaluate for autoimmune thyroid disease, which can sometimes cause diffuse FDG uptake on PET. This comprehensive evaluation is important because incidental thyroid uptake on PET scans carries a significant risk of malignancy (approximately 30-50% when focal), even in the absence of palpable abnormalities, as noted in the guidelines for thyroid cancer diagnosis and treatment 1.
Some key points to consider in the workup include:
- The use of FDG-PET combined with CT for assessing the extent of disease and defining the prognosis, with a sensitivity of around 94% and specificity between 80% and 84% 1
- The importance of characterizing nodules based on ultrasound findings, such as size, echogenicity, margins, and vascularity, to determine the risk of malignancy
- The role of FNA biopsy in diagnosing thyroid cancer, with a recommendation for its use in suspicious nodules identified on ultrasound
- The potential for autoimmune thyroid disease to cause diffuse FDG uptake on PET, and the usefulness of thyroid antibody tests in evaluating this condition
If initial workup is negative but clinical suspicion remains high, follow-up imaging in 6-12 months may be warranted, as suggested by the guidelines for thyroid cancer diagnosis and treatment 1. It is essential to prioritize the most recent and highest quality studies, such as those published in 2019 1, to guide the workup and management of patients with increased uptake on PET in the thyroid gland without a palpable mass.
From the Research
Incidental Thyroid Uptake on PET
- Incidental thyroid uptake on PET scans is a common finding, with a prevalence of 3.4% to 3.8% in patients with non-thyroidal malignancies 2, 3, 4.
- The majority of these cases are focal uptake, with a significant proportion being malignant, ranging from 27.8% to 63.6% 2, 3, 4, 5.
- The intensity of FDG uptake, measured by SUVmax, does not always correlate with the risk of malignancy, although some studies suggest that higher SUVmax values may be associated with malignant lesions 2, 4, 5.
Further Workup
- Fine-needle aspiration cytology (FNAC) and ultrasonography are recommended for further evaluation of incidental thyroid uptake on PET scans 3, 4, 5.
- Surgery may be indicated in cases with suspicious cytology or high SUVmax values 3, 5.
- The role of SUVmax in differentiating benign from malignant lesions is still being investigated, with some studies suggesting a cutoff value of 9.1 for malignancy 5.
Clinical Significance
- Incidental thyroid uptake on PET scans can be a significant finding, with a high risk of malignancy, particularly in cases with focal uptake 2, 3, 4, 5.
- Prompt and appropriate follow-up is warranted in these cases, including further imaging and cytological evaluation 3, 5.
- The use of FDG-PET/CT in the follow-up of differentiated thyroid cancer patients with negative iodine-131 whole-body scan and elevated anti-Tg level may be useful in detecting recurrence/metastasis 6.