The endocrine factor that plays the most vital role in stimulating bone growth during the pediatric years is Growth Hormone.

- During childhood and adolescence, bone growth primarily occurs at the epiphyseal growth plates, where new cartilage is continuously produced and then ossified to increase bone length.
- Among the various hormones influencing this process, growth hormone (GH) is the key stimulator.
- GH is secreted by the anterior pituitary gland and acts both directly and indirectly on bones.

- Direct Effects: GH stimulates the proliferation of chondrocytes (cartilage cells) in the growth plate.
- Indirect Effects: GH promotes the production of insulin-like growth factor 1 (IGF-1) in the liver and locally in bones, which further stimulates chondrocyte proliferation and bone matrix deposition.

While other hormones also influence bone metabolism:
- Thyroid hormone is important for normal skeletal development and maturation but acts more as a permissive factor rather than a primary stimulator of growth.
- Parathyroid hormone (PTH) regulates calcium homeostasis and influences bone remodeling but does not directly stimulate linear bone growth.
- Calcitonin inhibits osteoclastic activity, thus reducing bone resorption, but its role in pediatric bone growth is minimal.

Reference: Gardner & Gray's Anatomy, 2nd Edition, Volume 1, Chapter 5: The Endocrine System and Skeletal Growth