**Core Concept**
Primary thyrotoxicosis, also known as hyperthyroidism, is a condition characterized by the excessive production of thyroid hormones, primarily triiodothyronine (T3) and thyroxine (T4), due to an intrinsic defect within the thyroid gland itself. This can result from various causes, including Graves' disease, toxic multinodular goiter, or thyroiditis.

**Why the Correct Answer is Right**
The correct answer is related to the fact that primary thyrotoxicosis is often associated with an elevated level of thyroid-stimulating hormone receptor antibodies (TRAb). These antibodies stimulate the thyroid gland to produce excessive amounts of thyroid hormones, leading to hyperthyroidism. Additionally, primary thyrotoxicosis can also result in an elevated level of free T4 (FT4) and free T3 (FT3) in the blood.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it is true that primary thyrotoxicosis is often associated with an elevated level of thyroid-stimulating hormone (TSH) in the blood, as the pituitary gland attempts to stimulate the thyroid gland to produce more thyroid hormones.
**Option B:** Incorrect because it is true that primary thyrotoxicosis can result in an elevated level of free T4 (FT4) and free T3 (FT3) in the blood.
**Option C:** Incorrect because it is true that primary thyrotoxicosis can be caused by an intrinsic defect within the thyroid gland itself, such as Graves' disease.

**Clinical Pearl / High-Yield Fact**
It is essential to differentiate between primary and secondary hyperthyroidism, as the underlying cause and treatment approach differ significantly. Secondary hyperthyroidism is caused by an excess of thyroid-stimulating hormone (TSH) from the pituitary gland, whereas primary hyperthyroidism is caused by an intrinsic defect within the thyroid gland itself.

**Correct Answer:** A.