**Core Concept:** Transferrin is an iron-carrying protein in the blood that plays a crucial role in iron transport and regulation, ensuring iron distribution to cells and tissues where it is needed.

**Why the Correct Answer is Right:** Normal transferrin, specifically the α-isoform, carries iron in a tightly bound form called holotransferrin, which is saturated with iron. This saturation allows holotransferrin to bind to transferrin receptors on the cell surface, facilitating iron uptake into cells.

**Why Each Wrong Option is Incorrect:**

A. **Transferrin saturation is not normally less than 20%**: This is incorrect because the question asks for the saturation level of normal transferrin, not an absolute saturation percentage.

B. **Transferrin is not saturated with iron in polycythemic states**: Although the saturation may increase due to increased production of transferrin, the core concept is that normal transferrin is saturated with iron as mentioned in the correct answer.

C. **Transferrin saturation is not related to iron deficiency**: This is incorrect because the question is about the saturation of normal transferrin, not iron deficiency. Transferrin saturation may be reduced in iron deficiency, but the focus here is on the saturated form in healthy individuals.

D. **Iron saturation is unrelated to plasma volume**: The core concept is about the saturation of transferrin with iron, not plasma volume. Although plasma volume may alter iron saturation, the question addresses the iron content of transferrin, not its relationship to plasma volume.

**Clinical Pearl:** A transferrin saturation below 20% or above 60% may suggest underlying conditions such as iron deficiency or polycythemia, respectively. Assessing transferrin saturation along with other iron studies can help diagnose and monitor these conditions.