CT scans can show thyroid size, nodules, calcifications, and extension into the chest, and help stage thyroid cancer and evaluate lymph nodes. However, ultrasound is the preferred first-line test for thyroid nodules because it better characterizes small lesions. CT involves radiation and iodinated contrast can affect future radioactive‑iodine therapy, so CT is used when ultrasound is limited or for surgical planning and staging.

To prepare for a thyroid CT scan: bring ID and a list of medications/allergies, and tell staff if pregnant or diabetic. If IV contrast is planned, you may be asked to fast 4–6 hours, stop metformin at the time of scan and for 48 hours afterwards (restart only after renal function is checked), and have recent kidney tests. Remove jewelry, wear loose clothing, and arrive early.

Warning signs of thyroid cancer include a new lump or swelling in the front of the neck, a painless thyroid nodule, or a rapidly growing neck mass. Other red flags are persistent hoarseness or voice changes, difficulty swallowing or breathing, neck or throat pain that can radiate to the ear, and swollen lymph nodes. A long-lasting unexplained cough not caused by a cold may also be concerning.

For thyroid evaluation, ultrasound is the first‑line scan to assess gland size, nodules and cysts. For functional assessment and to distinguish “hot” versus “cold” nodules, thyroid scintigraphy (a radioactive iodine uptake scan using I‑123 or technetium‑99m) is performed. PET‑CT or radioactive‑iodine whole‑body scans are reserved for thyroid cancer staging or recurrence.

Ultrasound is the first-line test for thyroid nodules and guides fine-needle aspiration. CT and MRI are not routine for initial thyroid evaluation: CT is useful for assessing large substernal goiters, intrathoracic extent and calcifications; MRI provides superior soft-tissue detail without radiation and helps evaluate local invasion or mediastinal extension. Avoid iodine-containing CT contrast if radioactive iodine therapy may follow.

A CT scan of the thyroid itself usually takes about 5–10 minutes. Including arrival, registration, positioning and IV placement for contrast, the appointment commonly lasts 30–60 minutes. If contrast dye is used, allow extra time for injection, scanning and brief observation afterward. Your clinic will give specific instructions on arrival time and any fasting or medication restrictions.