**Core Concept**
Growth hormone (GH) is a peptide hormone produced by the anterior pituitary gland that plays a crucial role in regulating growth and metabolism. The effects of GH on growth are mediated through a complex interplay of hormones and growth factors.

**Why the Correct Answer is Right**
Growth hormone exerts its effects on growth by stimulating the production of insulin-like growth factor 1 (IGF-1), also known as somatomedin C. IGF-1 is a protein hormone that is produced in the liver and other tissues in response to GH. IGF-1 then acts on various tissues, including cartilage and bone, to promote growth and cell proliferation. The GH-IGF-1 axis is a critical signaling pathway that regulates growth and development during childhood and adolescence. IGF-1 binds to its receptor, which triggers a cascade of intracellular signaling events that ultimately lead to the activation of genes involved in cell growth and division.

**Why Each Wrong Option is Incorrect**
**Option A:** Growth hormone does not have a direct effect on growth; rather, it stimulates the production of IGF-1, which then mediates its effects on growth.
**Option C:** Thyroxine is a hormone produced by the thyroid gland that regulates metabolism, but it is not directly involved in the effects of growth hormone on growth.
**Option D:** Growth hormone does not act through intranuclear receptors; instead, it binds to a membrane receptor on the surface of target cells, triggering a signaling cascade that ultimately leads to the activation of genes involved in cell growth and division.

**Clinical Pearl / High-Yield Fact**
The GH-IGF-1 axis is a critical regulatory pathway that is often disrupted in growth disorders, such as growth hormone deficiency or excess. Understanding the role of IGF-1 in mediating the effects of GH on growth is essential for the diagnosis and treatment of these disorders.

**✓ Correct Answer: B. IG1-1**