Hormone
Hormone
Hormone
Parathyroid Hormone
- Protein hormone produced by chief cells of the parathyroid gland
- Secretion stimulated by a decrease in serum calcium levels
- Normal levels are 1.3 – 6.8 pmol/L
- Acts to increase serum calcium and decrease serum phosphate levels through a number of mechanisms:
- Kidney: Increases calcium reabsorption
- Bone: Mobilises calcium from bone stores by stimulating osteoclast activity and inhibiting osteoblast activity
- GI Tract: Increases GI absorption of calcium and phosphate indirectly by production active vitamin D metabolites
Vitamin D3
(cholecalciferol)
(cholecalciferol)
- Originates from:
- Dietary intake
- Synthesis in the skin through effects of UV light on 7-dehydrocholesterol
- Modified by liver and kidney to form active 1-25-hydroxycholecalciferol - under the influence of parathyroid hormone
- Levels also increase in response to low phosphate levels
- Acts to increase serum calcium levels
- GI Tract: Increases GI absorption by upregulating number and activity of transport proteins
- Kidneys: Weak effect to increase calcium and phosphate reabsorption
- Plays a minimal role in calcium homeostasis, secreted only when levels rise above normal limits
Calcitonin
(minimal role)
(minimal role)
- Peptide hormone produced by parafollicular cells of the thyroid gland
- Secretion stimulated by an increase in serum calcium levels
- Acts to decrease serum calcium levels when elevated (physiological antagonist to PTH)
- Bone: Prevents resorption by inhibiting osteoclast activity and stimulating osteoblast activity
- Kidneys: Weak action to decrease calcium reabsorption