Effect of iodine excess on the serum thyroid hormone and the expressi on of thyroid TPO,Tg mRNA in rats

Objective To explore the relation of iodine excess and serum thyroi d hormones, the expression of thyroid peroxidase (TPO) and thyroglobulin (Tg) mR NA. Methods One-month weaning Wistar rats were divided into six groups: low i odine group(LI); normal iodine group(NI); five-fold high iodine(5HI); ten-fold h igh iodine (10HI); fifty-fold high iodine(50HI); one hundred-fold iodine(100HI), and fed with water containing different iodine concentration by adding potassiu m iodide respectively. Rats were sacrificed after being fed for six months. Usin g radioimmunoassay and RT-PCR method, the serum thyroid hormones and the ratio o f TPO/β-actin and Tg/β-actin were measured. Results The levels of serum thy roid hormones in LI and 100HI were lower than that in NI(P 0.01); the level of serum thyroxine was decreased gradually from NI to 50HI group, but there were n o remarkable differences among the remaining four groups(P 0.05); the levels o f TT4, FT4, rT3 in 100HI were higher than those in LI(P 0.01). The TPO mRNA ex pression decreased gradually from LI to 10HI group, but increased from 50HI to 1 00HI. The expressions of Tg mRNA among six groups had no significant differences (P 0.05). Conclusions Chronic iodine excess leads to hypothyroidism, simila r to the consequence of iodine deficiency. Rats have the ability of resisting to iodine excess, only severe iodine excess for a long time can lead to hypothyroi dism. Hypothyroidism can promote the expression of the TPO mRNA whether it is in duced by iodine deficiency or iodine excess.