Why is free T4 sometimes preferred over total T4 in certain clinical scenarios?

• A. Free T4 reflects the binding protein levels and correlates with protein status.
• B. Free T4 reflects the unbound, biologically active hormone and is less affected by binding protein levels.
• C. Total T4 is more accurate in pregnancy.
• D. Free T4 is not affected by assay interference.

Answer: (B)

The key idea is that the unbound portion of thyroid hormone is the one that actually affects tissues. In the blood, most T4 is bound to transport proteins like thyroxine-binding globulin, albumin, and transthyretin, serving as a reservoir. Because of that binding, a person can have a normal or even high total T4 simply because binding protein levels are high, without the tissues experiencing more thyroid activity. In conditions that alter binding proteins—such as pregnancy, estrogen therapy, liver disease, or nephrotic syndrome—total T4 can be misleading. Free T4 measures the fraction that is not bound and is free to enter cells, so it more accurately reflects the current, biologically active thyroid status in those scenarios. While free T4 measurements can still be affected by assay issues, the main reason for preferring them is that they track the active hormone independently of binding protein fluctuations.