Determination of atomic local order in thyroid hormones by extended x-ray absorption fine structure [EXAFS] for radiation dose estimates.

Dosimetry from 125I emitting Auger electrons in the follicles of the thyroid will improve when their complete interaction with the molecular structure about the Auger emitting atoms is known to Angstrom distances. Extended x-ray absorption fine structure (EXAFS) can provide this information. EXAFS experiments gave intermolecular and intramolecular distance for pure solid L-thyroxine (T4) (C15H11I4NO4) using a model based on crystalline T4 hydrochloride monohydrate (C15H11I4NO4HCl.H2O). For a solution of L-thyroxine the structure consists of the intramolecular distances found for T4 and an additional shell of four carbon atoms distances of 2.01 A. Atomic density functions from Fourier transformed EXAFS measurements are suitable for estimating very short range interactions of Auger electrons in thyroid thyroglobulin over a radial distance of 7.71 A. Examination of density functions provided a plausible argument for the difference in biological effect between 131I and 125I in the thyroid.