| labelled atom | A radioactive atom, or a stable but rare one, which by its presence in a molecule helps localization or measurement of that molecule. Synonym: tagged atom. (05 Mar 2000) |
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| labelled thyroxine | Thyroxine in which a radioisotope of iodine (125I or 131I) is incorporated into its molecule; used in experiments tracing the metabolism of thyroxine. Synonym: labelled thyroxine, radiolabelled thyroxine, radiothyroxin. (05 Mar 2000) |