What is the reference range for serum T4 in females?

• A. 0.9-2.3 ng/dL
• B. 5.5-12.5 ng/dL
• C. 1.0-2.0 ng/dL
• D. 3.0-5.0 ng/dL

Answer: (A)

Understanding reference ranges for T4 starts with recognizing which form and unit are being used. Clinically, the relevant value is often free T4, the unbound hormone, which is reported in ng/dL. A typical reference interval for free T4 in adult females is about 0.8 to 2.3 ng/dL, though exact bounds can vary by assay and lab. The range 0.9–2.3 ng/dL sits right within that commonly accepted interval, capturing values from near the lower limit up to the upper end seen in many laboratories. Total T4, by contrast, is usually given in μg/dL (around 5–12 μg/dL); those units and ranges don’t align with ng/dL values, making the other options inconsistent with the standard free T4 reference. So this range reflects the usual adult female free T4 interval, allowing for lab-specific variation.