Effects of gentamicin on the thyroid function of rats

The effect of gentamicin (GM) on the thyroid function was investigated in relation with its stimulating action to the growth of rats at a very small dose level. The results may be summarized as follows: (1) Changes in 131-I-uptake of the thyroid after administration of GM were unremarkable. The change in wet weight of the thyroid was also unremarkable. Therefore, GM may not affect the thyroid itself. (2) GM-treated rats clearly showed a decrease in 131-I-triiodothyronine resine sponge uptake (trisorb test) as compared with the control animals, viz.,GM-treated rats indicated hypofunction of the thyroid. (3) Thyroxin (t4) levels in theserum, determined by 125-I-T4 resine sponge uptake (tetrasorb test), were apparentlylower in the GM-treated rats than in the control. (4) The thyroid function test for synthesis of the hormones revealed an increase in diiodotyrosine (DIT) value and decreases in 3, 5, 3'-triiodothyronine (T3) and T4 values in the GM-treated groups as compared with the values in the control. (5) In view of the above results iit was obvious that GM does not cause abnormalities on iodine-uptake, iodine oxidation, organic conversion to iodinated tyrosines, deiodination and thyroglobulin. In contrast, GM may induce abnormality on coupling reaction from DIT to T3 and T4 because the increase in DIT value and decreases in T3 and T4 values are seen following administration of GM. The whole of these facts seems to play an important role in stimulation to growth of rats after administration of the drug.