How do we know that a thyroid cancer spreads to other glands?

The spread of cancer to other parts of the body can be easily detected with the use of X-Rays, CT scans or PET scans. These tests are useful in cases where the recurrence is suspected based on new symptoms and patient is examined by their physicians first. The spread of Thyroid cancer usually goes to the lungs, Bones or Liver(not to any other glands).

With differentiated thyroid cancers (Papillary or Follicular), a PET-CT scan or a Radioiodine Whole Body Scan can discover thyroid cancer cells anywhere in the body.

We know that Thyroid Cancer metastasizes to other organ systems by means of years of research being reported in well refereed journals over decades of investigations. If you are asking if you have cancer has it spread, see an Oncologist. There are appropriate imaging techniques to help make that determination.