**Core Concept**
Thyroid hormones, such as triiodothyronine (T3) and thyroxine (T4), play a crucial role in regulating metabolism, growth, and development. Their intracellular transport is mediated by specific proteins that facilitate their entry into cells.

**Why the Correct Answer is Right**
Thyroid hormones are transported into cells via the thyroid hormone transport protein, transthyretin (previously known as prealbumin), and to a lesser extent, by albumin. However, the primary mechanism of intracellular uptake involves the thyroid hormone transport protein, which binds to T4 and T3, allowing them to cross the cell membrane. This process is crucial for the regulation of thyroid hormone levels within the cell.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to thyroid hormone transport.
**Option B:** This option is incorrect as it does not accurately describe the primary mechanism of intracellular thyroid hormone uptake.
**Option C:** This option is incorrect as it is not a known protein involved in thyroid hormone transport.

**Clinical Pearl / High-Yield Fact**
One key point to remember is that thyroid hormone transport proteins, such as transthyretin, are often decreased in conditions like nephrotic syndrome, leading to decreased levels of free thyroid hormones in the blood.

**Correct Answer:** C. Transthyretin.