Immunoassay method for thyroxine in which antibody-bound labeled thyroxine rotates slowly and emits lower energy light is known as?

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Fluorescence polarization is based on how fast a fluorescent probe rotates in solution. A small fluorescently labeled thyroxine molecule rotates rapidly when free, which leads to emission that is less polarized. When this labeled thyroxine binds to a large antibody, the complex tumbles more slowly, causing the emitted light to be more polarized (higher polarization). In a competitive thyroxine immunoassay, more native thyroxine in the sample displaces the labeled tracer from the antibody, reducing the fraction that is bound and thus lowering the polarization signal. This combination of binding-dependent rotation and measured polarized emission is the hallmark of a fluorescent polarization immunoassay, a homogeneous method commonly used for small molecules like thyroxine.

Other immunoassay approaches rely on different signal generations (such as chemiluminescence) or different readouts (enzyme labels with microparticle formats), which do not use the rotation–polarization principle at all.