Activity of Thyroid of Cold-Exposed Rats Evaluated by I131 Uptake and Histometric Studies

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Abstract
In cold-exposed rats (3[degree]C) as well as in controls (24-26[degree]C), an inverse relationship between the daily amt. of dietary I intake and the collection of I131 tracer dose by the thyroid gland was established. The mean acinar cell height was not altered if the daily I intake was maintained at least at the minimal level of 1.5-2 [mu]g. The 24-hr. collection values of I131 are the same for cold-exposed rats as for controls (24-26[degree]C), suggesting a dissociation between uptake and the high mean acinar cell height observed. Accumulation-rate studies, however, indicate a rapid accumulation of I131 by the thyroid of cold-exposed animals, thus revealing the hyperplasia of the thyroid gland. In cold-exposed animals, the turnover and utilization of thyroid hormone are greater than in rats treated with TSH. Purina chow, due to its high I content, hinders the collection of I131 by the thyroid gland; this effect can be eliminated by fasting the animals for 24 hrs. immediately after administering a tracer dose.