**Core Concept**
Pituitary gland enlargement, also known as pituitary hypertrophy, can occur due to various reasons such as adenomas, hyperplasia, or inflammation. This enlargement can lead to compression of adjacent structures, including the optic chiasm, which can result in visual disturbances.

**Why the Correct Answer is Right**
The pituitary gland is located in the sella turcica, a small bony structure in the sphenoid bone. When the pituitary gland enlarges, it can compress the optic chiasm, leading to visual field defects, such as bitemporal hemianopia. This is because the optic chiasm is located above the sella turcica and can be compressed by an enlarged pituitary gland. The compression of the optic chiasm can also lead to other visual disturbances, such as loss of peripheral vision.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to pituitary gland enlargement. While it is true that the pituitary gland is involved in various hormonal processes, its enlargement does not directly cause changes in thyroid function.
**Option B:** While it is true that the pituitary gland is involved in the regulation of growth hormone, its enlargement does not directly cause changes in growth rate.
**Option C:** This option is not relevant to pituitary gland enlargement. While it is true that the pituitary gland is involved in various hormonal processes, its enlargement does not directly cause changes in blood pressure.

**Clinical Pearl / High-Yield Fact**
One important clinical correlation to remember is that pituitary gland enlargement can be a cause of visual disturbances, such as bitemporal hemianopia. This is often seen in patients with pituitary adenomas or other conditions that lead to pituitary hypertrophy.

**Correct Answer:** D.