Abstract
Incorporation of radioiodine into intrathyroidal iodoamino acids was studied in Purina-fed rats at intervals ranging from 15 min to 12 hr after TSH administration. As has previously been reported, there is a latent period of several hours before TSH stimulates thyroidal radioiodine uptake. An initial decrease in 3-hr thyroidal 131I uptake was found when TSH was injected 2 hr before radioiodine. Progressive increases in uptake were seen when TSH was given 4, 8 and 24 hr before radioiodine, but these were significantly different from non-TSH treated controls only at 8 and 24 hr. In contrast to the lack of an early effect on 131I uptake, TSH administered immediately before 131I- strikingly altered the composition of radioiodinated substances within the thyroid gland within 30 min. There was a marked stimulation of T4* synthesis and a corresponding decrease in DIT* synthesis. A lesser increase in T3* and a questionable reduction in MIT* formation were also found. Stimulation of labeled iodothyronine synthesis...
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