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.

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.