Thyroid Scan A thyroid scan evaluates the size, shape and location of your thyroid gland. When you arrive for your appointment, a small amount of radioactive tracer will be injected into a vein in your arm. Then, several images are taken over your neck. A thyroid scan takes about 60 to 90 minutes. Preparation You should be off all thyroid hormones for six weeks prior to your appointment unless specifically instructed by your physician. You should not have any exams where X-ray dye was injected, such as a CT scan, IVP X-rays, etc., for at least four weeks before your appointment. Kelp (seaweed) or sushi wrapped in kelp should not be eaten four to six weeks before the scan. You may eat and drink as usual the day of your appointment. You may take your medications as directed by your physician. Remember to bring the written order (prescription), for this test, given to you by your doctor