| After successfully establishing animal models of iodine deficiency and iodine excess in Wistar rats and Balb/c mice,we observed the changes of thyroid morphorlogy and its function under the status of different iodine intakes,and then the mRNA and protein expression of IGF-1 and TGF-β1 were measured in thyroid tissue with the purpose to explore the possible effect of these two growth factors(IGF-1 and TGF-β1)involved in the thyroid growth and function caused by iodine deficiency and iodine excess.Wistar rats and Balb/c mice were randomly divided into three groups according to the gender and body weight:①low iodine(LI);②normal iodine(NI);③fifty-fold high iodine(50HI).The iodine content in animal diet for LI group was averagelly 50μg/kg,and 300μg/kg for both NI and 50HI groups.Aanimals of LI and NI groups drank deionized water and those of 50HI drank deionized water containing a certain content of potassium iodide.Based on the daily food-intake and drinking water of rats and mice,we calculated the total iodine intake per day for rats as 1μg/d(LI),6μg/d (NI),300μg/d(50HI);and for mice as 0.25μg/d(LI),1.5μg/d(NI),75μg/d(50HI), respectively.At the three and six months of experiment,the urinay iodine,thyroidal iodine,and thyroid tissue T3 and T4 levels were measured;the thyroid morphological changes were observed,and the storage and content of thyroid colloid(thyropid globulin)were anylazed by morphometry.The thyroid IGF-1 and TGF-β1 mRNA and their protein expressions were detected by Real time fluorescent quantitation PCR, half-quantitation PCR and immunohistochemistry techniques,respectively.Results:1.Iodine in urine and thyroid tissue:Urinary iodine levels in LI animals were much lower than that in the other groups(p＜0.01),nearly unable to detect;and the urinary iodine levels in NI and 50HI groups were paralleled with their iodine intake. The iodine contents in thyroid tissue increased from iodine deficiency to iodine excess,but were not paralleled with their iodine intake.2.Hormone levels in thyroid tissue:Thyroid tissue T3 and T4 levels of LI animals were significantly lower and T3/T4 were higher than those of NI animals (p＜0.01).No significant difference in T4 level was found between 50HI and NI in rats, but T3 and T3/T4 levels in 50HI were lower significantly than those of NI rats at both 3 and 6 months.In 50HI mice,thyroid tissue T3 and T4 levels increased obviously at 3 month,and at 6 month T3 level approached to be normal but T4 level still increased; and T3/T4 level decreased at both 3 and 6 months with no statistical significance.3.Thyroid morphological changes:We observed the histological changes of thyroid tissue by microscope.Most of NI animals showed that thyroid follicles were moderate in size,epitheliums were cubical or flat and colloid was abundant in the follicular lumen.In all LI groups,small follicular-hyperplasia was found in rats and mice thyroid glands.In contrast to NI group rats,50HI group appeared polytypism changes having a concurrence of increased large follicles with colloid accumulation and small follicular-hyperplasia.While iodine excess mice showed complete difference from rats,scarcely any thyroid hyperplasia and obvious colloid accumulation in enlarged follicular lumen with flat epitheliums,which indicated that a typical colloid-accumulation goiter occurred in 50HI mice.4.The morphometry analysis of thyroid colloid accumulative level:There were no evident difference between 50HI and NI rats in the sectional area of thyroid follicular cavity and the ratios of different size follicles at 3 and 6 months,which didn't indicate that big follicular colliodal accumulation occurred in 50 HI rats;in contrast with NI group,the area ratios of colloid and field view increased in 50HI rats at 3 month,and decreased at 6 month;the average gray density of colloid decreased in 50 HI rats at 3 month compared with NI group,and there was no significant difference between 50HI and NI groups at 6 month;therefore it showed that the total thyroid globulin content had no obvious change in rats under iodine excess intake. However,the sectional area of thyroid follicular cavity of 50HI rats was higher than that of NI group at 3 and 6 months,in contrast with NI group,the ratio of small and moderate folliclular cavity decreased,and the ratio of big follicular cavity increased, which indicated that big follicular colloidal accumulation occurred in 50HI mice;the area ratios of colloid and field view and the average gray density were higher than those of NI group at 3 and 6 months,therefore it showed that the total thyroid globulin content increased significantly in mice under iodine excess intake. 