**Core Concept**
The most common cause of hypothyroidism in pregnancy is related to the increased thyroid hormone requirements during gestation. **Thyroid-stimulating hormone (TSH)** levels need to be tightly regulated to ensure proper fetal development. However, certain conditions can disrupt this balance, leading to hypothyroidism.

**Why the Correct Answer is Right**
The correct answer is due to the high prevalence of **Hashimoto's thyroiditis** in pregnant women. Hashimoto's is an autoimmune condition where the body's immune system attacks the thyroid gland, leading to inflammation and eventual hypothyroidism. This condition is more common in women of childbearing age and can be triggered or exacerbated by pregnancy. The underlying mechanism involves **antithyroid antibodies**, which target the thyroid gland and disrupt thyroid hormone production.

**Why Each Wrong Option is Incorrect**
* **Option A:** **Subclinical hypothyroidism** is a condition where TSH levels are elevated, but free thyroxine (FT4) levels are within the normal range. While it can be a precursor to overt hypothyroidism, it is not the most common cause of hypothyroidism in pregnancy.
* **Option B:** **Postpartum thyroiditis** is a condition that occurs after childbirth, characterized by transient hypothyroidism. It is not a common cause of hypothyroidism during pregnancy.
* **Option C:** **Iodine deficiency** can cause hypothyroidism, but it is relatively rare in developed countries with adequate iodine intake.

**Clinical Pearl / High-Yield Fact**
It is essential to diagnose and manage hypothyroidism promptly in pregnant women to ensure proper fetal development and prevent complications such as **fetal growth restriction** and **preterm labor**.

**Correct Answer:** C. Hashimoto's thyroiditis.