Understanding the Parathyroid Glands: Location, Function, and Clinical Significance
Introduction
Minute yellowish-brown endocrine glands, the parathyroid glands are flattened and oval in shape. The majority of people have four parathyroid glands, although variation in number is common.
- Location: Posterior aspect of lateral lobes of thyroid gland
- 2 on each side (superior and inferior)
- Within the sheath (fibrous capsule) of the thyroid gland
- Dimension:
- Length – about 6mm
- Width – 3.5mm
- Weight – about 50mg each
Microanatomy
Parathyroid parenchyma consists of principal (or chief) cells.
- Chief cells are arranged in columns or cords, separated by sinusoids.
- They produce parathyroid hormone.
Anatomical Division
Anatomically, the glands can be divided into 2 pairs:
- Superior parathyroid glands
- Usually midway along the posterior border of the thyroid gland
- Approximately 1cm superior to the entry of the inferior thyroid arteries
- At the level of the inferior border of the cricoid cartilage
- It is relatively constant in position
- It is derived from the 4th pharyngeal pouch
- It is thus also called parathyroid IV
- Inferior parathyroid gland
- Usually behind the lower pole of the thyroid lobe
- It is more variable in position
- May be found in the superior mediastinum, close to the thymus
- Such variation is of surgical importance
- It develops from the 3rd pharyngeal pouch
- It is thus also called parathyroid III
Neurovasculature
- Arterial Supply: It is supplied by branches of the inferior thyroid arteries (which supply the posterior aspect of the thyroid gland).
- Collateral circulation is delivered by:
- The superior thyroid arteries
- Thyroid ima artery
- Laryngeal artery
- Tracheal artery; and
- Oesophageal artery
- Venous Drainage: The parathyroid veins drain into the thyroid plexus of veins (on the anterior aspect of the thyroid gland and trachea).
- Lymphatic Drainage: The lymphatic vessels drain along with those of the thyroid gland into the:
- Paratracheal lymph nodes; and
- Deep cervical lymph nodes
- Nerve Supply: Nerve fibres reach the gland from the cervical sympathetic ganglia via the plexus around the thyroid arteries.
- These fibers are vasomotor, not secretomotor.
- Endocrine secretion of parathyroid hormone is controlled hormonally.
Importance Of Parathyroid Gland
- It produces parathyroid hormone (parathormone).
- Parathyroid hormone maintains calcium levels in the body:
- By facilitating the mobilization of calcium from bones into the plasma
- A rise in plasma calcium levels inhibits production
- A fall stimulates it
- Its effect is countered by calcitonin:
- Produced by parafollicular cells (C cells) of the thyroid gland
- Calcitonin lowers blood Calcium levels
Clinical Correlates
- Removal of the parathyroid gland:
- The inconsistency in their location increases their risk of being damaged or removed during neck surgery (e.g., in total thyroidectomy).
- This results in hypocalcaemia and tetany (involuntary muscle contraction).
- Because tetany involves respiratory muscles, death may occur as well.
- Hyperparathyroidism:
- Excess production of parathyroid hormone occurs in:
- Parathyroid adenocarcinoma or
- Primary hyperparathyroidism (with calcaemia)
- This results in osteitis fibrosa cistica:
- Owing to excessive endosteal osteoclastic erosion of bone and removal of calcium ions from bone
- Calcium could also be deposited in renal tubules (nephrocalcinosis) and arteries
- Causing renal and cardiovascular diseases (atherosclerosis, myocardial infarction, stroke, etc.)
- Parathyroidectomy could be performed to remove the enlarged hyperactive gland.
- With some risks such as injury to the laryngeal nerve.
- Excess production of parathyroid hormone occurs in:
