**Core Concept**
Gestational hyperthyroidism refers to the development of hyperthyroidism during pregnancy, which can be caused by various factors. This condition can lead to maternal and fetal complications if left untreated. The underlying principle being tested is the pathophysiology of thyroid hormone regulation during pregnancy.

**Why the Correct Answer is Right**
Gestational hyperthyroidism is often due to the stimulation of the thyroid gland by human chorionic gonadotropin (hCG) produced by the placenta. hCG has a similar structure to thyroid-stimulating hormone (TSH) and can bind to the TSH receptor, leading to an increase in thyroid hormone production. This condition is most commonly seen in pregnancies where the hCG levels are elevated, such as in multiple gestations or in the presence of a molar pregnancy.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not specify the underlying cause of gestational hyperthyroidism. While thyroiditis can cause hyperthyroidism, it is not the primary cause during pregnancy.
* **Option B:** This option is incorrect because Graves' disease is an autoimmune cause of hyperthyroidism and is not specific to pregnancy.
* **Option C:** This option is incorrect because iodine deficiency can cause hypothyroidism, not hyperthyroidism.

**Clinical Pearl / High-Yield Fact**
It is essential to diagnose and manage gestational hyperthyroidism promptly to prevent maternal and fetal complications. A thorough history and physical examination, along with laboratory tests, are necessary to establish the diagnosis.

**Correct Answer:** D.