Parathyroid

A

Embryology

• 1.

  Superior parathyroid glands—derived from fourth branchial pouch (same as thyroid)

  • a.

    Most common location: posterior aspect of upper and middle thyroid lobes at level of cricoid cartilage (80%)

  • b.

    Alternative locations: tracheoesophageal groove, paraesophageal or retroesophageal, middle or posterior mediastinum, intrathyroidal, carotid sheath

• 2.

  Inferior parathyroid glands—derived from third branchial pouch (same as thymus)

  • a.

    Most common location: within 1 cm of intersection of inferior thyroid artery and recurrent laryngeal nerve

  • b.

    Alternative locations: in the thymus, mediastinal, intrathyroidal, thyrothymic ligament, submandibular; variable due to long descent pathway

B

Anatomy

• 1.

  Most patients have four glands; 13% have more than four; 3% have fewer than 4.

• 2.

  Golden yellow to light brown color

• 3.

  Measure up to 7 mm

• 4.

  Weigh 40–50 mg each

• 5.

  Superior—dorsal to recurrent laryngeal nerve

• 6.

  Inferior glands—ventral to recurrent laryngeal nerve

• 7.

  Main blood supply: branches of inferior thyroid artery, branch of superior thyroid artery may supply upper glands; drain via ipsilateral thyroid veins

• 8.

  Histology: composed of chief cells (which produce parathyroid hormone [PTH]) and oxyphil cells

C

Physiology

• 1.

  Total serum calcium level: 8.5–10.5 mg/dL

• 2.

  PTH acts to increase calcium at:

  • a.

    Bone—stimulates osteoclasts, releasing calcium and phosphate

  • b.

    Kidney—absorption in proximal convoluted tubule, limits excretion in distal convoluted tubule

  • c.

    Intestine—enhanced 1-hydroxylation of 25-hydroxyvitamin D, indirectly increases absorption

A

General

• 1.

  Increased PTH production from abnormal parathyroid glands leads to increased enteral absorption, increased hydroxylation of vitamin D, and decreased renal clearance.

• 2.

  Incidence: 0.1%–0.3%; more common in women

• 3.

  Single gland or parathyroid adenoma (80%); multiple adenomas or hyperplasia (15%–20%); parathyroid carcinoma (1%)

B