The Parathyroid Gland

1
The Parathyroid Gland Calcium Balance

• Felix E. Grissom, Ph.D.
• 2219 Numa P.G. Adams Bldg
• Howard University
• Tel. [phone]

2
Parathyroid Gland Anatomy

• Four Parathyroid glands are usually found
  posterior to the thyroid gland
• Total weight of parathyroid tissue is about 150mg
• Parathyroid hormone (PTH) is made by these glands

3
Biosynthesis, Storage Secretion of PTH

• PTH is synthesized as the preprohormone
  (Preproparathyroid Hormone) by parathyroid gland
  chief cells
• The active form of PTH is cleaved from the
  preprohormone before release from the gland
• PTH is synthesized continously (it is either
  released from the gland or degraded)
• PTH is released by exocytosis in response to
  reduced plasma calcium
• Vitamin D feeds back to reduce PTH secretion as a
  secondary mechanism

4
Biological Activity of PTH

• BONE
• PTH stimulates bone osteoblasts to increase
  growth metabolic activity
• PTH stimulated bone resorption releases calcium
  phosphate into blood
• KIDNEY
• PTH increases reabsorption of calcium reduces
  reabsorption of phosphate
• Net effect of its action is increased calcium
  reduced phosphate in plasma
• INTESTINE
• Increases calcium reabsorption via vitamin D

5
Calcium Regulation of PTH Release

• Parathyroid gland chief cell calcium receptor has
  two signal transduction systems
• Inositol triphosphate which reduces PTH release
• cAMP which increases PTH release

6
Calcitonin

• Calcitonin is a peptide hormone secreted by the
  parafollicular or C cells of the thyroid gland
• It is synthesized as the preprohormone released
  in response to high plasma calcium
• Calcitonin acts on bone osteoclasts to reduce
  bone resorption.
• Net result of its action is a decline in plasma
  calcium phosphate

7
Synthesis, Release Activity of Active Vitamin D

• Vitamin D3 is may be obtained from the diet or
  made in the skin
• It is converted to the active form (1,25-OH-D3 by
  sequential enzymatic reactions in the liver and
  kidney (stimulated by PTH)
• Vitamin D3 stimulates intestinal calcium uptake,
  increased bone calcium resorption increased
  kidney phosphate uptake

8
Bone Growth and Calcium Metabolism

• Epiphyseal plate new bone growth site
• Chondrocytes, osteoblasts calcification build
  bone

9
Bone Growth and Calcium Metabolism
Figure 23-19 Bone growth at the epiphyseal plate
10
Calcium Metabolism
Figure 23-20 Calcium balance in the body
11
Control of Calcium Balance Metabolism

• Parathyroid H
• Vitamin D
• Sun/diet
• Calcitonin
• Thyroid
• C-cells
• (Phosphate balance)

Figure 23-23 Endocrine control of calcium balance
12
Hormonal Regulation of Calcium and Phosphate
Balance

• Decreased Plasma Calcium Causes
• Increased PTH
• Resulting in mobilization of bone Ca phosphate,
  increased renal phosphate excretion Ca
  retention and increased Vitamin D3 synthesis
• The outcome is a rise in plasma Ca levels
  maintenance of normal phosphate levels

13
Osteoporosis Disease of Bone Growth Calcium
Metabolism

• Bone reabsorption exceeds deposition
• Osteoclasts mobilize Ca to plasma
• Factors inadequate Ca intake, genes, hormones,
  smoking

Figure 23-21 Osteoclasts are responsible for
bone resorption
14
Summary

• PTH calcitonin release are regulated by plasma
  Ca levels
• Bone Ca phosphate serve as a ready reserve for
  maintenance of plasma levels
• Bone, kidney intestine participate in the
  regulation of plasma calcium
• PTH, Vitamin D, calcitonin balance plasma
  Ca for bone synthesis, muscle contraction,
  cell signaling
• Endocrine diseases result from pathway or
  glandular hypo or hyper secretion