Thorotrast is an obsolete contrast medium containing a colloidal suspension of thorium dioxide. Contrast agents based on thorium have been abandoned since they turned out to be carcinogenic.

Thorium (232Th) is a naturally radioactive element. Thorium is the parent nuclide of one radioactive series, and specific thorium nuclides are decay products of three radionuclide series.

See also Thorotrast.