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

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.