**Core Concept**
Pendred syndrome is a genetic disorder characterized by congenital bilateral sensorineural hearing loss and euthyroid goiter due to mutations in the SLC26A4 gene, which encodes pendrin, a protein involved in iodide transport in the thyroid gland and chloride transport in the inner ear.

**Why the Correct Answer is Right**
The SLC26A4 gene mutations lead to the production of a dysfunctional pendrin protein, resulting in impaired iodide uptake in the thyroid gland and chloride transport in the inner ear. This disruption causes the thyroid gland to become enlarged (goiter) and the inner ear to develop abnormal structures, leading to hearing loss. The euthyroid state is maintained due to the presence of other mechanisms compensating for the impaired iodide uptake.

**Why Each Wrong Option is Incorrect**
**Option A:** Pendred syndrome is not caused by a deficiency of iodine in the diet. While iodine deficiency can cause thyroid disorders, it is not directly related to Pendred syndrome.
**Option B:** Pendred syndrome is not characterized by hyperthyroidism. The euthyroid state is maintained due to compensatory mechanisms, but hyperthyroidism is not a feature of this condition.
**Option C:** Pendred syndrome is not caused by mutations in the SLC26A4 gene. While this is the correct gene involved, the question is likely asking for a different option, and this distractor is incorrect.

**Clinical Pearl / High-Yield Fact**
Pendred syndrome is the most common cause of congenital hearing loss in children, and it is often associated with other features such as cleft palate and cardiac abnormalities.

**Correct Answer:** D.