Final answer:
The normal parathyroid glands, usually numbering four and located on the posterior surface of the thyroid gland, are not typically visible sonographically. They produce parathyroid hormone (PTH) to regulate blood calcium levels. Dysfunction can lead to conditions like hypocalcemia or hypercalcemia.
Step-by-step explanation:
The normal parathyroid glands are typically not visible sonographically in healthy individuals because they are very small and located behind the thyroid gland, which can obscure them on an ultrasound. The parathyroid glands, which are most often four in number, are situated on the posterior surface of the thyroid gland. They are embedded within the thyroid tissue and are surrounded by a thick connective tissue capsule. These glands are vital as they produce parathyroid hormone (PTH), which regulates blood calcium levels by enhancing reabsorption of calcium by the kidneys, stimulating the activity of osteoclasts, and promoting the synthesis and secretion of calcitriol by the kidneys, thus influencing calcium absorption by the digestive system.
Hypocalcemia, a condition characterized by abnormally low calcium blood levels, may occur in hypoparathyroidism, often after thyroid surgery due to the accidental removal of parathyroid tissue. Conversely, hypercalcemia, or abnormally high calcium blood levels, can be indicative of primary hyperparathyroidism. The glands are made up of two cell types: chief cells, which produce PTH, and oxyphil cells, whose function is not clearly understood. Proper functioning of the parathyroid glands is essential for maintaining homeostatic calcium levels in body fluids.