Parathyroid Glands & Calcium Homeostasis
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Hormonal regulation of extracellular calcium and phosphate concentrations by the parathyroid glands.
Key Principles
Free calcium participates in various cellular processes, including:
- Skeletal, cardiac, and smooth muscle contraction
- Nerve conduction
- Blood clotting
- Bone and tooth formation
- Enzyme activation and deactivation
- Participates in cellular metabolism
- Plays a role in enzyme activation and deactivation
Storage and Release:
Calcium and phosphate are stored within hydroxyapatite crystals of bone
- When bone is resorbed, calcium and phosphate are released into the extracellular fluid
- Calcium and phosphate levels are regulated by the same hormones:
- Parathyroid hormone, which is secreted by chief cells of the parathyroid glands
- Vitamin D (in activated form)
(The physiologic role of calcitonin, a hormone released by the thyroid gland in response to increased calcium concentrations, is as of yet uncertain, and, therefore, omitted in this tutorial.)
Parathyroid Hormone Pathways:
In response to lowered extracellular calcium concentration, the parathyroid glands secrete parathyroid hormone (PTH).
Bone:
- In bone, episodic, transient binding of parathyroid hormone causes an increase in new bone synthesis
- Prolonged exposure to parathyroid hormone promotes resorption of old bone, and, therefore, the release of calcium and phosphate into extracellular fluid
Clinical consequences of these dichotomous effects:
- Osteoporosis, which is characterized by loss of bone density, can be treated with intermittent PTH administration
- Continuous release of PTH in individuals with hyperPARAthyroidism causes excessive bone resorption
Kidneys:
- Increased calcium reabsorption in the distal convoluted tubule of the nephrons
- Decreased phosphate reabsorption in the proximal convoluted tubule, which leads to phosphaturia, an increase in phosphate in the urine
PTH stimulates renal activation of Vitamin D
Kidney
- Vitamin D acts increases renal reabsorption of both calcium and phosphate
Small Intestine
- Vitamin D increases calcium and phosphate reabsorption
Bones
- Vitamin D works with parathyroid hormone to facilitate skeletal remodeling, which requires both synthesis and resorption of bone.
Clinical correlation:
Vitamin D deficiency in children causes rickets, in which skeletal development is impaired, the bones are weak, and, consequently, growth is often stunted.
System-wide consequences of calcium imbalances:
Hypocalcemic individuals experience hyperreflexia, muscle twitching and cramping, numbness and tingling
- Trousseau's sign, characterized by involuntary hand and feet spasms, carpopedal spasms, which can be provoked by the examiner by inflating a blood pressure cuff to cause prolonged brachial artery occlusion.
- The Chvostek sign, characterized by hyper excitable facial muscle twitching in response to tapping the facial nerve.