Acute Effects of Thyrotropin on Thyroid Hormone Biosynthesis in the Rat

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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 [thyroid stimulatory hormone] 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 I131 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 I131 uptake, TSH administered immediately before I131-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 within 30 min. after TSH administration was also seen in hypophysectomized animals. It is suggested that TSH rapidly stimulates the coupling of freshly iodinated iodotyrosines to form iodothyronines.