RELATIONSHIP BETWEEN PEROXIDASE ACTIVITY AND SERUM TSH, T4 AND T3 LEVELS IN RATS IN THE COURSE OF IODINE DEFICIENCY

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Abstract
The relationship between peroxidase activity and serum TSH [thyrotropin], T4 [thyroxine] and T3 [triiodothyronine] levels was investigated in the course of I deficiency in rats. Rats were maintained either on a control diet with a relatively high I content (600 .mu.g/kg of 127I), or on a low I diet (30 .mu.g/kg of 127I). Twenty days after the low treatment, the thyroid I [127I]concentration was half that of control value (647 .+-. 52 and 1241 .+-. 72 .mu.g/g of wet weight, respectively). However, no significant changes in serum T4, T3, TSH were found at 20 days even though an early increase in peroxidase activity was observed. It was only at 35 days of I deficiency, when the concentration of I in the gland averaged 260 .mu.g/g of wet weight that serum T4 and TSH levels started to be significantly modified. From day 35-day 70, a significant and progressive decrease of plasma T4 concentration was observed, and it leveled off thereafter. The changes of serum T3 were much smaller than those of T4. A significant increase in serum TSH level was noted at 35 days. Thereafter TSH levels increased rapidly and progressively (205% increase over control at 70 days and 643% at 80 days). From day 35 until day 80 of the low I treatment, the thyroid peroxidase activity and the serum TSH level varied concomitantly. The results suggest that for an I content between 5 and 2 .mu.g per thyroid gland, the high cellular peroxidase activity observed could be correlated with an increase in circulating TSH, due to a decrease of T4. In contrast, in the early period of I deficiency, no correlation was found between peroxidase activity and serum T4, T3 and TSH levels.

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