Parathyroid glands

The parathyroid glands are small, oval-shaped endocrine glands located on the posterior aspect of the thyroid gland. They are primarily responsible for regulating calcium and phosphate levels in the blood by secreting parathyroid hormone (PTH). Despite their close anatomical relationship to the thyroid gland, their function is entirely distinct.

Location

Typically, there are four parathyroid glands—two superior and two inferior—embedded in the posterior surface of the thyroid lobes. They are usually found within the thyroid capsule but outside the thyroid parenchyma. Their position can vary, especially the inferior glands, which may be located as low as the mediastinum.

Structure

Each parathyroid gland is a flattened, oval structure measuring about 4–6 mm in length and weighing approximately 30–50 mg. Histologically, the glands are composed of:

• Chief cells: Most abundant; secrete parathyroid hormone (PTH)
• Oxyphil cells: Larger cells with unknown function; appear after puberty

Number and Variations

• Typical: 4 glands (2 superior, 2 inferior)
• Common variation: 5 or more glands, sometimes up to 8
• Ectopic locations: Thymus, pericardium, or even carotid sheath

Function

The main function of the parathyroid glands is to maintain serum calcium and phosphate homeostasis through secretion of parathyroid hormone (PTH). PTH acts on:

• Bones: Increases osteoclast activity, releasing calcium into the bloodstream
• Kidneys: Enhances calcium reabsorption and phosphate excretion; stimulates activation of vitamin D
• Intestine: Indirectly increases calcium absorption by promoting active vitamin D synthesis

Hormonal Control

Parathyroid hormone secretion is regulated via negative feedback:

• ↓ Serum calcium → ↑ PTH secretion
• ↑ Serum calcium → ↓ PTH secretion

Blood Supply

• Primary: Inferior thyroid artery (branch of thyrocervical trunk)
• Secondary: Superior thyroid artery (may contribute to upper glands)

Venous Drainage

• Drains into the thyroid venous plexus → internal jugular vein → brachiocephalic vein

Lymphatic Drainage

• Paratracheal, pretracheal, and deep cervical lymph nodes

Innervation

The parathyroid glands are innervated by autonomic fibers from the cervical sympathetic chain, but hormone secretion is not under direct neural control.

Relations

• Anterior: Thyroid gland
• Posterior: Longus colli muscle and prevertebral fascia
• Lateral: Carotid sheath
• Medial: Trachea and esophagus

Clinical Relevance

• Hyperparathyroidism: Excess PTH causes hypercalcemia, bone resorption, kidney stones, and fatigue
• Hypoparathyroidism: Often occurs after thyroidectomy; leads to hypocalcemia, tetany, and neuromuscular symptoms
• Parathyroid adenoma: A common cause of primary hyperparathyroidism; usually requires surgical removal
• Parathyroidectomy: Surgical removal of one or more glands in hyperfunctioning states
• Ectopic parathyroid glands: May lead to persistent disease post-surgery if not identified