The parathyroids are endocrine glands intimately involved with calcium balance. They secrete parathormone (PTH) in response to low serum calcium and secrete less PTH when the calcium is normal or high. The incidence of hyperparathyroidism in the general population is 25/100,000 people. It is most common in women after menopause and in women greater than 65 years of age the incidence rises to 2.5/1000.

Anatomy and Physiology

• The parathyroid glands are typically four in number. (Figure1) The glands may be found anywhere from the mandible to the pericardium (sac that covers the heart) but are mostly behind the middle or upper third of the thyroid gland. The parathyroid glands are not found behind the thyroid gland in about 10% of cases. They are yellow-brown to red-brown in color, 5-6 x 3 x 1-2 millimeters in size and 35 - 40 grams in weight
  1. The glands may be in the capsule of the thyroid, beneath it or within the thyroid
  2. The blood supply to the parathyroid glands is from the superior and inferior thyroid arteries
  3. The glands have an intimate relationship with the recurrent laryngeal nerves that control the vocal cords
• Under the microscope, parathyroid tissue in adults shows three kinds of cells:
  1. Chief cells that secrete Parathormone (PTH). The usual adenoma (tumor) is of chief cell origin
  2. Oxyphil cells, appear at puberty, and can cause hypercalcemia (increased blood calcium)
  3. Fat cells are few in number but increase at puberty
• Serum calcium is regulated as follows:
  1. A decrease in serum calcium causes a rise in PTH which in turn causes a rise in 1,25-(OH)2D3 (metabolite of the natural form of vitamin D3)
  2. PTH and 1,25-(OH)2D3 then cause calcium and phosphorus to come out of bone and enhance intestinal absorption of calcium and phosphorous. For vitamin D3 to be active it is converted in the liver to 25-hydroxyvitamin D3 (25-OH D3). The 250OH D3 is further converted in the kidney to 1,25-dihydroxy-vitamin D 3 (1,25-(OH)2D3). People who have had their kidneys removed cannot convert vitamin D3 to its active form and consequently cannot raise their serum calcium
  3. High serum calcium results in lower levels of PTH and 1,25-(OH)2D3.
  4. The usual dietary intake of calcium is between 500 milligrams to 1000 milligrams per day. Calcium is excreted in the urine normally 100 - 200 milligrams per day, sweat up to 100 milligrams per day, and the remainder in the stool. Calcium absorption is mainly in the duodenum and jejunum
  5. Calcium is frequently tied to bone density alone but it has many other functions. Calcium has major roles in nerve function, muscle contraction, cardiac muscle contraction, hormone release and function of body cells

Pathology

Hyperparathyroidism (increased PTH with elevated calcium) is the most common reason for parathyroid surgery and comes in several forms:

• Parathyroid adenoma - a benign (noncancerous) tumor with unregulated release of PTH. A single adenoma is found in 70-80% of cases and double adenoma in 4-5%
• Parathyroid hyperplasia - a diffuse enlargement of all the parathyroid glands with unregulated release of PTH. It is found in10-15% of cases and is a difficult operative diagnosis
• Multiple Endocrine Neoplasia Type - familial syndrome with abnormalities of thyroid, adrenal, and parathyroid glands usually producing hyperplasia of the parathyroid glands
• Parathyroid carcinoma (cancer) - a rare cause and less than 1%
• Familial Hyperparathyroidism - genetic in nature, inherited and rare

Figure 1 - Side view of the parathyroid glands on the back side of the thyroid gland. Note the relationship of the recurrent laryngeal nerve.© C. Scalici