The principal blood supply to the parathyroid glands is which of the following?

b. Inferior thyroid arteries

The principal blood supply to both parathyroid glands is the inferior thyroid artery. Parathyroid glands invariably have a single end artery supplying them, and if the main trunk of the inferior thyroid artery is ligated during thyroidectomy, there is no collateral blood supply to maintain their viability. It is preferable to divide only the branch of the inferior thyroid artery medial to those that supply either of the parathyroid glands. This requires individual clamping of smaller vessels under the thyroid sheath as these vessels penetrate into the thyroid capsule. Ligation of the main trunk of the inferior thyroid artery was commonly used for bilateral subtotal thyroidectomy in the past. It did not routinely cause hypoparathyroidism only because enough collateral blood supply was maintained to each end artery to one or more parathyroid glands. This is to be avoided.