Endocrine: Parathyroid Anatomy & Physiology

Parathyroid Anatomy

Embryology Mn

Superior Parathyroids: From Fourth Pharyngeal Pouch
- Along with Lateral Thyroid
Inferior Parathyroid: From Third Pharyngeal Pouch
- Along with Thymus

Average Size

Typical Gland Location

Superior Parathyroids
- Posterolateral to Recurrent Laryngeal Nerve
- Above Inferior Thyroid Artery
Inferior Parathyroids
- Anteromedial to Recurrent Laryngeal Nerve
- Below Inferior Thyroid Artery

Abnormal Gland Location

Missing Gland Locations:
- Most Common Location of a Missed Gland: Normal Position
- Most Common Location of an Ectopic Gland: Thymus
Most Common Ectopic Sites:
- Superior: Retroesophageal or Paraesophageal Space
  - 80% of Superior Parathyroid Glands Will Appear in the Same Position as the Contralateral Gland
- Inferior: Tail of Thymus
  - Position is More Variable (Further Travel)
Different Number Occurrence
- Supernumerary (Extra Glands)
  - Frequency: 13% (More Common than Infranumerary)
  - Most Common Site: Thymus
- Infranumerary (≤ 4 Glands)
  - Frequency: 3%

Blood Supply

Inferior Thyroid Artery
- Supplies Both Superior & Inferior Parathyroid Glands
- From the Thyrocervical Trunk (Off Subclavian Artery)
- Artery Enters Gland Medially – If Preforming a Partial Resection Should Resect the Lateral Half (Spare the More Perfused Portion)

Parathyroid Glands and Inferior Thyroid Artery ¹

Parathyroid Gland (Arrow) ²

Parathyroid Hormone (PTH)

Secreted by Parathyroid Chief Cells
Short Half-Life: 2-4 Minutes
Overall Effects:
- Increased Calcium, Hydrogen (Acidosis) & Chloride
- Decreased Phosphate Mn
Renal Effects:
- Increase Calcium Absorption (Overall Increased Urinary Calcium)
- Decrease Phosphate Absorption
- Increase Vitamin D Production
- Inhibits Na/H Antiporter > Inhibits Bicarb Reabsorption > Acidosis > High Chloride
- Urine cAMP Increased
Bone Effects:
- Increase Osteoclast Calcium Secretion Mn
GI Effects:
- No Direct Effect
- Increased Vitamin D from Kidney Increases GI Absorption of Calcium

Calcium Physiology

Parathyroid Embryology

P-P: Parathyroids from Pouch (Not Arch or Cleft)
4-4: 4 Glands – First Think of the Fourth Pouch (4)
- Superior: Bigger Number (4) – Superior Parathyroids from Fourth Pharyngeal Pouch
- Inferior: Smaller Number (3) – Inferior Parathyroids from Third Pharyngeal Pouch

PTH Effects on Phosphate

Bone Cells Affected by PTH/Calcitonin

C-C: Calcium Alters by Changes in Osteo-Clasts (Not Osteoblasts)
- For Both PTH and Calcitonin