Intrathyroidal parathyroid adenoma or hyperplasia. An occasionally overlooked cause of surgical failure in primary hyperparathyroidism.

In six surgically proved intrathyroidal parathyroid tumors, neither thyroid scan nor gross appearance of the thyroid at surgery allowed reliable detection of parathyroid glands wholly within the thyroid. Preoperative selective catheterization of thyroid veins with radioimmunoassay for parathyroid hormone facilitated successful removal of hyperfunctioning intrathyroidal parathyroid tissue in all five cases in which it was performed.