What does a thyroid scan check for?

Different things. We use thyroid scans to help distinguish between functioning and non-functioning thyroid nodules, to assess activity of the thyroid for purposes of ablation in cases of grave's disease, to follow up patients who've had their thyroid glands removed because of cancer.
Goiter & cancer. A thyroid scan evaluates the size and consistency of the thyroid gland. The scan detects how some areas of the thyroid are working either more or less when compared to adjacent areas of the gland. This information is useful in assessing the presence of a goiter, a thyroid noduie or thyroid cancer.