**Core Concept**
Thyroxine (T4) is a lipophilic hormone synthesized by the thyroid gland, and its transport in the bloodstream is crucial for maintaining its bioavailability. The primary mechanism of thyroxine transport involves specific proteins that bind to the hormone, protecting it from degradation and ensuring its delivery to target tissues.

**Why the Correct Answer is Right**
Thyroxine is primarily transported in the bloodstream bound to thyroxine-binding globulin (TBG), a protein produced by the liver. TBG has a high affinity for T4, with a binding capacity of approximately 99.97%. This binding is essential for maintaining the hormone's stability and facilitating its transport to tissues, such as the liver, kidneys, and muscles, where it can be converted into its active form, triiodothyronine (T3). Additionally, thyroxine is also bound to albumin and prealbumin (transthyretin), but to a lesser extent.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not a correct answer, as it does not accurately describe the primary transport mechanism of thyroxine.
**Option B:** While thyroxine is indeed lipophilic, this option does not specifically address its transport mechanism in the bloodstream.
**Option C:** This option is incorrect, as it does not accurately describe the primary transport mechanism of thyroxine.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that alterations in TBG levels can affect thyroxine transport and lead to changes in T4 levels, even in the absence of thyroid dysfunction. For example, conditions like nephrotic syndrome or severe liver disease can lead to decreased TBG levels, resulting in lower T4 levels.

**Correct Answer: C. Thyroxine-binding globulin (TBG).**