How soon can you have a thyroid scan after a cat scan with contrast?

3 months. It depends on the contrast medium used during the scan. When it contains iodine, it is recommended to wait 3 months before performing a thyroid uptake measurement and scan.
8 weeks. If you had a ct with IV contrast, you should wait 8 weeks prior to having an iodine thyroid scan. Ideally you could also measure plasma inorganic iodine or urinary iodine prior to the thyroid study, although these tests are not widely available. I know they you can get them through the mayo clinic laboratory.