THE EARLY STAGE OF THYROID HORMONE FORMATION: STUDIES ON RAT THYROIDSIN VIVO1

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Abstract
Labeled components of thyroids from normal and iodine-deficient rats have been studied 2 minutes to 24 hours after subcutaneous injection of I131. Normal glands collected 1% of dose at 120 minutes, whereas the iodinedeficient group collected 12%. Both groups accumulated I131 in excess of binding during the earl) minutes of observation. Transfer of PBI131 from particulatc to supernatant fraction was not proved. However, in studies on the distribution of total gland homogenate PBI131 between supernatant fraction (soluble cell proteins and colloid) and particulate fractions (nuclear, mitochondria], and microsomal), it was observed in some groups of animals that the proportion of PBI131 present in the supernatant fraction increased rapidly during the first 20 minutes, and then plateaued. MIT and DIT were present within 2 minutes of labeling, and in hyperplastic glands thyroxine was present after 14 minutes. The ratio of DIT/MIT was reduced by iodine deficiency, but in all groups the ratio remained relatively constant during the experiment. Thyroglobulin, and, in iodine-deficient glands a second distinct protein, were the only soluble iodinated proteins detected.