1,25-dihydroxyvitamin D3 (calcitriol) is derived from vitamin D precursors in the skin and diet, and acts in the intestine, bone, and kidney to promote calcium absorption in the gut and mobilize calcium from bone to blood. Insufficient calcitriol can lead to rickets in children and osteomalacia in adults due to impaired calcium deposition in bone. Erythropoietin is a glycoprotein produced in the bone marrow that stimulates red blood cell formation, enabling more oxygen to reach muscles.

2. Hormone Type Target
Tissue
Principal
Action
1,25-
dihydroxyvita
min D3
(Calcitriol)
Sterol Intestine,
bone, kidney
Ga2†
absorption in
gut
synergistic
with RTH
Erythropein Glycoprotein Bone Marrow Erythrocyte
formation

3. It is derived from:
• calciferol (vitamin D3) which is synthesized in skin exposed to
the ultraviolet rays of the sun
• precursors ("vitamin D") ingested in the diet.
Calcitriol acts on:
• the cells of the intestine to promote the absorption of calcium
from food
• bone to mobilize calcium from the bone to the blood

4. • Calcitriol enters cells and, if they contain receptors for
it (intestine cells do), it binds to them. The calcitriol
receptors are zinc-finger transcription factors.
• Insufficient calcitriol prevents normal deposition of
calcium in bone.
• In childhood, this produces the deformed bones
characteristic of rickets (softening of bones).
• In adults, it produces weakened bones causing
osteomalacia.

5. • Erythropoietin is a glycoprotein. It acts on the
bone marrow to increase the production of red
blood cells. Stimuli such as bleeding or moving
to high altitudes (where oxygen is scarcer)
trigger the release of EPO.
• Because EPO increases the hematocrit, it
enables more oxygen to flow to the skeletal
muscles.