Linked from
The 13 pages that link to Thorium, each with the reason it gives.
Thorium-232Narrower topic: Thorium-232 is thorium’s most abundant naturally occurring isotope.
PitchblendeRelated: Thorium may occur as a minor constituent in uranium-rich mineral deposits.
ThorianiteNarrower topic: Thorianite is a concentrated natural source of thorium.
AllaniteRelated: Thorium can substitute into allanite and contribute to its radioactivity.