THE EFFECT OF IODINE DEFICIENCY AND SUPPRESSION OF TSH SECRETION ON RADIOIODINE LABELLED THYROXINE AND TRIIODOTHYRONINE IN THE THYROID OF RATS

ABSTRACT It has been suggested that TSH stimulation of the thyroid gland is accompanied by an alteration in the ratio of newly synthesized thyroxine (*T4)1)/triiodothyronine (*T3) in favour of *T3. Evidence in support of this hypothesis is provided here by the finding that suppression of TSH secretion in rats alters this ratio in the other direction, i. e. in favour of *T4. Thus, endogenous TSH stimulation was increased for 4 weeks by iodine deficiency. Its suppression was performed by the administration of T3, using a dose of 0.05-0.25 μg/100 g body weight which was injected subcutaneously every 12 hours for three days. The effect of TSH stimulation and suppression could be assessed from the following parameters: thyroid weight and histology, thyroid 131I uptake, 131I conversion ratio, hormonal iodine concentration, and TSH level in the plasma. After iodine deficiency the ratio of *T4/*T3 in the thyroid gland changed in favour of *T3. This may compensate for the iodine deficiency, since the oxygen consumption and the heart rate of the animals remained in the normal range. After suppression the ratio of *T4/*T3 changed in the opposite direction, i. e. in favour of *T4. The extent of the suppression of *T4 and *T3 was dependent on the suppression dose used.