The parathyroid glands, as the prefix "para" implies, lie at the side of the thyroid gland, actually on the transverse or posterior side of the thyroid. They impart no effect on thyroid function, but because of their anatomic proximity, may be damaged by thyroid treatments.
Humans usually have four parathyroid glands that are responsible for regulating the calcium levels in circulation. If the thyroid gland needs to be surgically removed due to a cancerous growth or is treated with radiation to treat cancer, occasionally the parathyroid glands are damaged or even removed as well.