GLOSSARY

The parathyroid glands are usually four small glands (each about 30–50 milligrams, the size of a grain of rice) on the posterior surface of the thyroid gland; by secreting parathyroid hormone (PTH), they are the main, minute-to-minute regulators of blood calcium.

Structure

• Chief cells: secrete PTH and carry the calcium-sensing receptor (CaSR), which monitors ionised calcium in the blood.
• Oxyphil cells: increase with age; their role is unclear.
• Variable location: about 10–15 percent of people have more than four glands or ectopic glands (in the thymus or within the thyroid).

Vital Importance

Accidental removal of all four glands during thyroid surgery → severe hypocalcaemia → tetany, muscle spasm, seizures, and arrhythmia; preserving them in surgery is therefore vital.

Clinical Application

• Primary hyperparathyroidism: the most common cause of high calcium in outpatients (about 85 percent a single adenoma); the classic features are “stones (kidney), bones (pain and osteoporosis), abdominal groans (constipation and ulcers), and psychic moans (depression and fatigue)”, though today it is often asymptomatic and found incidentally on blood tests.
• Hypoparathyroidism: usually after neck surgery; low calcium and high phosphate.
• Secondary hyperparathyroidism: in kidney failure and vitamin D deficiency.

Relevance to Sport

• Practical example: dermal calcium loss through sweating during prolonged endurance exercise may raise PTH and stimulate bone resorption; studies in cyclists have shown that taking about 1,000 milligrams of calcium 90 minutes before exercise reduces the rise in PTH and in bone resorption markers, one possible explanation of the low bone density seen in professional cyclists.