The concentration and the distribution of iodine in various sized follicles of rat thyroid glands have been analyzed by electron-probe microanalysis. The results of the iodine analysis were grouped according to uncorrected lumen diameter size. No significant differences in iodine concentration were observed among the various size categories. When the results for all follicles from a given sample were pooled, the standard error of the mean was approximately 4%. Usually 40–50 follicles per animal were analyzed. The concentration of iodine ranged from 0.9 to 2.1% by weight among individual animals. Scanning pictures and step-scan analysis showed the iodine distribution to be quite uniform across the colloid area. Several techniques of sample preparation were used; they produced no significant differences in the iodine concentrations observed. Sodium concentration, also determined in all samples, was found to vary from 2 to 9% by weight. Because of the mobility of the sodium ion, its distribution was greatly affected by the method of sample preparation. The technique that best preserved the natural chemistry of the sample was that of freezing the tissue, sectioning, and then freeze-drying.

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