**Core Concept**
Primary hypothyroidism refers to a condition where the thyroid gland fails to produce sufficient thyroid hormones, leading to an imbalance in the body's metabolic processes. This condition is often caused by autoimmune thyroiditis, thyroid surgery, or thyroid radiation.

**Why the Correct Answer is Right**
In primary hypothyroidism, the thyroid gland is unable to produce sufficient thyroid-stimulating hormone (TSH), leading to decreased levels of thyroxine (T4) and triiodothyronine (T3). The pituitary gland, in response to low T4 and T3 levels, produces more TSH to stimulate the thyroid gland, but this is insufficient to overcome the gland's dysfunction. The correct answer, elevated TSH, is a hallmark of primary hypothyroidism.

**Why Each Wrong Option is Incorrect**
* **Option A:** This is incorrect because in secondary hypothyroidism, TSH is low due to pituitary dysfunction.
* **Option B:** This is incorrect because in tertiary hypothyroidism, TSH is low due to hypothalamic dysfunction.
* **Option D:** This is incorrect because in euthyroid sick syndrome, thyroid hormone levels are usually normal, but non-thyroidal illness can cause elevated reverse T3.

**Clinical Pearl / High-Yield Fact**
Remember that primary hypothyroidism is characterized by elevated TSH levels, while secondary and tertiary hypothyroidism present with low TSH levels.

**Correct Answer: A. Elevated TSH.**