In CT scan of the thyroid, focal and diffuse thyroid abnormalities are commonly encountered. These findings can often lead to a diagnostic dilemma, as the CT reflects nonspecific appearances. Ultrasound (US) examination has a superior spatial resolution and is considered the modality of choice for thyroid evaluation. Nevertheless, CT detects incidental thyroid nodules (ITNs) and plays an important role in the evaluation of thyroid cancer.

This pictorial review covers a wide spectrum of common and uncommon, incidental and non-incidental thyroid findings from CT scans. It will also include the most common incidental thyroid findings. In addition, the role of imaging in the assessment of thyroid carcinoma (before and after treatment) and preoperative thyroid goiter is explored, as well as localization of ectopic and congenital thyroid tissue.

Thyroid ultrasonography is the modality of choice for thyroid evaluation. Yet, focal and diffuse thyroid abnormalities are commonly encountered during the interpretation of computed tomography (CT) exams performed for various clinical purposes. For example, CT often detects incidental thyroid nodules (ITNs). It plays an important role in the evaluation of abnormal structures including thyroid cancer.

Thyroid disorders are common and include many entities. They can be symptomatic, asymptomatic, diffuse, focal, neoplastic, or non-neoplastic processes. Neck ultrasound (US), with the prospect of proceeding to fine needle aspiration (FNA), is the first line of investigation; however, other options are available. Thyroid Uptake Scans using Tc-99 m or I-123 are typically reserved for specific clinical scenarios. Cross-sectional imaging including computed tomography (CT) and magnetic resonance imaging (MRI) detect incidental thyroid nodules (ITNs) and can be used in the evaluation of thyroid cancers and goiter. The aim of this article is to provide a pictorial review of a broad spectrum of incidental and non-incidental thyroid findings on CT scans.

The thyroid gland is a vascular, encapsulated structure made up of right and left lobes, which are connected at the midline by the isthmus. Each lobe is about 2 cm thick, 3 cm wide, and 5 cm long. The thyroid apex is located superiorly at the level of the mid-thyroid cartilage. The inferior margin of the gland is at the level of the fifth or sixth tracheal ring. The thyroid gland is encapsulated by the middle layer of deep cervical fascia and is part of the visceral space in the infrahyoid neck. It wraps around the trachea and is separated from the esophagus by the tracheoesophageal groove on each side, which houses the recurrent laryngeal nerves. The thyroid has variable lymphatic drainage to the internal jugular chain, para-tracheal region, mediastinum, and retropharyngeal area. It has homogeneous high attenuation values on a CT scan, as compared to adjacent muscles, due to its high iodine concentration. It shows avid iodine contrast enhancement due to its hypervascularity.

Multi-detector volumetric acquisition from the skull base to the tracheal bifurcation is usually obtained. Multiplanar 2-mm axial, coronal, and sagittal images are typically available. Examination can be acquired with or without administration of intravenous (IV) iodinated contrast.

The thyroid gland can have variable CT scan findings, such as calcifications, single or multiple nodules, cysts, or diffuse enlargement.

Thyroid calcifications on a CT scan can be seen in both benign and malignant thyroid lesions. Sonographic examination of the thyroid can differentiate between micro-calcifications, which are highly associated with papillary thyroid carcinoma, and eggshell calcifications, which favour a benign process such as colloid cysts (Figs. 1 and 2). In a retrospective review of preoperative CT scan, 35% (135 of 383) of the patients had detectable intrathyroidal calcifications. Among them, 48% had a histopathologically proven thyroid cancer. Calcified nodules had a significantly higher incidence of thyroid cancer and lymph node metastases. The incidence of thyroid cancer among nodules with different calcifications patterns was 79% of nodules with multiple punctate calcifications, 58% of nodules with a single punctate calcification, 21% of nodules with coarse calcification, and 22% of nodules with peripheral calcification. Most of the single calcified nodules were malignant. However, this did not include patients with ITNs and the sample is skewed towards malignancy. Another study evaluated the presence of ITNs on CT scans and found that 12% of thyroid nodules were calcified, with no significant correlation between malignant or potentially malignant histology and punctate calcifications. As a result, some researchers believe that calcification per se is not a suspicious CT sign, and have suggested that calcified thyroid nodules on CT scans should be treated the same as non-calcified nodules.