5.Thyroid IGF-1 and TGF-β1 mRNA and protein expressions in rats and mice: In 3 month rats,IGF-1 mRNA expression level increased in LI and 50HI groups, especially more obviously in 50HI group compared to the NI group.In 6 month rats, IGF-1 mRNA expression level was significantly lower in LI group than that of NI group,and 50HI group was nearly normal.In 3 month rats,in contrast with NI group, the thyroid IGF-1 protein expression level increased significantly in LI and 50HI groups,we could see obviously positive particles in the thyroid epithelial cytoplasm. In 6 month rats,the IGF-1 protein expression level of LI group obviously decreased and 50HI group showed no significant difference compared to NI group.In mice,the IGF-1 mRNA expression level was higher in LI group and was no statistical difference in 50HI group compared with the NI group at both 3 and 6 months.Similar change's trend in protein expression was found that IGF-1 protein expression remarkably increased in LI group and no much change in 50HI group when compared to the NI group during whole experiment.Our findings demonstrated that the thyroid IGF-1 mRNA expression level was in accordance with its protein in all groups of rats and mice.In contrast to NI group,thyroid TGF-β1 mRNA expression level increased in LI and 50HI groups with more obviously in LI group of 3 and 6 month rats.In 3 month rats,the thyroid TGF-β1 protein expression level was lower in LI group than that in NI group,and there was no significant difference between 50HI and NI groups.In 6 month rats,the TGF-β1 protein expression level increased in LI group and decreased in 50HI group compared to NI group.In 3 and 6 months mice,the thyroid TGF-β1 mRNA expression levels were higher in LI and 50HI groups than that in NI group.In 3 month mice,there was no difference in thyroid TGF-β1 protein expression level between LI and NI groups,while that of 50HI was lower than the NI group.In 6 month mice,the TGF-β1 protein expression level increased obviously in LI group and no significant difference in 50HI compared with NI group.The results indicated that the thyroid TGF-β1 mRNA was in accordance with its protein only in LI groups of 6 month rats and mice. Conclusions:1 In our present study,we comparatively observed thyroid morphological and functional changes in two species of rodent,Wistar rat and Balb/c mouse,caused by iodine deficiency and iodine excess.The results showed that iodine deficiecy produced overt hypothyroidism and consequent hyperplastic goiter in both rat and mouse.Iodine excess,however,caused only polytypism change without goiter in rats, but a typical colloid accumulation goiter in mice.Combined other obtained results, we could conclude that Wistar rats and Balb/c mice all had strong adaptive capacity to iodine excess,but a certain variance existed between rats and mice in the regulation ability of stabilizing thyroidal iodine content,hormone level and thyroid structure change;and on the whole,Wistar rats had stronger adaptability than Balb/c mice.2 We observed that thyroid tissue IGF-1 mRNA expression level transiently increased in iodine deficiency rats(at 3 month)and persistently increased in iodine deficiency mice(at 3 and 6 month),therefore it could confirm that thyroid autocrine IGF-1 may be involved in the formation of thyroid hyperplastic goiter caused by iodine deficiency.In the status of iodine excess,the thyroid tissue IGF-1 expression levels were incompletely different between rats and mice,however,the changes in IGF-1 expression level were consistent to their respective thyroid morphological change.Regarding to the relationship between thyroid autocrine IGF-1 and thyroid function change in iodine deficiency and iodine excess still need be further studied.3 Thyroid autocrine TGF-β1,inhibiting thyroid follicular hyperplasia,increased in all LI animals,which indicated the mechanism of thyroid homeostasis.Thyroid TGF-β1 mRNA expression levels were increased in 50HI rats and mice,which could demonstrate the thyroid TGF-β1 may participate in the regulation of rat thyroid polytypism change and mouse colloid accumulative goiter formation.
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