Which analyte can only be run on serum (gold top) tubes?

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The key idea is that the type of blood sample and the tube it’s collected in can affect how some hormone tests perform. Free T4 measurements are particularly sensitive to the sample matrix, because the assay is designed to quantify the unbound hormone and can be influenced by anticoagulants or other components present in plasma. Serum obtained from a gold top tube (which clots and separates away clotting factors) provides a cleaner matrix for these immunoassays, reducing potential interference and yielding more reliable results for free T4.

So, among the options, free T4 is the one typically validated and run on serum samples to ensure accuracy, whereas the other hormones listed can be measured in serum as well, but this question highlights serum as the preferred, standard sample type for free T4 due to the assay’s reliance on a serum matrix.