The main purpose of this study is to see how well FDG-PET scans can determine the malignancy of thyroid nodules that have already been tested (and come back positive) by fine needle aspiration.

While FNA is a sensitive test for diagnosing thyroid tumors, it cannot differentiate benign from malignant follicular nodules and sometimes yields equivocal results due to inadequate sampling or indeterminate cytology. The standard of care for patients with equivocal or follicular histology is surgical removal of these nodules, most of which are benign in nature. FDG-PET, as evidenced by our prior experience and studies from other groups, may have application in discriminating benign from malignant disease in these patients with equivocal or follicular FNA results using standardized uptake value determination. We have demonstrated the feasibility and preliminary clinical utility of using limited neck FDG-PET exams in patients with indeterminate thyroid nodules in a pilot study. The purpose of this trial is to prospectively evaluate a larger series of patients with equivocal or follicular histology on FNA to more accurately define the sensitivity and specificity of FDG-PET for diagnostic imaging of these nodules. In addition, the utility of this modality in identifying metastatic foci in patients with thyroid cancer having follicular or equivocal histology on FNA will be assessed. If the sensitivity and specificity of this modality are determined to be high (≥95%) for diagnosing malignant nodules in these patients, many patients with benign disease may potentially benefit by avoiding unnecessary operations.

Inclusion Criteria:

• Documented history of a solitary thyroid nodule or a dominant nodule within multinodular disease, with fine needle aspiration demonstrating a follicular or indeterminate cytologic examination. If a core needle biopsy was performed instead of a fine needle aspiration, demonstrating follicular or indeterminate cytology, the patient is eligible if the biopsy procedure was felt to be minimally disruptive to the nodule architecture, based on a review by the PI or nuclear medicine investigator.
• Thyroid nodule must be palpable on physical examination or have a minimum size of 1 cm in diameter by ultrasonography, CT or MRI. The minimum size criterion was established to address the spatial resolution limitations of PET/CT imaging.
• Scheduled for surgical excision of thyroid nodules within 3 months of the date of the FDG-PET/CT scan.
• Ability to tolerate lying supine for a FDG-PET/CT examination.
• Age >/= 18 and </= 105 (This disease is rare in children and therefore the study will be limited to adults.)
• Willing to participate in all aspects of the study (patient may opt out of the tissue collection portion.)
• Patient must be euthyroid with a serum TSH or a free T4 level within the institutional upper and lower limits of normal, measured within 6 months of registration. NOTE: mild deviations from the institutional normal limits may be considered acceptable if the patient has achieved a clinically euthyroid state with medication at a stable dose for >3 months, and the TSH is considered to be at target by the patient's treating physician. In patients with hyperthyroidism requiring treatment, this euthyroid state may be achieved with administration of a thionamide such as propylthiouracil prior to FDG-PET/CT exam. Patients with hyperthyroid inflammatory conditions such as thyroiditis and toxic multinodular goiter often exhibit increased glucose uptake resulting in diffuse uptake of FDG which may obscure visualization of a thyroid tumor.
• If female, patient must have a negative pregnancy test at the time of registration, be post-menopausal (with no period in the last twelve months), have had a tubal ligation at least twelve months ago, or have had a hysterectomy.
• In patients with multinodular disease and a dominant nodule, the nuclear medicine physician responsible for FDG-PET/CT scan interpretation must determine whether the indeterminate nodule can be discriminated on FDG-PET/CT imaging prior to enrollment.
• A signed and dated written informed consent obtained from the patient or the patient's legally acceptable representative prior to study participation.

Exclusion Criteria:

• Patient has a fasting glucose level > 200 mg/dL at the time of the PET/CT scan
• Patient has had prior neck surgery or radiation that in the opinion of the investigator has disrupted tissue architecture of the thyroid
• Patient has evidence of infection localized to the neck in the 14 days prior to the FDG-PET/CCT scan
• Patient does not meet any of the inclusion criteria