**Core Concept**
Sheehan syndrome is a rare endocrine disorder that occurs in postpartum women, characterized by hypopituitarism due to necrosis of the pituitary gland. This condition is often caused by severe postpartum hemorrhage leading to hypovolemic shock, resulting in pituitary gland ischemia.

**Why the Correct Answer is Right**
The correct answer is related to the pathophysiology of Sheehan syndrome. Severe postpartum hemorrhage leads to hypovolemic shock, which in turn causes a decrease in blood flow to the pituitary gland. This reduction in blood flow results in ischemia, which can cause necrosis of the pituitary gland, leading to hypopituitarism. This condition can affect the production of various hormones, including ACTH, TSH, FSH, and LH.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the pathophysiology of Sheehan syndrome. While thyroid dysfunction can occur in Sheehan syndrome, it is not the primary cause of the condition.
**Option B:** This option is not accurate. Sheehan syndrome is not a form of thyroiditis, and the thyroid gland is not directly affected by the condition.
**Option C:** This option is incorrect. While pituitary tumors can cause hypopituitarism, Sheehan syndrome is a specific condition caused by postpartum hemorrhage and pituitary gland ischemia, not by a tumor.

**Clinical Pearl / High-Yield Fact**
It is essential to recognize the risk factors for Sheehan syndrome, including severe postpartum hemorrhage, multiparity, and previous history of postpartum hemorrhage. Early recognition and treatment of postpartum hemorrhage can help prevent this condition.

**Correct Answer: D. Postpartum hemorrhage leading to pituitary gland ischemia.**