**Core Concept**
Diffuse hair loss in a postpartum woman is a common condition that can be caused by hormonal changes, particularly the decrease in estrogen levels after childbirth. This condition is often referred to as postpartum alopecia or telogen effluvium.

**Why the Correct Answer is Right**
The decrease in estrogen levels after childbirth leads to a shift from the anagen phase to the telogen phase of the hair growth cycle. As a result, a large number of hairs enter the resting phase and are then shed, leading to diffuse hair loss. This condition is usually self-limiting and resolves on its own within 6-12 months after delivery.

**Why Each Wrong Option is Incorrect**
* **Option A:** Postpartum thyroiditis is an autoimmune condition that can cause hair loss, but it is not the most common cause of diffuse hair loss in this scenario.
* **Option B:** Anagen effluvium is a condition that causes hair loss due to damage to the hair follicles, often due to chemotherapy or radiation. It is not related to the postpartum period.
* **Option C:** Alopecia areata is an autoimmune condition that causes patchy hair loss, but it is not associated with the postpartum period.

**Clinical Pearl / High-Yield Fact**
Postpartum hair loss is a common condition that can be distressing for new mothers. It is essential to reassure them that it is a normal and temporary condition that resolves on its own within a few months.

**Correct Answer: D. Telogen effluvium.**