**Question:** Thyroid hormones in blood is transported by:

**Core Concept:** Thyroid hormones are essential for proper development and regulation of various physiological processes, and they are transported in the bloodstream. The primary thyroid hormones are triiodothyronine (T3) and thyroxine (T4). The thyroid hormones are bound to carrier proteins in the blood, ensuring their stability and safe delivery to target tissues.

**Why the Correct Answer is Right:** Thyroid hormones are transported in the blood by thyroxine-binding globulin (TBG), also known as thyroxine-binding protein (TBP), which is an albumins-like protein. TBG binds to T4, enabling it to be transported efficiently throughout the body. This binding is highly specific, ensuring that the thyroid hormones remain bound to TBG until they reach the target tissues where they are released.

**Why Each Wrong Option is Inaccurate:**

A. Option A (albumin) is incorrect because it is not specific for thyroid hormones; it binds other proteins and does not have the same affinity for thyroid hormones as TBG.

B. Option B (transport proteins) is not the correct answer because it does not name the specific protein responsible for thyroid hormone transport - TBG.

C. Option C (free T4) is incorrect because free T4 refers to T4 not bound to any carrier protein, not the carrier protein itself.

D. Option D (free T3) is also incorrect as it refers to T3 not bound to any carrier protein, not the carrier protein itself.

**Clinical Pearl:** The specificity of TBG ensures that thyroid hormones reach their target tissues effectively, preventing premature release and potentially harmful effects when bound to other proteins or tissues. Additionally, the transport of thyroid hormones by TBG helps regulate their concentration in the bloodstream to maintain their steady-state levels, preventing excessive stimulation of target tissues.

**Correct Answer:** C. Thyroxine-binding globulin (TBG) is responsible for transporting thyroid hormones in the blood.